A PRIMARY
I SUBJECT Agriculture AHTA-0000-G732 CLASSI-
SECO1ADARY1972 MiPests of plants--Philippines(TIsN
2 TITLE AND SUBTITLE
Reproduction and seasonal abundance of the ricefield rat (Rattus rattus mindanensis
Mearns) at SiniloarLaguna
3 AUTHORSMarges1B1E
OF PAMES ARC NUME 1NUMBER BS 4 DOCUENT DATE1972I 52pI ARC
7 REFERENCE ORGANIZATION NAME AND ADDRESS
Interior
Publishers Availability)n1 SUPPLEMENTARY NOTES (Sponsoring Organization
(Thesis MS--Univof the Philippines)
9 ABSTRACT
11 PRICE OF DOCUMENT10 CONTROL NUMBER
PN-RAA-624
12 DESCRIPTORS 13 PROJECT NUMBER Fecundity LagunaPhilippines Seasonal variations 14 CONTRACT NUMBER
Philippines PASA RACID) I-67 Res TYPE OF DOCUMENTRats 1s
HEMOUCTIO AND SEASOML ABUNDANCE OF THE RIMML~D MT (RAk1US EATTUS AIMNNIS -REAMS)
AT sInIIAm IAMUA
MRMMABD E 4RCES
SUB4~DTO THE WADUAE FCUIIf OF THE COLIECE OF AMIUIIL UNJMlVRSITY CF TIJE PHLIPPflEs 3NI
BkRTIA FUILILI NqT OF TOE IUIIEITS
LSTE OF SCIENCE (Applied Zooloar)
pRomber -X972
The thesis attached hereto enitled Reproduction and Seasoal
Abundance of the Riceflield Rat -(1Rttusrattus mindanensis Marns) at Siniloan Lagunma prepared and submitted by Bernardo E mirges in
Partial fulfillment of the requirements for the degree of iMster of
Science in Applied Zoology is hereby accepted
Wamber Guidance Comittee Ymber Guidance Comittee
(Date signed) (Date signed)
1vmber Guidance Committee Adviser and chairman Guidance Committee
(Da-te iigued) (Date signed)
Accepted as partial fulnil wn of the requirements for the
degree of aster of Science (Applied Zoology)
FT ORILLO Dean
College of Agriculture University of the Philippines
on (Date)
iernrdo Eifrittu rges WOB -M on August 19- 1937 in Thda6ig
14 thethird sonof Proceso Mrges and Mesta Espitu He took up his- primarycourse at Caiguchi Barrio School at
IfdAngCvjte Hefihisthd his intermediate (with hontors) and
coreesat LunM9nd~i~ n - lmfrbtZJ -School nd -Indan H~ Sdhool respectively IHe attended the University of the pailppines3
College of Agriculturd in 1955 and obtained his Bachelor of Science
in Agriculture in 1959
He joined the Bureau of Plant industry in 1960 Plant Itst Control Officer and mas assiged in Zamboanga del Sur In 1968 he
as treisferred to Davao City Where he assumed supervisory f nztions along withthe Regionis regulatory activities later he Masappointed as Regioal Vegetable ProgramICbordinator for Region No 7 with station
at Zambanga ityi
In 1a9yv as avarded aColombo training grant -o th United Kingdom-ih6r hd ccmpleteafthe Past1inagement coursep He was admitted o he U1A 6afECUSAI graduate fellow later thatyeapr H
qualified as a college scholar kort ie f rst semestet-of the school
year 19704-7 Theb author-k aa to D Escuata j deli ed Thbi o tgos Dao
Sur Their daughters are Ifzel and 1 Cecilia
ERMPDO E YA)GES
ii
ACKN(3IDGEMNTS
I am grateful to nanypeople who helped me throughout the study
Profound gratitude and sincere WpprcIation U expre6ssed to ny major
Prof Pablo J Alfonso for the years of instruction guidance bdviser( i
Thanks areand ndrstanding he bas devqWd to m graduate program
also due to the Bureau of Plant Industry especially to Director Eliseo
C Carandang Messrs Jose I Males Hipolito A Cutodio and Jesus
Po Suangil for their continued encouragement and financial support
I am also grateful to NEC and US-AID for the financial assistance
during nr academic and research enrollment at UPA
Sincere thanks and appreciation are alsoftd to thle Dinector and
members of theRodent Research Center staff for theirassisiaiice and
cooperation during the collection f specimens and in the preparation
of-the manuscript For the geperWs help extend1ed by the teaching
staf of Bkvbay National Agricultural and Vocational School especially
to Messrs Bienvenido RomerosaVnd Roqulo Aloaa for allowing us the
use of their qurters and ohet facilities I am extremely grateful
I wish to thank Wn conmittee DrB Benja~iin L Cariaso and
Vicente G -M0onn and Mr Saztiago M Alviar for their suggestions
and helpful criticisms of the manuscript
In conclusion- I owe a debt of gratitude to zqvife for her
patience understandipg and encpuragement vhen morle s low
---
TABLE OF CONTENTS
Chapter Page
I INTRODUCTION------ --- ----------- -- 1
II TAXONOMY AND DISTRIBUTION- -------------- 4
III DESCRIPTION OF THE STUDY AREA- ----------- - 5
IV MATERIALS AND METHODS 9
A Collection of rats------ ------------ 9
B Autopsy -I----- ---- ---- --- ---- 10
C Statistical analyses - ---- 11i-----
V RESULTS AND DISCUSSION- 1
A Criteria of maturity - ----- ---- ----- 11
1 Testes position size and maturity ------ 14
B -Reproduction -- - - - - - ---- -- 16
1 Breeding season ------------ ----- 16
2 Prevalence of pregnancy ------------- 23
3 Incidence of pregnancy -- -------------- 24
4 Litter size ------ ----- ------- 25
5 Productivity - ------ --- ---- --- 29
6 Juvenile incidence --------- -- ---- 30 C Sex ratio- - shy - - -
D Rat population indices and seasonal fluctuations 36
VI LITEATURE CITED -- 40
iv
LIST OF TALES
Table
i Monthly mean rainfall and average temperature in Siniloan Laguna------------ 8
2 Size of the ricefield rat t various stages of mturity ------ ---- ---- ---- ----- 13
3 Moithly composition of adult male population of Hattus rattus mindanensis with respect to the positionof testes - -------- - ------- 15
4 Monthly changes in the prevalence of pregnant Rattus rattus mindanensis collected in Siniloan August 1971 through Jul 1972 - - - - 22
5 Monthly mean distribution of fetuses in the two horns of the uterus of pregnant rats examined in Siniloan August 1971 through July 1972 ------ 26
6 Analysis of variance for litter size ------- --- 28
7 Mean litter size separation for all pregnant rats examined for this study in Siniloan August 1971 through July 1972 ---- ------- -- ------ 28
8 Productivity of Rattus rattus mindanensis captured during the monthly trapping in Siniloan Auguit C 1971 through July 1972 ------- --- ----- 29
9 Monthly changes in the young-adult ratio of all rats collected for this study in Siniloapi Laguna- - 31
10 Mothly changes in the sex ratio of all rats trapped in Siniloan August 1971 through July 1972 - - - - 33
11 Number of pregnant females aptured in relation to staesof DremancY 35
12 Sex ratios in various weight classes of attus rgttus mindanensis ------------ - --- - 36
13 Population density index of ricefield rats in Siniloan based on monthly trapline catches - ---- 38
v
LIST OF FIGURES
Page
1 Portion ofthebull qtudy area showing condition of the rice paddies after harvest- ------------- 6
2 Scatter diagram of length of testis against weight of rats showing presence or absence of sperm 17
3- Graph showing extent of breeding season Judged by percent pregnant and immature ----------- 1
4 Monthly changes in the prevalence of pregnancy in -relation to total monthly rainfall and number of rainy days per month -- ---- ---- -- ---- 21
5 Graph showing relationship between breeding and monthly sex ratio - ------------------- 34
6 Monthly trapline catches of field rats in relation to rainfall and temperature -------- -- 39
ABSTACT
Mrges Bernardo Espiritu University of the Tilippines
November 1972 Reproduction and Seasonal Abundance of the Ricefield
it (lMttus rattus mindanensis Yearns) at Siniloan laguna
ljor Professor blo J Alfonso
The reproductive patterns and seasonal abundance of the common
ricefield rat were studied in an irrigated ricefield in Siniloan
IAguna from August 1971 through July 1972 The study area ias
planted to two crops of rice every year Of the two rat species
found the ricefield rat dominated the entire riceland comunity
comprising more than 99 percent of the total collection
Two age classes were recognized Juveniles and adults Adults
were further subdivided into breeding and non-breeding The vaginal
orifice became perforate at 5445 grams among fenales while the
testes descended to the scrotum at 13493 grams among males For
both sexes the skull ossified at 9079 grams The length of the testes
vas significantly correlated with the body weight Active sperms
were present in the cauda epididymis when the testes reached lengtha
of 2229 mm Once sexual maturity was attained males remained fertile
throughout the year
Reproductive pattern was sarply bimodal corresponding very
closely with the crop cycle Most litters were produced during the
reproductive and ripening stages of rice plants with little or no
breeding activity when the fields lie fallow There was no direct
relationship between breeding and seasonal or climtic chages Mean
vii
litter size was 967 but decreased significantly during the beginning and termination of the breedinp period There vas no tendency for the number of embryos to increase with body weight The distribution of embryos between the right and left horns of the uterus vas entirely independent with no marked tendency for all embryos to develop on one side or the other The annual incidence of pregnancy was 5 67 litters the number of litters produced during the dry season breeding as pore than twice that of the wet season breeding With few exceptions the sex ratio approximated equality Of the exceptions most could be shown due to differential tra Pability of both-
sexes at certain star of the
breeding period Productivity was high about 44 vouni bInft ThMiwaA
per female per year
Seasonal reproduction resulted in high PopulAtion density by August September and October sarp decline in density was observed
in January February and Mrch
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
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41
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43
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HEMOUCTIO AND SEASOML ABUNDANCE OF THE RIMML~D MT (RAk1US EATTUS AIMNNIS -REAMS)
AT sInIIAm IAMUA
MRMMABD E 4RCES
SUB4~DTO THE WADUAE FCUIIf OF THE COLIECE OF AMIUIIL UNJMlVRSITY CF TIJE PHLIPPflEs 3NI
BkRTIA FUILILI NqT OF TOE IUIIEITS
LSTE OF SCIENCE (Applied Zooloar)
pRomber -X972
The thesis attached hereto enitled Reproduction and Seasoal
Abundance of the Riceflield Rat -(1Rttusrattus mindanensis Marns) at Siniloan Lagunma prepared and submitted by Bernardo E mirges in
Partial fulfillment of the requirements for the degree of iMster of
Science in Applied Zoology is hereby accepted
Wamber Guidance Comittee Ymber Guidance Comittee
(Date signed) (Date signed)
1vmber Guidance Committee Adviser and chairman Guidance Committee
(Da-te iigued) (Date signed)
Accepted as partial fulnil wn of the requirements for the
degree of aster of Science (Applied Zoology)
FT ORILLO Dean
College of Agriculture University of the Philippines
on (Date)
iernrdo Eifrittu rges WOB -M on August 19- 1937 in Thda6ig
14 thethird sonof Proceso Mrges and Mesta Espitu He took up his- primarycourse at Caiguchi Barrio School at
IfdAngCvjte Hefihisthd his intermediate (with hontors) and
coreesat LunM9nd~i~ n - lmfrbtZJ -School nd -Indan H~ Sdhool respectively IHe attended the University of the pailppines3
College of Agriculturd in 1955 and obtained his Bachelor of Science
in Agriculture in 1959
He joined the Bureau of Plant industry in 1960 Plant Itst Control Officer and mas assiged in Zamboanga del Sur In 1968 he
as treisferred to Davao City Where he assumed supervisory f nztions along withthe Regionis regulatory activities later he Masappointed as Regioal Vegetable ProgramICbordinator for Region No 7 with station
at Zambanga ityi
In 1a9yv as avarded aColombo training grant -o th United Kingdom-ih6r hd ccmpleteafthe Past1inagement coursep He was admitted o he U1A 6afECUSAI graduate fellow later thatyeapr H
qualified as a college scholar kort ie f rst semestet-of the school
year 19704-7 Theb author-k aa to D Escuata j deli ed Thbi o tgos Dao
Sur Their daughters are Ifzel and 1 Cecilia
ERMPDO E YA)GES
ii
ACKN(3IDGEMNTS
I am grateful to nanypeople who helped me throughout the study
Profound gratitude and sincere WpprcIation U expre6ssed to ny major
Prof Pablo J Alfonso for the years of instruction guidance bdviser( i
Thanks areand ndrstanding he bas devqWd to m graduate program
also due to the Bureau of Plant Industry especially to Director Eliseo
C Carandang Messrs Jose I Males Hipolito A Cutodio and Jesus
Po Suangil for their continued encouragement and financial support
I am also grateful to NEC and US-AID for the financial assistance
during nr academic and research enrollment at UPA
Sincere thanks and appreciation are alsoftd to thle Dinector and
members of theRodent Research Center staff for theirassisiaiice and
cooperation during the collection f specimens and in the preparation
of-the manuscript For the geperWs help extend1ed by the teaching
staf of Bkvbay National Agricultural and Vocational School especially
to Messrs Bienvenido RomerosaVnd Roqulo Aloaa for allowing us the
use of their qurters and ohet facilities I am extremely grateful
I wish to thank Wn conmittee DrB Benja~iin L Cariaso and
Vicente G -M0onn and Mr Saztiago M Alviar for their suggestions
and helpful criticisms of the manuscript
In conclusion- I owe a debt of gratitude to zqvife for her
patience understandipg and encpuragement vhen morle s low
---
TABLE OF CONTENTS
Chapter Page
I INTRODUCTION------ --- ----------- -- 1
II TAXONOMY AND DISTRIBUTION- -------------- 4
III DESCRIPTION OF THE STUDY AREA- ----------- - 5
IV MATERIALS AND METHODS 9
A Collection of rats------ ------------ 9
B Autopsy -I----- ---- ---- --- ---- 10
C Statistical analyses - ---- 11i-----
V RESULTS AND DISCUSSION- 1
A Criteria of maturity - ----- ---- ----- 11
1 Testes position size and maturity ------ 14
B -Reproduction -- - - - - - ---- -- 16
1 Breeding season ------------ ----- 16
2 Prevalence of pregnancy ------------- 23
3 Incidence of pregnancy -- -------------- 24
4 Litter size ------ ----- ------- 25
5 Productivity - ------ --- ---- --- 29
6 Juvenile incidence --------- -- ---- 30 C Sex ratio- - shy - - -
D Rat population indices and seasonal fluctuations 36
VI LITEATURE CITED -- 40
iv
LIST OF TALES
Table
i Monthly mean rainfall and average temperature in Siniloan Laguna------------ 8
2 Size of the ricefield rat t various stages of mturity ------ ---- ---- ---- ----- 13
3 Moithly composition of adult male population of Hattus rattus mindanensis with respect to the positionof testes - -------- - ------- 15
4 Monthly changes in the prevalence of pregnant Rattus rattus mindanensis collected in Siniloan August 1971 through Jul 1972 - - - - 22
5 Monthly mean distribution of fetuses in the two horns of the uterus of pregnant rats examined in Siniloan August 1971 through July 1972 ------ 26
6 Analysis of variance for litter size ------- --- 28
7 Mean litter size separation for all pregnant rats examined for this study in Siniloan August 1971 through July 1972 ---- ------- -- ------ 28
8 Productivity of Rattus rattus mindanensis captured during the monthly trapping in Siniloan Auguit C 1971 through July 1972 ------- --- ----- 29
9 Monthly changes in the young-adult ratio of all rats collected for this study in Siniloapi Laguna- - 31
10 Mothly changes in the sex ratio of all rats trapped in Siniloan August 1971 through July 1972 - - - - 33
11 Number of pregnant females aptured in relation to staesof DremancY 35
12 Sex ratios in various weight classes of attus rgttus mindanensis ------------ - --- - 36
13 Population density index of ricefield rats in Siniloan based on monthly trapline catches - ---- 38
v
LIST OF FIGURES
Page
1 Portion ofthebull qtudy area showing condition of the rice paddies after harvest- ------------- 6
2 Scatter diagram of length of testis against weight of rats showing presence or absence of sperm 17
3- Graph showing extent of breeding season Judged by percent pregnant and immature ----------- 1
4 Monthly changes in the prevalence of pregnancy in -relation to total monthly rainfall and number of rainy days per month -- ---- ---- -- ---- 21
5 Graph showing relationship between breeding and monthly sex ratio - ------------------- 34
6 Monthly trapline catches of field rats in relation to rainfall and temperature -------- -- 39
ABSTACT
Mrges Bernardo Espiritu University of the Tilippines
November 1972 Reproduction and Seasonal Abundance of the Ricefield
it (lMttus rattus mindanensis Yearns) at Siniloan laguna
ljor Professor blo J Alfonso
The reproductive patterns and seasonal abundance of the common
ricefield rat were studied in an irrigated ricefield in Siniloan
IAguna from August 1971 through July 1972 The study area ias
planted to two crops of rice every year Of the two rat species
found the ricefield rat dominated the entire riceland comunity
comprising more than 99 percent of the total collection
Two age classes were recognized Juveniles and adults Adults
were further subdivided into breeding and non-breeding The vaginal
orifice became perforate at 5445 grams among fenales while the
testes descended to the scrotum at 13493 grams among males For
both sexes the skull ossified at 9079 grams The length of the testes
vas significantly correlated with the body weight Active sperms
were present in the cauda epididymis when the testes reached lengtha
of 2229 mm Once sexual maturity was attained males remained fertile
throughout the year
Reproductive pattern was sarply bimodal corresponding very
closely with the crop cycle Most litters were produced during the
reproductive and ripening stages of rice plants with little or no
breeding activity when the fields lie fallow There was no direct
relationship between breeding and seasonal or climtic chages Mean
vii
litter size was 967 but decreased significantly during the beginning and termination of the breedinp period There vas no tendency for the number of embryos to increase with body weight The distribution of embryos between the right and left horns of the uterus vas entirely independent with no marked tendency for all embryos to develop on one side or the other The annual incidence of pregnancy was 5 67 litters the number of litters produced during the dry season breeding as pore than twice that of the wet season breeding With few exceptions the sex ratio approximated equality Of the exceptions most could be shown due to differential tra Pability of both-
sexes at certain star of the
breeding period Productivity was high about 44 vouni bInft ThMiwaA
per female per year
Seasonal reproduction resulted in high PopulAtion density by August September and October sarp decline in density was observed
in January February and Mrch
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
The thesis attached hereto enitled Reproduction and Seasoal
Abundance of the Riceflield Rat -(1Rttusrattus mindanensis Marns) at Siniloan Lagunma prepared and submitted by Bernardo E mirges in
Partial fulfillment of the requirements for the degree of iMster of
Science in Applied Zoology is hereby accepted
Wamber Guidance Comittee Ymber Guidance Comittee
(Date signed) (Date signed)
1vmber Guidance Committee Adviser and chairman Guidance Committee
(Da-te iigued) (Date signed)
Accepted as partial fulnil wn of the requirements for the
degree of aster of Science (Applied Zoology)
FT ORILLO Dean
College of Agriculture University of the Philippines
on (Date)
iernrdo Eifrittu rges WOB -M on August 19- 1937 in Thda6ig
14 thethird sonof Proceso Mrges and Mesta Espitu He took up his- primarycourse at Caiguchi Barrio School at
IfdAngCvjte Hefihisthd his intermediate (with hontors) and
coreesat LunM9nd~i~ n - lmfrbtZJ -School nd -Indan H~ Sdhool respectively IHe attended the University of the pailppines3
College of Agriculturd in 1955 and obtained his Bachelor of Science
in Agriculture in 1959
He joined the Bureau of Plant industry in 1960 Plant Itst Control Officer and mas assiged in Zamboanga del Sur In 1968 he
as treisferred to Davao City Where he assumed supervisory f nztions along withthe Regionis regulatory activities later he Masappointed as Regioal Vegetable ProgramICbordinator for Region No 7 with station
at Zambanga ityi
In 1a9yv as avarded aColombo training grant -o th United Kingdom-ih6r hd ccmpleteafthe Past1inagement coursep He was admitted o he U1A 6afECUSAI graduate fellow later thatyeapr H
qualified as a college scholar kort ie f rst semestet-of the school
year 19704-7 Theb author-k aa to D Escuata j deli ed Thbi o tgos Dao
Sur Their daughters are Ifzel and 1 Cecilia
ERMPDO E YA)GES
ii
ACKN(3IDGEMNTS
I am grateful to nanypeople who helped me throughout the study
Profound gratitude and sincere WpprcIation U expre6ssed to ny major
Prof Pablo J Alfonso for the years of instruction guidance bdviser( i
Thanks areand ndrstanding he bas devqWd to m graduate program
also due to the Bureau of Plant Industry especially to Director Eliseo
C Carandang Messrs Jose I Males Hipolito A Cutodio and Jesus
Po Suangil for their continued encouragement and financial support
I am also grateful to NEC and US-AID for the financial assistance
during nr academic and research enrollment at UPA
Sincere thanks and appreciation are alsoftd to thle Dinector and
members of theRodent Research Center staff for theirassisiaiice and
cooperation during the collection f specimens and in the preparation
of-the manuscript For the geperWs help extend1ed by the teaching
staf of Bkvbay National Agricultural and Vocational School especially
to Messrs Bienvenido RomerosaVnd Roqulo Aloaa for allowing us the
use of their qurters and ohet facilities I am extremely grateful
I wish to thank Wn conmittee DrB Benja~iin L Cariaso and
Vicente G -M0onn and Mr Saztiago M Alviar for their suggestions
and helpful criticisms of the manuscript
In conclusion- I owe a debt of gratitude to zqvife for her
patience understandipg and encpuragement vhen morle s low
---
TABLE OF CONTENTS
Chapter Page
I INTRODUCTION------ --- ----------- -- 1
II TAXONOMY AND DISTRIBUTION- -------------- 4
III DESCRIPTION OF THE STUDY AREA- ----------- - 5
IV MATERIALS AND METHODS 9
A Collection of rats------ ------------ 9
B Autopsy -I----- ---- ---- --- ---- 10
C Statistical analyses - ---- 11i-----
V RESULTS AND DISCUSSION- 1
A Criteria of maturity - ----- ---- ----- 11
1 Testes position size and maturity ------ 14
B -Reproduction -- - - - - - ---- -- 16
1 Breeding season ------------ ----- 16
2 Prevalence of pregnancy ------------- 23
3 Incidence of pregnancy -- -------------- 24
4 Litter size ------ ----- ------- 25
5 Productivity - ------ --- ---- --- 29
6 Juvenile incidence --------- -- ---- 30 C Sex ratio- - shy - - -
D Rat population indices and seasonal fluctuations 36
VI LITEATURE CITED -- 40
iv
LIST OF TALES
Table
i Monthly mean rainfall and average temperature in Siniloan Laguna------------ 8
2 Size of the ricefield rat t various stages of mturity ------ ---- ---- ---- ----- 13
3 Moithly composition of adult male population of Hattus rattus mindanensis with respect to the positionof testes - -------- - ------- 15
4 Monthly changes in the prevalence of pregnant Rattus rattus mindanensis collected in Siniloan August 1971 through Jul 1972 - - - - 22
5 Monthly mean distribution of fetuses in the two horns of the uterus of pregnant rats examined in Siniloan August 1971 through July 1972 ------ 26
6 Analysis of variance for litter size ------- --- 28
7 Mean litter size separation for all pregnant rats examined for this study in Siniloan August 1971 through July 1972 ---- ------- -- ------ 28
8 Productivity of Rattus rattus mindanensis captured during the monthly trapping in Siniloan Auguit C 1971 through July 1972 ------- --- ----- 29
9 Monthly changes in the young-adult ratio of all rats collected for this study in Siniloapi Laguna- - 31
10 Mothly changes in the sex ratio of all rats trapped in Siniloan August 1971 through July 1972 - - - - 33
11 Number of pregnant females aptured in relation to staesof DremancY 35
12 Sex ratios in various weight classes of attus rgttus mindanensis ------------ - --- - 36
13 Population density index of ricefield rats in Siniloan based on monthly trapline catches - ---- 38
v
LIST OF FIGURES
Page
1 Portion ofthebull qtudy area showing condition of the rice paddies after harvest- ------------- 6
2 Scatter diagram of length of testis against weight of rats showing presence or absence of sperm 17
3- Graph showing extent of breeding season Judged by percent pregnant and immature ----------- 1
4 Monthly changes in the prevalence of pregnancy in -relation to total monthly rainfall and number of rainy days per month -- ---- ---- -- ---- 21
5 Graph showing relationship between breeding and monthly sex ratio - ------------------- 34
6 Monthly trapline catches of field rats in relation to rainfall and temperature -------- -- 39
ABSTACT
Mrges Bernardo Espiritu University of the Tilippines
November 1972 Reproduction and Seasonal Abundance of the Ricefield
it (lMttus rattus mindanensis Yearns) at Siniloan laguna
ljor Professor blo J Alfonso
The reproductive patterns and seasonal abundance of the common
ricefield rat were studied in an irrigated ricefield in Siniloan
IAguna from August 1971 through July 1972 The study area ias
planted to two crops of rice every year Of the two rat species
found the ricefield rat dominated the entire riceland comunity
comprising more than 99 percent of the total collection
Two age classes were recognized Juveniles and adults Adults
were further subdivided into breeding and non-breeding The vaginal
orifice became perforate at 5445 grams among fenales while the
testes descended to the scrotum at 13493 grams among males For
both sexes the skull ossified at 9079 grams The length of the testes
vas significantly correlated with the body weight Active sperms
were present in the cauda epididymis when the testes reached lengtha
of 2229 mm Once sexual maturity was attained males remained fertile
throughout the year
Reproductive pattern was sarply bimodal corresponding very
closely with the crop cycle Most litters were produced during the
reproductive and ripening stages of rice plants with little or no
breeding activity when the fields lie fallow There was no direct
relationship between breeding and seasonal or climtic chages Mean
vii
litter size was 967 but decreased significantly during the beginning and termination of the breedinp period There vas no tendency for the number of embryos to increase with body weight The distribution of embryos between the right and left horns of the uterus vas entirely independent with no marked tendency for all embryos to develop on one side or the other The annual incidence of pregnancy was 5 67 litters the number of litters produced during the dry season breeding as pore than twice that of the wet season breeding With few exceptions the sex ratio approximated equality Of the exceptions most could be shown due to differential tra Pability of both-
sexes at certain star of the
breeding period Productivity was high about 44 vouni bInft ThMiwaA
per female per year
Seasonal reproduction resulted in high PopulAtion density by August September and October sarp decline in density was observed
in January February and Mrch
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
iernrdo Eifrittu rges WOB -M on August 19- 1937 in Thda6ig
14 thethird sonof Proceso Mrges and Mesta Espitu He took up his- primarycourse at Caiguchi Barrio School at
IfdAngCvjte Hefihisthd his intermediate (with hontors) and
coreesat LunM9nd~i~ n - lmfrbtZJ -School nd -Indan H~ Sdhool respectively IHe attended the University of the pailppines3
College of Agriculturd in 1955 and obtained his Bachelor of Science
in Agriculture in 1959
He joined the Bureau of Plant industry in 1960 Plant Itst Control Officer and mas assiged in Zamboanga del Sur In 1968 he
as treisferred to Davao City Where he assumed supervisory f nztions along withthe Regionis regulatory activities later he Masappointed as Regioal Vegetable ProgramICbordinator for Region No 7 with station
at Zambanga ityi
In 1a9yv as avarded aColombo training grant -o th United Kingdom-ih6r hd ccmpleteafthe Past1inagement coursep He was admitted o he U1A 6afECUSAI graduate fellow later thatyeapr H
qualified as a college scholar kort ie f rst semestet-of the school
year 19704-7 Theb author-k aa to D Escuata j deli ed Thbi o tgos Dao
Sur Their daughters are Ifzel and 1 Cecilia
ERMPDO E YA)GES
ii
ACKN(3IDGEMNTS
I am grateful to nanypeople who helped me throughout the study
Profound gratitude and sincere WpprcIation U expre6ssed to ny major
Prof Pablo J Alfonso for the years of instruction guidance bdviser( i
Thanks areand ndrstanding he bas devqWd to m graduate program
also due to the Bureau of Plant Industry especially to Director Eliseo
C Carandang Messrs Jose I Males Hipolito A Cutodio and Jesus
Po Suangil for their continued encouragement and financial support
I am also grateful to NEC and US-AID for the financial assistance
during nr academic and research enrollment at UPA
Sincere thanks and appreciation are alsoftd to thle Dinector and
members of theRodent Research Center staff for theirassisiaiice and
cooperation during the collection f specimens and in the preparation
of-the manuscript For the geperWs help extend1ed by the teaching
staf of Bkvbay National Agricultural and Vocational School especially
to Messrs Bienvenido RomerosaVnd Roqulo Aloaa for allowing us the
use of their qurters and ohet facilities I am extremely grateful
I wish to thank Wn conmittee DrB Benja~iin L Cariaso and
Vicente G -M0onn and Mr Saztiago M Alviar for their suggestions
and helpful criticisms of the manuscript
In conclusion- I owe a debt of gratitude to zqvife for her
patience understandipg and encpuragement vhen morle s low
---
TABLE OF CONTENTS
Chapter Page
I INTRODUCTION------ --- ----------- -- 1
II TAXONOMY AND DISTRIBUTION- -------------- 4
III DESCRIPTION OF THE STUDY AREA- ----------- - 5
IV MATERIALS AND METHODS 9
A Collection of rats------ ------------ 9
B Autopsy -I----- ---- ---- --- ---- 10
C Statistical analyses - ---- 11i-----
V RESULTS AND DISCUSSION- 1
A Criteria of maturity - ----- ---- ----- 11
1 Testes position size and maturity ------ 14
B -Reproduction -- - - - - - ---- -- 16
1 Breeding season ------------ ----- 16
2 Prevalence of pregnancy ------------- 23
3 Incidence of pregnancy -- -------------- 24
4 Litter size ------ ----- ------- 25
5 Productivity - ------ --- ---- --- 29
6 Juvenile incidence --------- -- ---- 30 C Sex ratio- - shy - - -
D Rat population indices and seasonal fluctuations 36
VI LITEATURE CITED -- 40
iv
LIST OF TALES
Table
i Monthly mean rainfall and average temperature in Siniloan Laguna------------ 8
2 Size of the ricefield rat t various stages of mturity ------ ---- ---- ---- ----- 13
3 Moithly composition of adult male population of Hattus rattus mindanensis with respect to the positionof testes - -------- - ------- 15
4 Monthly changes in the prevalence of pregnant Rattus rattus mindanensis collected in Siniloan August 1971 through Jul 1972 - - - - 22
5 Monthly mean distribution of fetuses in the two horns of the uterus of pregnant rats examined in Siniloan August 1971 through July 1972 ------ 26
6 Analysis of variance for litter size ------- --- 28
7 Mean litter size separation for all pregnant rats examined for this study in Siniloan August 1971 through July 1972 ---- ------- -- ------ 28
8 Productivity of Rattus rattus mindanensis captured during the monthly trapping in Siniloan Auguit C 1971 through July 1972 ------- --- ----- 29
9 Monthly changes in the young-adult ratio of all rats collected for this study in Siniloapi Laguna- - 31
10 Mothly changes in the sex ratio of all rats trapped in Siniloan August 1971 through July 1972 - - - - 33
11 Number of pregnant females aptured in relation to staesof DremancY 35
12 Sex ratios in various weight classes of attus rgttus mindanensis ------------ - --- - 36
13 Population density index of ricefield rats in Siniloan based on monthly trapline catches - ---- 38
v
LIST OF FIGURES
Page
1 Portion ofthebull qtudy area showing condition of the rice paddies after harvest- ------------- 6
2 Scatter diagram of length of testis against weight of rats showing presence or absence of sperm 17
3- Graph showing extent of breeding season Judged by percent pregnant and immature ----------- 1
4 Monthly changes in the prevalence of pregnancy in -relation to total monthly rainfall and number of rainy days per month -- ---- ---- -- ---- 21
5 Graph showing relationship between breeding and monthly sex ratio - ------------------- 34
6 Monthly trapline catches of field rats in relation to rainfall and temperature -------- -- 39
ABSTACT
Mrges Bernardo Espiritu University of the Tilippines
November 1972 Reproduction and Seasonal Abundance of the Ricefield
it (lMttus rattus mindanensis Yearns) at Siniloan laguna
ljor Professor blo J Alfonso
The reproductive patterns and seasonal abundance of the common
ricefield rat were studied in an irrigated ricefield in Siniloan
IAguna from August 1971 through July 1972 The study area ias
planted to two crops of rice every year Of the two rat species
found the ricefield rat dominated the entire riceland comunity
comprising more than 99 percent of the total collection
Two age classes were recognized Juveniles and adults Adults
were further subdivided into breeding and non-breeding The vaginal
orifice became perforate at 5445 grams among fenales while the
testes descended to the scrotum at 13493 grams among males For
both sexes the skull ossified at 9079 grams The length of the testes
vas significantly correlated with the body weight Active sperms
were present in the cauda epididymis when the testes reached lengtha
of 2229 mm Once sexual maturity was attained males remained fertile
throughout the year
Reproductive pattern was sarply bimodal corresponding very
closely with the crop cycle Most litters were produced during the
reproductive and ripening stages of rice plants with little or no
breeding activity when the fields lie fallow There was no direct
relationship between breeding and seasonal or climtic chages Mean
vii
litter size was 967 but decreased significantly during the beginning and termination of the breedinp period There vas no tendency for the number of embryos to increase with body weight The distribution of embryos between the right and left horns of the uterus vas entirely independent with no marked tendency for all embryos to develop on one side or the other The annual incidence of pregnancy was 5 67 litters the number of litters produced during the dry season breeding as pore than twice that of the wet season breeding With few exceptions the sex ratio approximated equality Of the exceptions most could be shown due to differential tra Pability of both-
sexes at certain star of the
breeding period Productivity was high about 44 vouni bInft ThMiwaA
per female per year
Seasonal reproduction resulted in high PopulAtion density by August September and October sarp decline in density was observed
in January February and Mrch
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
ACKN(3IDGEMNTS
I am grateful to nanypeople who helped me throughout the study
Profound gratitude and sincere WpprcIation U expre6ssed to ny major
Prof Pablo J Alfonso for the years of instruction guidance bdviser( i
Thanks areand ndrstanding he bas devqWd to m graduate program
also due to the Bureau of Plant Industry especially to Director Eliseo
C Carandang Messrs Jose I Males Hipolito A Cutodio and Jesus
Po Suangil for their continued encouragement and financial support
I am also grateful to NEC and US-AID for the financial assistance
during nr academic and research enrollment at UPA
Sincere thanks and appreciation are alsoftd to thle Dinector and
members of theRodent Research Center staff for theirassisiaiice and
cooperation during the collection f specimens and in the preparation
of-the manuscript For the geperWs help extend1ed by the teaching
staf of Bkvbay National Agricultural and Vocational School especially
to Messrs Bienvenido RomerosaVnd Roqulo Aloaa for allowing us the
use of their qurters and ohet facilities I am extremely grateful
I wish to thank Wn conmittee DrB Benja~iin L Cariaso and
Vicente G -M0onn and Mr Saztiago M Alviar for their suggestions
and helpful criticisms of the manuscript
In conclusion- I owe a debt of gratitude to zqvife for her
patience understandipg and encpuragement vhen morle s low
---
TABLE OF CONTENTS
Chapter Page
I INTRODUCTION------ --- ----------- -- 1
II TAXONOMY AND DISTRIBUTION- -------------- 4
III DESCRIPTION OF THE STUDY AREA- ----------- - 5
IV MATERIALS AND METHODS 9
A Collection of rats------ ------------ 9
B Autopsy -I----- ---- ---- --- ---- 10
C Statistical analyses - ---- 11i-----
V RESULTS AND DISCUSSION- 1
A Criteria of maturity - ----- ---- ----- 11
1 Testes position size and maturity ------ 14
B -Reproduction -- - - - - - ---- -- 16
1 Breeding season ------------ ----- 16
2 Prevalence of pregnancy ------------- 23
3 Incidence of pregnancy -- -------------- 24
4 Litter size ------ ----- ------- 25
5 Productivity - ------ --- ---- --- 29
6 Juvenile incidence --------- -- ---- 30 C Sex ratio- - shy - - -
D Rat population indices and seasonal fluctuations 36
VI LITEATURE CITED -- 40
iv
LIST OF TALES
Table
i Monthly mean rainfall and average temperature in Siniloan Laguna------------ 8
2 Size of the ricefield rat t various stages of mturity ------ ---- ---- ---- ----- 13
3 Moithly composition of adult male population of Hattus rattus mindanensis with respect to the positionof testes - -------- - ------- 15
4 Monthly changes in the prevalence of pregnant Rattus rattus mindanensis collected in Siniloan August 1971 through Jul 1972 - - - - 22
5 Monthly mean distribution of fetuses in the two horns of the uterus of pregnant rats examined in Siniloan August 1971 through July 1972 ------ 26
6 Analysis of variance for litter size ------- --- 28
7 Mean litter size separation for all pregnant rats examined for this study in Siniloan August 1971 through July 1972 ---- ------- -- ------ 28
8 Productivity of Rattus rattus mindanensis captured during the monthly trapping in Siniloan Auguit C 1971 through July 1972 ------- --- ----- 29
9 Monthly changes in the young-adult ratio of all rats collected for this study in Siniloapi Laguna- - 31
10 Mothly changes in the sex ratio of all rats trapped in Siniloan August 1971 through July 1972 - - - - 33
11 Number of pregnant females aptured in relation to staesof DremancY 35
12 Sex ratios in various weight classes of attus rgttus mindanensis ------------ - --- - 36
13 Population density index of ricefield rats in Siniloan based on monthly trapline catches - ---- 38
v
LIST OF FIGURES
Page
1 Portion ofthebull qtudy area showing condition of the rice paddies after harvest- ------------- 6
2 Scatter diagram of length of testis against weight of rats showing presence or absence of sperm 17
3- Graph showing extent of breeding season Judged by percent pregnant and immature ----------- 1
4 Monthly changes in the prevalence of pregnancy in -relation to total monthly rainfall and number of rainy days per month -- ---- ---- -- ---- 21
5 Graph showing relationship between breeding and monthly sex ratio - ------------------- 34
6 Monthly trapline catches of field rats in relation to rainfall and temperature -------- -- 39
ABSTACT
Mrges Bernardo Espiritu University of the Tilippines
November 1972 Reproduction and Seasonal Abundance of the Ricefield
it (lMttus rattus mindanensis Yearns) at Siniloan laguna
ljor Professor blo J Alfonso
The reproductive patterns and seasonal abundance of the common
ricefield rat were studied in an irrigated ricefield in Siniloan
IAguna from August 1971 through July 1972 The study area ias
planted to two crops of rice every year Of the two rat species
found the ricefield rat dominated the entire riceland comunity
comprising more than 99 percent of the total collection
Two age classes were recognized Juveniles and adults Adults
were further subdivided into breeding and non-breeding The vaginal
orifice became perforate at 5445 grams among fenales while the
testes descended to the scrotum at 13493 grams among males For
both sexes the skull ossified at 9079 grams The length of the testes
vas significantly correlated with the body weight Active sperms
were present in the cauda epididymis when the testes reached lengtha
of 2229 mm Once sexual maturity was attained males remained fertile
throughout the year
Reproductive pattern was sarply bimodal corresponding very
closely with the crop cycle Most litters were produced during the
reproductive and ripening stages of rice plants with little or no
breeding activity when the fields lie fallow There was no direct
relationship between breeding and seasonal or climtic chages Mean
vii
litter size was 967 but decreased significantly during the beginning and termination of the breedinp period There vas no tendency for the number of embryos to increase with body weight The distribution of embryos between the right and left horns of the uterus vas entirely independent with no marked tendency for all embryos to develop on one side or the other The annual incidence of pregnancy was 5 67 litters the number of litters produced during the dry season breeding as pore than twice that of the wet season breeding With few exceptions the sex ratio approximated equality Of the exceptions most could be shown due to differential tra Pability of both-
sexes at certain star of the
breeding period Productivity was high about 44 vouni bInft ThMiwaA
per female per year
Seasonal reproduction resulted in high PopulAtion density by August September and October sarp decline in density was observed
in January February and Mrch
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
---
TABLE OF CONTENTS
Chapter Page
I INTRODUCTION------ --- ----------- -- 1
II TAXONOMY AND DISTRIBUTION- -------------- 4
III DESCRIPTION OF THE STUDY AREA- ----------- - 5
IV MATERIALS AND METHODS 9
A Collection of rats------ ------------ 9
B Autopsy -I----- ---- ---- --- ---- 10
C Statistical analyses - ---- 11i-----
V RESULTS AND DISCUSSION- 1
A Criteria of maturity - ----- ---- ----- 11
1 Testes position size and maturity ------ 14
B -Reproduction -- - - - - - ---- -- 16
1 Breeding season ------------ ----- 16
2 Prevalence of pregnancy ------------- 23
3 Incidence of pregnancy -- -------------- 24
4 Litter size ------ ----- ------- 25
5 Productivity - ------ --- ---- --- 29
6 Juvenile incidence --------- -- ---- 30 C Sex ratio- - shy - - -
D Rat population indices and seasonal fluctuations 36
VI LITEATURE CITED -- 40
iv
LIST OF TALES
Table
i Monthly mean rainfall and average temperature in Siniloan Laguna------------ 8
2 Size of the ricefield rat t various stages of mturity ------ ---- ---- ---- ----- 13
3 Moithly composition of adult male population of Hattus rattus mindanensis with respect to the positionof testes - -------- - ------- 15
4 Monthly changes in the prevalence of pregnant Rattus rattus mindanensis collected in Siniloan August 1971 through Jul 1972 - - - - 22
5 Monthly mean distribution of fetuses in the two horns of the uterus of pregnant rats examined in Siniloan August 1971 through July 1972 ------ 26
6 Analysis of variance for litter size ------- --- 28
7 Mean litter size separation for all pregnant rats examined for this study in Siniloan August 1971 through July 1972 ---- ------- -- ------ 28
8 Productivity of Rattus rattus mindanensis captured during the monthly trapping in Siniloan Auguit C 1971 through July 1972 ------- --- ----- 29
9 Monthly changes in the young-adult ratio of all rats collected for this study in Siniloapi Laguna- - 31
10 Mothly changes in the sex ratio of all rats trapped in Siniloan August 1971 through July 1972 - - - - 33
11 Number of pregnant females aptured in relation to staesof DremancY 35
12 Sex ratios in various weight classes of attus rgttus mindanensis ------------ - --- - 36
13 Population density index of ricefield rats in Siniloan based on monthly trapline catches - ---- 38
v
LIST OF FIGURES
Page
1 Portion ofthebull qtudy area showing condition of the rice paddies after harvest- ------------- 6
2 Scatter diagram of length of testis against weight of rats showing presence or absence of sperm 17
3- Graph showing extent of breeding season Judged by percent pregnant and immature ----------- 1
4 Monthly changes in the prevalence of pregnancy in -relation to total monthly rainfall and number of rainy days per month -- ---- ---- -- ---- 21
5 Graph showing relationship between breeding and monthly sex ratio - ------------------- 34
6 Monthly trapline catches of field rats in relation to rainfall and temperature -------- -- 39
ABSTACT
Mrges Bernardo Espiritu University of the Tilippines
November 1972 Reproduction and Seasonal Abundance of the Ricefield
it (lMttus rattus mindanensis Yearns) at Siniloan laguna
ljor Professor blo J Alfonso
The reproductive patterns and seasonal abundance of the common
ricefield rat were studied in an irrigated ricefield in Siniloan
IAguna from August 1971 through July 1972 The study area ias
planted to two crops of rice every year Of the two rat species
found the ricefield rat dominated the entire riceland comunity
comprising more than 99 percent of the total collection
Two age classes were recognized Juveniles and adults Adults
were further subdivided into breeding and non-breeding The vaginal
orifice became perforate at 5445 grams among fenales while the
testes descended to the scrotum at 13493 grams among males For
both sexes the skull ossified at 9079 grams The length of the testes
vas significantly correlated with the body weight Active sperms
were present in the cauda epididymis when the testes reached lengtha
of 2229 mm Once sexual maturity was attained males remained fertile
throughout the year
Reproductive pattern was sarply bimodal corresponding very
closely with the crop cycle Most litters were produced during the
reproductive and ripening stages of rice plants with little or no
breeding activity when the fields lie fallow There was no direct
relationship between breeding and seasonal or climtic chages Mean
vii
litter size was 967 but decreased significantly during the beginning and termination of the breedinp period There vas no tendency for the number of embryos to increase with body weight The distribution of embryos between the right and left horns of the uterus vas entirely independent with no marked tendency for all embryos to develop on one side or the other The annual incidence of pregnancy was 5 67 litters the number of litters produced during the dry season breeding as pore than twice that of the wet season breeding With few exceptions the sex ratio approximated equality Of the exceptions most could be shown due to differential tra Pability of both-
sexes at certain star of the
breeding period Productivity was high about 44 vouni bInft ThMiwaA
per female per year
Seasonal reproduction resulted in high PopulAtion density by August September and October sarp decline in density was observed
in January February and Mrch
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
LIST OF TALES
Table
i Monthly mean rainfall and average temperature in Siniloan Laguna------------ 8
2 Size of the ricefield rat t various stages of mturity ------ ---- ---- ---- ----- 13
3 Moithly composition of adult male population of Hattus rattus mindanensis with respect to the positionof testes - -------- - ------- 15
4 Monthly changes in the prevalence of pregnant Rattus rattus mindanensis collected in Siniloan August 1971 through Jul 1972 - - - - 22
5 Monthly mean distribution of fetuses in the two horns of the uterus of pregnant rats examined in Siniloan August 1971 through July 1972 ------ 26
6 Analysis of variance for litter size ------- --- 28
7 Mean litter size separation for all pregnant rats examined for this study in Siniloan August 1971 through July 1972 ---- ------- -- ------ 28
8 Productivity of Rattus rattus mindanensis captured during the monthly trapping in Siniloan Auguit C 1971 through July 1972 ------- --- ----- 29
9 Monthly changes in the young-adult ratio of all rats collected for this study in Siniloapi Laguna- - 31
10 Mothly changes in the sex ratio of all rats trapped in Siniloan August 1971 through July 1972 - - - - 33
11 Number of pregnant females aptured in relation to staesof DremancY 35
12 Sex ratios in various weight classes of attus rgttus mindanensis ------------ - --- - 36
13 Population density index of ricefield rats in Siniloan based on monthly trapline catches - ---- 38
v
LIST OF FIGURES
Page
1 Portion ofthebull qtudy area showing condition of the rice paddies after harvest- ------------- 6
2 Scatter diagram of length of testis against weight of rats showing presence or absence of sperm 17
3- Graph showing extent of breeding season Judged by percent pregnant and immature ----------- 1
4 Monthly changes in the prevalence of pregnancy in -relation to total monthly rainfall and number of rainy days per month -- ---- ---- -- ---- 21
5 Graph showing relationship between breeding and monthly sex ratio - ------------------- 34
6 Monthly trapline catches of field rats in relation to rainfall and temperature -------- -- 39
ABSTACT
Mrges Bernardo Espiritu University of the Tilippines
November 1972 Reproduction and Seasonal Abundance of the Ricefield
it (lMttus rattus mindanensis Yearns) at Siniloan laguna
ljor Professor blo J Alfonso
The reproductive patterns and seasonal abundance of the common
ricefield rat were studied in an irrigated ricefield in Siniloan
IAguna from August 1971 through July 1972 The study area ias
planted to two crops of rice every year Of the two rat species
found the ricefield rat dominated the entire riceland comunity
comprising more than 99 percent of the total collection
Two age classes were recognized Juveniles and adults Adults
were further subdivided into breeding and non-breeding The vaginal
orifice became perforate at 5445 grams among fenales while the
testes descended to the scrotum at 13493 grams among males For
both sexes the skull ossified at 9079 grams The length of the testes
vas significantly correlated with the body weight Active sperms
were present in the cauda epididymis when the testes reached lengtha
of 2229 mm Once sexual maturity was attained males remained fertile
throughout the year
Reproductive pattern was sarply bimodal corresponding very
closely with the crop cycle Most litters were produced during the
reproductive and ripening stages of rice plants with little or no
breeding activity when the fields lie fallow There was no direct
relationship between breeding and seasonal or climtic chages Mean
vii
litter size was 967 but decreased significantly during the beginning and termination of the breedinp period There vas no tendency for the number of embryos to increase with body weight The distribution of embryos between the right and left horns of the uterus vas entirely independent with no marked tendency for all embryos to develop on one side or the other The annual incidence of pregnancy was 5 67 litters the number of litters produced during the dry season breeding as pore than twice that of the wet season breeding With few exceptions the sex ratio approximated equality Of the exceptions most could be shown due to differential tra Pability of both-
sexes at certain star of the
breeding period Productivity was high about 44 vouni bInft ThMiwaA
per female per year
Seasonal reproduction resulted in high PopulAtion density by August September and October sarp decline in density was observed
in January February and Mrch
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
LIST OF FIGURES
Page
1 Portion ofthebull qtudy area showing condition of the rice paddies after harvest- ------------- 6
2 Scatter diagram of length of testis against weight of rats showing presence or absence of sperm 17
3- Graph showing extent of breeding season Judged by percent pregnant and immature ----------- 1
4 Monthly changes in the prevalence of pregnancy in -relation to total monthly rainfall and number of rainy days per month -- ---- ---- -- ---- 21
5 Graph showing relationship between breeding and monthly sex ratio - ------------------- 34
6 Monthly trapline catches of field rats in relation to rainfall and temperature -------- -- 39
ABSTACT
Mrges Bernardo Espiritu University of the Tilippines
November 1972 Reproduction and Seasonal Abundance of the Ricefield
it (lMttus rattus mindanensis Yearns) at Siniloan laguna
ljor Professor blo J Alfonso
The reproductive patterns and seasonal abundance of the common
ricefield rat were studied in an irrigated ricefield in Siniloan
IAguna from August 1971 through July 1972 The study area ias
planted to two crops of rice every year Of the two rat species
found the ricefield rat dominated the entire riceland comunity
comprising more than 99 percent of the total collection
Two age classes were recognized Juveniles and adults Adults
were further subdivided into breeding and non-breeding The vaginal
orifice became perforate at 5445 grams among fenales while the
testes descended to the scrotum at 13493 grams among males For
both sexes the skull ossified at 9079 grams The length of the testes
vas significantly correlated with the body weight Active sperms
were present in the cauda epididymis when the testes reached lengtha
of 2229 mm Once sexual maturity was attained males remained fertile
throughout the year
Reproductive pattern was sarply bimodal corresponding very
closely with the crop cycle Most litters were produced during the
reproductive and ripening stages of rice plants with little or no
breeding activity when the fields lie fallow There was no direct
relationship between breeding and seasonal or climtic chages Mean
vii
litter size was 967 but decreased significantly during the beginning and termination of the breedinp period There vas no tendency for the number of embryos to increase with body weight The distribution of embryos between the right and left horns of the uterus vas entirely independent with no marked tendency for all embryos to develop on one side or the other The annual incidence of pregnancy was 5 67 litters the number of litters produced during the dry season breeding as pore than twice that of the wet season breeding With few exceptions the sex ratio approximated equality Of the exceptions most could be shown due to differential tra Pability of both-
sexes at certain star of the
breeding period Productivity was high about 44 vouni bInft ThMiwaA
per female per year
Seasonal reproduction resulted in high PopulAtion density by August September and October sarp decline in density was observed
in January February and Mrch
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
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43
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1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
ABSTACT
Mrges Bernardo Espiritu University of the Tilippines
November 1972 Reproduction and Seasonal Abundance of the Ricefield
it (lMttus rattus mindanensis Yearns) at Siniloan laguna
ljor Professor blo J Alfonso
The reproductive patterns and seasonal abundance of the common
ricefield rat were studied in an irrigated ricefield in Siniloan
IAguna from August 1971 through July 1972 The study area ias
planted to two crops of rice every year Of the two rat species
found the ricefield rat dominated the entire riceland comunity
comprising more than 99 percent of the total collection
Two age classes were recognized Juveniles and adults Adults
were further subdivided into breeding and non-breeding The vaginal
orifice became perforate at 5445 grams among fenales while the
testes descended to the scrotum at 13493 grams among males For
both sexes the skull ossified at 9079 grams The length of the testes
vas significantly correlated with the body weight Active sperms
were present in the cauda epididymis when the testes reached lengtha
of 2229 mm Once sexual maturity was attained males remained fertile
throughout the year
Reproductive pattern was sarply bimodal corresponding very
closely with the crop cycle Most litters were produced during the
reproductive and ripening stages of rice plants with little or no
breeding activity when the fields lie fallow There was no direct
relationship between breeding and seasonal or climtic chages Mean
vii
litter size was 967 but decreased significantly during the beginning and termination of the breedinp period There vas no tendency for the number of embryos to increase with body weight The distribution of embryos between the right and left horns of the uterus vas entirely independent with no marked tendency for all embryos to develop on one side or the other The annual incidence of pregnancy was 5 67 litters the number of litters produced during the dry season breeding as pore than twice that of the wet season breeding With few exceptions the sex ratio approximated equality Of the exceptions most could be shown due to differential tra Pability of both-
sexes at certain star of the
breeding period Productivity was high about 44 vouni bInft ThMiwaA
per female per year
Seasonal reproduction resulted in high PopulAtion density by August September and October sarp decline in density was observed
in January February and Mrch
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
litter size was 967 but decreased significantly during the beginning and termination of the breedinp period There vas no tendency for the number of embryos to increase with body weight The distribution of embryos between the right and left horns of the uterus vas entirely independent with no marked tendency for all embryos to develop on one side or the other The annual incidence of pregnancy was 5 67 litters the number of litters produced during the dry season breeding as pore than twice that of the wet season breeding With few exceptions the sex ratio approximated equality Of the exceptions most could be shown due to differential tra Pability of both-
sexes at certain star of the
breeding period Productivity was high about 44 vouni bInft ThMiwaA
per female per year
Seasonal reproduction resulted in high PopulAtion density by August September and October sarp decline in density was observed
in January February and Mrch
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
MTRODUp7ON
1ts have alwys been a problem in TiJippine rcuizure here
early reports of serious rat depredations in the Bicol region andwere
in the Mountain Province (Crucillo otanes and Morales 1954) but these
lacked the details that would provide better meaning and importance to
such apparently localized and short-term irruptions Simiar severe -- 2 5 i
were Imown to exist ininfestations but notably of longer durations
large sugr estates in Mindoro and Negros islands (Utzon personal
cozunication) but loss of records during the last mar denies to pre
sent workers information on rat species and their patterns of growth
and abundance in cropland conditions The near simultaneous and widespreno
rodent outbreaks in Cotabato (Sarmiento 1953) Davao a4d adjacent i~ i bull -
provinces in Mindanao in 1952-53 provided the first Ehilippine exmprience
which considers rats as an agricultural pest of national econoic
invortance (crucillo et al 1954 and Villadolid 1954) Since then
rat outbreaks occurring at times to the proportion of a plague have
severely affected rice and corn roduction before harvest while also
affecting the potentials of sugar and coconut exports in all principal
islands and regions of the country
The Thilippine ricefield rat Rattus rattus mindanensis MRrns Is
the most prevalent rodent form affecting local agriculture (Barbehenn
Sunanil and Libay in preparation) It is chiefly responsible for most
of the danage in cereal crops where the value of crop losses to rice and
corn alone vas estimted to reach 160 million pesos annually (Sun gil
et al 1970)
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
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41
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42
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NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
2
Inspiteof its widspread distribuitiow the riceplusmneld rat has
been the-subject of relatively feWtailed studieson Aeproduction and
popUdtioi dnamics To Obtain relible records oa the breeding of most
wil ls onemutrely iinlj on Dbst-mortem ezingtions of an
adequate number of the-species collected at intervals over a period of
6iWVY more years These conditions were approached by Rosell (1955)
with otr 6onsecutive years of study on the breeding activity-of attus
ak~nftftte in ricefields and vastelids of Cotabat6and by
se 1and Mcaz (1963) on three intervelig-periods in the succeeding
(1958) further reported some beeding data years Singil and Aqdlno
on AR_r mindanensds and -argdentivente collected in cropl nds and
forest clearings inlknbuiAro M1idoroOccidental SiWpl6entary data
on the ontogeny and laboiatory breeding Were aiSo obtain by Rosell
(1959-)on R4 argentivefter and Medina et al (1972) on he7i mindanensis
M1iddetaill -6n the kijrodudtivity and breddingactivity of RI 4N
taidaneni-amp6 the basis of geoaaphic Iocachirwere repotd from
International Rice ReSearch Institute Colleg6 lagu (Uler 1967)
40oi cift dii lfties df lAguna (fan~bei di 41)171) land Mufiozt
N aEcj Ad(Afonsoand Castillo 1970)0 It is apliWehVr6n n thesw
reporbs- eii 6rddfed areas ampreiniclved -the in breetig period
coinede ith te rali y season and -pekiod ofrice dulti-iation (-( wto
Ddleml ii Vi~thucl reduced breeding abivity durIig-theAdVoiFufr
bull h 5 (3 btrty) This breeding pe dicitY46eS tllt 44dJ
seasonal build-up of population andMyat1o AtIdy = ther~adits
obtained by LaVoie et al (1970) who found that tiller destruction by
rats during the vegetative growth period of rice mas light but increased
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
3
appreciaby during thereproductiveAnd ripening Stages of development
Si~i1(971) observed-tb th presence of~oie foodand qiur~r cover offered bybooting rice at acts rapement fsuod i
wastelands into rice fielp Thus 1irrtIye populations are cozsuonl1y associat to seri uscrop dam gebefore Orest
The basic breeding cycle ofanibals may be affected either favorably
or~ ~ pnionentalpoundhctor~ Trpugh bis agicltr1~b9yLuP ~asqL
pratices nan has altered his enviroment tno fvor -the existence of many
species includng fi ld rate Etensive use ofirrigation in Iag= province duin~g te~c Beasqn hs converted astelands and lowed areas
ids Hence irrigated p3nted to
rice during the dry season This provides rats with adequate supply of
food and cover even during the dry months of the year Uhlpr (1967)
observed at IRRI where irrigstion is continous and rice is grown throughshy
out the year that R r mindnensis ehibitsa year-round breeding season
The effects of env onmntAl-fctors ariculrq olhnwte and food requireshy
ments on ed cycle or rats is emzphasized by Sunangii (1965) who
states that where food is alays plentiful and olimtemore or lespeven
throughout the year rats breed and reproduce anytime oF month of the year
Knowledge on the reproductive activity of the ricqfield rat
particul ry breeding season is needed before any realistic reduction
program pan be eyolved B understanding population changes inrelation
to season and habitat reducion measures pay be instituted at a critical
stage begore apMulatcn increase
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
4
The esent study vas undertaken from August 1971 through July
1912 to collect informtion on the reproductive pattern of rats in an
irriaatdk ricefield and to determine population changes in relation to
seasonal and habitat changes In a broader context the study offered -4
an opportunity to egamine in detail the mechanisms of population regulation
of a single mannalian species in a typical riceland ecosystem
TAXONONAID DISTRIBUTION
Rattus rattus mindanensis is the most widely distributed Philippine 7
species and is chiefly responsible for the destruction of rice and other
crops- in In Siniloan lAguna it represented thanLuzon0 and Visayas more i - 4-
99percent of the total rat collection all others are I0ttus exulans
Although originally described by Mearns as a distinct species (Taylor
1934) the ricefield rat is now designated Ratu rat mindanensis
(Barbehenn et al 1972 and Johnson 1962) It is medium-sized rat
with adults averaging about 180 gram The tail is usuilly longer than
the head and body ac S iiiforu7yblak dark brown scalation is
variable The hind foot averages 40 mm in length but the range of 36ste Jj4 owj Jsi)dE o -
to 45 overlaps that of many species of rats Plantar pads are well
developed Ears measured from the notch averaged 21 ma (range 19 to
23) 1elage color is quite variable ranging from grayish brown toV) i3 plusmn32 - r i j I
reddish chestnut dorsally and usully blending gpudually into a paler
venter which is even more variable = from crewr white to dirty grey
often with a wash of buff of variable intensity (Barbehenn et al 1972)
The ricefield rat typically hbas two pairs of pectoral and three pairs
of inguinsl e with the anterior pair actally being abdominl
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
5 and more distinct from the middle pair tban the ps-teri- i par To the
h
non-taxonomist the various sub-species are very similar and wheramp ranges
in mea6sreent overlap it is extremeey ditficult to s jedti3 the
In the whole isiands of Mndanao and some parts of Mind6r here
1~ttus argentiventer dominates ricefield commuitiesi becomd more
cormensal in is distribution (barbebieni it a 2 cording to
Sumsngil (1968) this specisi3 apastjcUr liking for hunmn dwellings
and generally destrcys backyard fruit crops particularly coconuts and
banana plantations as well as stored rieeand-ern- - n Coflitdt more
than 90 percent of those caughi near and inside houis belong to this species narbehem (1972)et al considered R r mindanensis to be an
ede icsi--s-pecies in the Philippines since it is presumed to occur
on vfirully every island The anizls are found from grasslwnd at sea
id~jL to ove 6 ~fd n Luzon and Mindoro although they are less
conon in hiter altitudes
DASCPI OP STDY APtS
The stuy area vs located in Siniloan laguna aboub tvo kilometers
from IAguna de Day shoreline at an approxicte elevation of slightly above sea level The topography is relatively flat with sligW dec~ine
to the east tO~rd the base of Sierra Jdre mountain- Anarea of
axp~oncltely one square kilometer within and around the peraiphery of I 7
Daybay National Agricultur and Vocatio chool was used as the
prinmry collection site (pigure 1) It is dissected by a number of
sM11 and shallow creeks and lrger tributaries that result in overflow
during the rasiny season with any nmrked rise in lake water
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
Elg 1 Portion of the study ae showing conditions of the rice pardies after harvest
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
The climate is characteristic of the lowland areas of Iaguna
province (Table 1) The average annual precipitation for Siniloan over
two-year period vas 19904 mn Average monthly precipitation did not
seem to vary widely from year to year the average for 1970 and 1971 va 18825 and 20982 anm respectively The bulk of the rainfall usually
occurred from July through December In 1971 much of the rainfall
occurred during the period from October to December while in 1972 the
effects of monsoon rain cused a peak rainfall in July with 7670 n of
rain This abnormQly high precipitation inundated and submerged the
entire study area a week after the last specimen collection was Dade
The monthly mean temerature for Siniloan did not vary widely The
monthly mean tenerature ranged from 2469 to 28320C with an average
of 26780 C during the whole periodf study
Although there are two cropping season annually rice production
in Siniloan is entirely dependent on the depth of ater in the paddy
fields Unlike in nany other Iaguna towns the first or regular rice
crop is transplanted in ampnuary and all the paddy fields are planted
in late februsry H rvest occurs mostly in My and June The second
crop or r gad rice is transplanted in July and August and harvested
in November and December Because most of the areas are titer-logged
during the rainy season only about half of the total hectarage is
Planted to rice Hence the amount of cereal food available to the
rats during the rainy season is coortively less than during the dry
season
Weeds are a constant problem on the YAddy fields The unplanted
wras remain largely covered with succulent gasses compsed mostly of
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
Table 1 Mont3 mean wa- -Iand ay ge t me tie J $inilcmn
aI deg - C
bullMn h z 70 3972 ~ ~ ~ ~ je 4 Ih1infal Ai ti14n~1 1ainfall
c ) -c)7
J niw 409 2555 125 2469 47o
February 168 2553 256 2541 25
M~rch 201 2718 827 2563 749
April 203 2828 183 272E 162
My 198 2926 2298 2832 1582
June 1775 a858 2511 277E 4280
Juy 1406 2773 33146 2747 7670
A t18204 147 1658 27714
September 3592 100 2085 27-90
October 5165 2-7025 418 2688 shy
5770 682 34Q 627
D376 Z8 654 4496 2597 shy
an 8825 2727 20982 2678 21340
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
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and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
9 bayakibok (Echinoclo crusgalli) and large stands of tikiw (Scrpu
ampdsibl) are equaljcspVuos ie banks iid nin~sIal area6 ire~
coveredwith thick stands ofta3AhbIL-(Sacchrunm spontaneum) while
pddy dikes- Iirrigtioi canalsrwere dnminatedby thick nattin of
para -grass (BraShArly ica) The presence of succulent weedsand
xtooning rice plants after harveit affords the rats anple cover nd
food throughout the yehr
YATMAIS AND MTHCDS
A Coflection of Rats
lost-Drtem exminAtion of 932 rats collected at two-week intervals
from August 1971 through Ju3Ly 1972 in irrigted rice fields fo vxd
the basis of this study Animals were generally collected by Victor
traps Suplemen tl collections were made by band capture
(blaneting) of aninals ip grassy areas Snap traps were placed along
PA dikes in two or three transects each composed of 50 or more raps
Distance between traps was about 5 meters and the space between transects
Sried IccoxI4ng to the distance between paddy dikes or about 50to 100 --trap -lines wereeg usii3yoperated for2 or3--consecutive nights
Bits conisted of fresh coconuts or sliced sweet potatoes The trapshy
ping lines vere moved into a different collection site for each trapping
period and were located systenatically to enable zanpling the entire
study area- 1he number averaga of spe ns exined per month was 776
with a range om 14 to 194 rats Generally the monthly s6Mles
consisted of more than fifty awimls
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
LO
B Autopsy
Necropsy was conducted iii the field as soon as specimens were
collectede Allan l wered identified sexed and weighed separately
In the of gravid fem3Js-the difference in scale after removingcase
the weight of the embryos and enlarge uterus was taken as the weight
of the parent In addition standard physical measurements (total
length- _tail length ear-length-and hind foot) were recorded
The reprodfobive condition of-the males was determined by examining
the position ofthe testes and redording them as scrotal or abdoiInal
Testicular length and -width was measuredin millimeters and the conditions ofthecauda epididymis werenoted FA s with enlarged scrotal testes
with visible tubules in the cauda epididymios -wcre -considered
reproductively active
The condition of the female reproductive organs were determined by
making a median incision on the -ventral surface of tbe abdomen and
examining the uterus id situ Females were classifiedas reproductively
active if they were pregnant lactatig orhad opened vWkgiolorifice
The presence of spots or placental scars in the mall of the uterus were
recorded However no attempt was made to count the uterine scars
because this criterion was not reliable for determining litter size
(Davis and Emlen 18) A rat as classed as pregnant if the embryos
are mcroscopicaly visoIle and lactating if milk can be expressed by
the fingers from the nipples For the embryos actual crown-rump length
As WDfsured when the specimen was large enough (3 mm or more) The
number of embryos contained in the right and left horn- of the uterus
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
were recorded Dead embryos and those being resorbed (snaller than the
mjority that developet is obviously abnornal) were noted and used to esiliate the loss of embryos duning pregnancy
Ossification of the skull Mas determined qua3itativel7 by resistance
to finner vreskikd (Dalvis and HB1 1951)
Statistical Aa6es
The annual incidence of pregmncy or the number of litters born
to a female as Appr~ciiat d fvom estimtes of prevalence of pregnancy
(Emlen sad Davis 1948 Davis and Hll 1951) Sex ratios were expressed
in percent males and were tested for statistical significance by Chi-sqar
analysis Mtuikty based on the perforation of the vaginal orifice in
females presence of scrotdi testes in uesand osugfication of the
skull in both sexes were determined by correlation and regression analysis
3h determinLg reproductive -trends the data were analyzed by
=oths$ anad also by seasons Mnthly Mean litter size vas tested for
statistical significance by the use of Anaiysis of Variance and mean
differences vasdetermined by Duncan id ltiple Mnge Test
ESUIEW AD DISCUSSION
A Criteria of laturit
Determination of hrnonPogical age inferal rats isdifficult
because growth and developent rates r y vary In ear individualnd
in different PO1)l3ltioflsa were it tioable to ObserV9PnIividus over
12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
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BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
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- and 0Hall 1948 Seasonal reproductive condition
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EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
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42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
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SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
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43
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12 a period of time the differentages at which they attained a given level of developent could be determined However since the aninals
in the present sudy were examined at but one stage in their development
onthe presenge or absence ofa given character could be observed
Characters ofteA used for differentiating adults from Juveniles are based on the capacity of the animals for breeding Though annals are adult
by chronological criteria they can play no part in the reproductive
phases of the population dynamics until motile sperm are contained in nale reproductive tracts or until ovulation has occurred in ferles
(Jackson 1952)
Use of weight at which 50 percent of the aninls possess a given
characteristic is preferrable to 100 percent level since the latter
TV be undul affect-ed b a few slow3y naturing individuals (Jackson
1952) these 50 perdent points were calculated from the regression of
percentages on the body weight
Table 2 shows the sizes of the ricefield rat at various stages of maturity The weight at which 50 percent of the females have the vagna
foAte wks 54145 g~rams althoug femnles mysaome become perforate at wikihtas ow ai 10 grams However these newly perforate femles sho 06d t ofrpductive tracts Perforation is not indicative
sexual lturct in temas=f due to the presence in the population of
iew Per~i~ate fensles which lack ovarian developeent tIn the mles rapid increase in weight is not normally acccqsnied
bymaturation of the gonads or accessory glands The testes become
scrotal posiion a6t 13493 gras while sperms were present in the
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
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41
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42
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NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
~13
V~ble 2J Sie of I ricefield rai at Varid stages of mturity
Weiwht in grams Characters No of rats 50 ercent poizt
TagIiriai iritibe per orate 450 5441
mvi1- osoiricition 90-797
st scrotalpoitn 477 134 3
Spe in -daud epidikvmis 55 15447
lAcental scars 450 18555
oauda epiidymis at 15445 grams Theyeight t which 50 percent
become fertile gives ainasure of the averageweight of onse of fershy
tility
Ossification of the skull occurred afterVt rats attined the
weight of 9079 grams
The presence of scars in the uterus is a good InUoplusmntion that
implantation has occurred Thus if the aninals are grouped by size
and classed as parous (indvidmils wi scars or visible embryos) or
non-rarous (individuals Vithout scars or visible embryos) a measure
of the size at first pre canyc beobtained adEmlen 1948)can (Davyi
The lowest weight at which first preuay ourred was 105 -grams which
can be taken asa measure ofpeurty 7pregnantof the age Of the lo+
females examined 8654 pe ighed re grams
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
14
1 Testis Dositionl size and fertility
ibsition and size of testes have been used as the most important
criteria f breeding in mles Testes in the ricefield rats may occupy
either abdionil or pscrotal Both testes generaly occupyositions
the same position Descent my occur at an early age depending upon
the season since scrotal testes were found among males weighing well
below 50 grams during the months of April and October The proportion
of rnles with scrotal testes increases with weight thus showing some
relationships between position of testes and seuml maturity Recent
studies by the Rodent Research Center indicate that rats with active
spernatozoa invariably had scrotal testes although 45 percent of the
adult mles with no apparent sperm activity (epididyml tubules not
visible) had scrotal testes (Swink et al 1970) Descent of testes
isnot a very good index of fertility in the ricefield rats
Table 3 shows that there isa high proportion of males with scrotal
testes throughout the year Considerable reduction inthe propoplusmniOn
of males with scrotal testes in June and July mas due to the presence
of more Juveniles in the-population Since this species is capable
of producing active speriatozoa throughout the year in both frrigated
and rainfed areas (Swink et al 1970) then the occurrence of seasonal
changes in the Pregnancy rates cannot be attributed to seasonal changes
inthe capacity of nales to fertilize
Length of testis was used as an index of testis size Miller
(1946) observed that length and weight of testes among pocket gophers
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
15
T~ble 3 Mnthr c~~sition of adult nle po tuJMon ojI rattas mindanensis with respect to position ot testes
Tota les stis 3ositiOn ercent Month exam er Abd inal Scrotal scrotal
Agst 1971 8 1 7 8750
September 40 12 28 700
October 30 8 22 7333
November 69 15 54 7826
December 114 28 86 7544
Januu 1972 22 8 14 6363
brw 10 3 7 7000
wxch 9 2 7 7777
April 43 13 30 6976
1y 38 14 24 6315
June 56 40 16 2857
4uly 43 27 16 5925
Total and average 8 171 311 6452
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
are closely correlated (r a 087) so that length isan adequate estimate
of size Laurie (1946) found among mice that if the tubules in the
cauda epididymis are visible to the naked they invariably contained
large number ok sperms and that the indlJdil would be fecund In the
ricefield rats the presenceof sperm is usually associated with a testes
length of between 20 and 32 mm (Figure 2) It was also found that a
testes weight of 22 mm or over was almost invariably associated with
fecundity that is with large number of sperm in the cauda epididymis
Below the length of 20 mm the cauda epididymis was nearly always
without sperms that is to say the males are non-fecund
B Reproduction
1 Ereeding season
The seasonal reproductive pattern of 1ttus rattus mindanensis
in Sinilcan Iguna are sunmrized in Table 4 and Figure 3 Although
the proportion of adult sales capable of producing active sperms
remained high throughout the year (Swink et al 1970) reproductive
activity of adult females show seasonal trends Analysis of the
thlyprevalence of visible pregnancy andincidence of juveniles in
the population indicates that this species features a sharp bimodal
breeding curve peaks occurring from March to April and October to
NoNvember This is in general agreement with Uhler (19) who also
observed bimodality in the breeding pattern characteristic of this
species at the International Rice Research Institute College Laguna
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
SPERM 0000 o NO ERM e e
0 0 0 o0 00
00S0 00
_6 0 00 U) 0 0 0
deg bull70 LI I t
Ishy
35 55 75 95 115 135155 175 195 215 235 255 275295 315 BODu Y WEi GH T (gin)
Fig 2 Scatter diagram of Iongth of losuis against woight ifrats showi~g presence or abaen~e of s~rw
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
0 PREGNANT
901 A JUVENILE
soshy
70
oshy
0
0 o L 1
) 2
A0 6 F
40 ~bg at l eadr e
Fig 3 Graph showing extent of breeding season judged percenp3 7 by g~tandifimatiire-
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
19
In other parts of the Philippines a unimodal curve has been found
In Cotabato the ricefield rat breeds actively during the rainy season
(June to December) followed by no breeding activity from January to
early part of Jmune (Sumangil e al 1963 Rosell 1959 and Sumngil
and Rosell 1959) Sanchez et al (1971) also observed that inNueva
Ecija this species breeds actively during the rainy season months but
ceases compltely during Mrch April My and June this period
corresponding to the dy season
Jackson (1952) stated that many environmental factors play a role
in the regulation of sall muml breeding The basis however for
the appirent relationship between these factors and increased breeding
is not well understood Bwruon (1936) observed that breeding season
appears to combine with the warm season in temperature and sub-temperature
clites But it appears that in monsoon areas the relation between
climate and breeding is not so simple It seems that breeding is largely
a matter of nutrition that my well depend on harvest and indirectly
on cliatic events
Harrison (1952) in Wayamprecorded a bi-monthly rhythm and attempted
to relate itto the lunar cycle No suchcorrelation has been attempted
with the present data Swmngil and Rosell (1959) generaly regrded
climate as the min factor affecting the rhythm of reproduction of the
ricefield rat They observed signifiant correlation between reproduction
and rainfall In laya HarriSon (1952) has shown that rats appeared
to have no breeding season although breeding activity my show fluctuations
related to raInfall
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
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1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
20
In this study correlation of monthly prevalence of pregnancy with
rainfall showed a slight negative relationship (r a 035) When the
monthly prevalence of pregnancy was correlated with the number of rainy
fays each month the relationship renained slightly negative (r u 0446)
indi(cting that seasonal changes has very little effect on the reproductive
activity of the rats This tends to show that in effectively irrigated
areas where rice can be grown at all months of the years rats can breed
continuously regardless of seasonal fluctuations As shown in Figure 4
the rainy seamon breeding seems to be significantly correlated with
rainfal (r a o447) while dry season breeding appears to be negatively
correlated with rafall (r a -030) This mY explain the significant
relationship between reproduction and rainfall reported by Sunsngil
et al (19j3) ad S ugil and Rosell (1955) Apparently their data
were gathered from rainfed fields where seasonal cycle and crop cycle
correspond more closely than in irrigated areas In rainfed areas the
farmers entirely depends from rain to start land preparation and planting
Table 4 indicates that active brPeding in rats is directly related
with the presence of rice in the field In Siniloan dry season breeding
appears to be of greater intensity and of longer duration than during
the rainy season This my be attributed to -he fact that wider area
was planted to rice during the dry season Hence more food is available
to the rats as a source of nutrition to maintain their Lxeeding activity
Breeding my be reduced or terminated when cover and food supply are
depleted or when convetition for resources such as food nest sites and
harborage is high (Sanchez et al 1971) Sunangil and osell (1959)
observed that thu rate of reproduction of lampborstopy-rowed rats diminiches
with the reduction of protein supply
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
1001
40
20
o A F M A M J
97F [972
600shy
500
S400
F306 200 _3sea
o 1 M 4 40100
AtS O N J F M A J J 197 1972
S20 bull bull i 4
4 4O
4
0 IS
6 bull 0 - 4 bull4 r= i
AS O N F MAMJ J 1971 192
Fig fMonthl o ge ithe preval~n~e of pregnancy in relation to total monthly rainfh11and n~mber of
rain~v days ter month
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
22
Table b pmeaence of pregnnat M us ratty-Mnthly obnges in t-he mindanensis inSinilan August 1971 through July 1
Month No of adult
Number Percent pregnt pregnAnt
Embryos per
Growth stagesof rice
females fegale I
August 3971 14 0 0 0 transplanting
1052 650 tilleringSeptember 38 4
21 6 3157 967 booting headingOctober
79 38 4810 968 mature harestingNovenber
Decenmer 80 7 875 886 post-harvest
0 0 seedbed land prepJanuary 1972 23 0
131 3 2727 70 transplantingFebrury
5 4 8000 1075 ti11eringIiMrch
April 32 20 625o 1o45 booting heading
My 30 11 3667 1118 mature harvesting
12 2553 916 post-harvestJune 7
sepdbed 3AIn 3repJuly 37 (f2 541 750
Total-aidaverwges417 107 2802 967
aJ Prevaltie of pregnancy bised on total number of feniles with perforated vaginal orifice
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
23
(1955) thiamine is essential forAccording to Nelson and Evans
in the rat Nelson and Evans (1955)the normal reproductive performwance
cited Even- ishob --(1922)-who itepbrtded the imediate cessation of
on vitamin B-deficient diets Ueno (1934)estrous cycles in rats placed
cited by Nelson and Evans (1955) iidependentlyand Cowrd et al (19 2) as
mentioned the occurrence iof reproductive distrUbance in rats deficient
in vitamin B or B1 These results strongly suggest that seasonal
fluctuaions inthe breeding activity is dependent on the quantity of
anIS3l and vegetAble food available to the rats
2 Prevalence of pregnancy
Prevalence of pregnancy (per cent pregnant) was based on the number
of adult teenles per month Annual prevalence mas calculated by
Table 4 shows thataveraging the estimates from the monthly samples
oriented around two peaksthe variability of the breeding intensity vas
of activity One occurred in November when about 48 percent of the
seually mture females were pregnant and a second and higher one in
Narch and April when about 65 percent of the mture females are pregnant
arch my not be reliableHigh perceamptige of pregdant females during
It is-since the number of individuals involved ws not large
possible that non-breeding animals coming from surrounding wastelands
rp hsnyaccountwre ificluded inthe samples during th~e ~aaa
This is infor the low )rproductive rate during the rainy season
agreement with the observation of Sumangil (personal cncm) at Midsayap
Cotabato where he found that one-third of the annual reproduction
occurred during the s planting
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
Although the data from the present study are Inadequate to determine
actulf-the factors rtesponsible for initiating active breeding it is
evident that the 0wevalence of pregnancy increased markedly during the
repr6ductive and ripening phases of therice growth A prevalence
of 526 and 875 percent during the months of July and December indicates
a shrp decline and termination of the breeding seasons
3 Incidence of Meoncy
The incidence of pregnancy or the number of litters born to a
female ddtwing a known 1eriod of time was calculated from estimates Of
the prevalence of pregnancy (Emlen and Davis) 1948 Davis and Hall
1951) by using the formula I Pt where I is the incidence during a d
time P isthe prevalence of pregnancy t isthe time of interval of
observation -i days and d is the number ofdays during the gestations
period of Tatsthat embryos are visible in the uterus which is about
18 days
The annal lujaidence of pregnancy of Siniloan field rats of 568
liters wnsbased on 1417 adult fenales This isnot sigpifieantly
different frmnIthose reported by Sanchez et al (1971) of 59 liters
for this Opeciel in three southern municipalities of guM It 1si
slightly higher (I -5) though nay not be statiticall- siifitcant
from the observations of Sumangil and Rosell (1959) for this nhspi
in Occidental Mindoro and for other Speciesnfor most oher-ares of the
world (Jackson 1952)
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
25 Ro compre the incidence between two crop cycles the data are
sumarized for seasnal differences as sugested by Davis (1953) The
pqocentage pregnapt for all months (or seasons) were summed and then
divided by the znumbprofmonthp The method essentlaiy assumes that qile numb)= of t (100 was caught in- each month --Te incidence
of pregnancy was 405 litterls during the dry season and 164 litters for
the rainy season It appears that ricefield rats in Siniloan are able
to produce more than twice-the w er f litters cbring the regular
cropplanting than during the palaad or second crop planting This
results in a rapid increasp in the population towards tb tormination
of the dry season breeding
4 Litter size
Estimates of litter size was derived from counts of presumed normal
fetuses Ased on fetus counts the mean number of embryos per female
is 967 with litters ranging fromu 5 to 17 (Table 5) Counts of very
young fetuses my be of less value in determining litter size as some
loss ofyoung my occur after this age (Wirtz 1970) Loss of ova before
fertilization or resorption of embryos are among the fctors that may
reduce littersize in ammls Resorbing embryos were found in 11 of
107 pregnnt females examined One fetus ws affected in more than half
of thp oases while 2 were affected in 3 cases Intrauterine mortalty
Of 212percent is signifant1 lowerthan that reported by Smwngil
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
26
Table 5 Monthly mean distribution of fetuses In the two horns of the uterus of pregoant rats examined in Sinilcan laguna August 1971 through July 1972
No of Total no enno of NMths pregnant
fenles of
eMgros Right horn
Left horn
embryos
Augut 1971 0 0 0 0 0
September 4 26 12 i4 650
October 6 58 28 30 967
November 38 368 179 L89 968
December 7 62 31 31 886
January 1972 0 0 0 0 0
February 3 21 1l 10 70
IMrch 4 43 21 22 1075
April 20 209 110 99 3o45
My 1 123 67 56 1118
June 12 110 56 51 916
July 2 15 10 5 750
Total 107 1035 525 510 967
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
aui )Rpslh 9)o 6pret This pay be attributed to tie
difficulty encountered in determi frmnnornal dresorbing fetuses
during the early stages of pregnancy
ble 5 shows the distribution of fetuses in the two horns of the
uterus Based on 107 pregnant femlesdisseotedfrom which 1035
embryos were counted 525 or 5072 percent were lodged on the right horn
of the uterus while 510 or 4928 percent were lodged on the left horn
of the uterus Except for one case where all of the 7 embryos developed
on the right side of the uterus there seems to be no tendency for the
embryos to develop on one side of the uterus or the other It is evideni
that embryo development among the ricefield rat is entirely random and
independent This is In contrast with the report of Harrison in Malaya
(195L) where one side of the uterus was favored for embryo developmnt
in Rattus argentiventer
Table 6 shows the analysis of variance for the number of embryos
of all pregnant females The mean separation are shown in Table 7 The
average litter size recorded during the months of 1ebruary Ju3y
September abd December are significantly smoller (pu 005) tba those
obtained for the other months This indicates that litter size produced
between crop cycles decreases when food is scarce nnd competition for
other resources is high There seems to be no correlation between the
weight of the paent and the size of litter (r a 02)
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
28
Table 6 Analysis of varlaae for litter size
Source of Degrees of Sun of mean
vaziatQI r-di i sams ic Los -value
th 9 12055 1319 249
97 IMr510I 537I
Total 106 61a56
ISignificant at 5 6 i
Table 7 Mean littir size iiaxation ofall _egait rats Ii aanined for this study iniSiniloan Z =a
Sept Feb July Dec June Oct Way Apr -Met Ift
L
6z50 70- 75 886 216
992-w
268it
ioI42 1075 n 18
SNoawn mcqui b 14
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
29
ratio~ - pro o of adult fem1esi
popltjtion Ason in Table - the embryo productivity vas 4396p shy
femle per year This values obtained bymultipJying the average
n mber of viable embryos per female and the incidence of pregnancy Bia
rate (birth per rat per year)ia calculated by multiplying the anal
Wbrm0 pozumy---he percentage of-ultIWales -itbe -populetion
and the sex ratio (Davis and Golle4r 1963) Tab4 8 shows that the
rate of birth among ricefield rats mas 931 young per rat This did nol
e 81InnI IFI Idanensin t captured du i n
rp mgmn nq mde l A 88
omonths vthout-b1fo O- it li- Vrig20
a shyAverage-tio of viable enbryosfe leaJ 774
Embryoaplusmne -ieyear 4396
Age ratio (adult) 7379
Bex ratio (nale) 5183 Adult females (of total) bull0
Birth rate (birthratyear) 931
a Base qpon in ut toconto and correeatedby an estLteof 20 uterine morl tfpibl~shedfr cl6sely -Jr tedspecies
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
30
differ significantly from the results obtained by Sanchez et al (1971)
for bothrainfed andirigated conditions In the real population
young and even nomeadult femles do not breed hence the rate is even
lowqr These results show that uder thd conditions obtaining in the
rice fields the rats have a high potential rateof increase
6 Juvenile incidence
Supplementary and confirmtory evidence on breeding may be furnished
from other sources The age composition particularly the incidence of
Juveniles may provide valuable clues A continuous stream of juveniles
contributes to the mnthly sanples but their relative frequencies that
the gestation period of rats ranges from 21 to 22 days Considering the
lactation period of 21 days (Medina et al 1972) the highest percentage
of Juveniles appeared in the collection about two months after the peak
of the breeding period The smllest rat captured weighs only 24 grams
whichmy be just about the size at weaning
There are two peaks in the incidence of rats less than 80 grams
in body weight (Table 9) These peaks confirms the existence of two
min breeding seasons of rats in Sinilcan Both occurred about two
months ater the peaks of pregnancy has been attained A Mrked increase
in the number of wal rats uas noted in January following the peak in
November An increase in the proportion of juveniles in June and July
substantiates the hypothesis of considerable breeding during the sumer
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
Table 9 Monthly changes inthe young - adult ratio of al raiiCollected for this study at Siniloan IASUnA
Months Toamp 1 Number Number -Pecent
Rats ofy omxl ofadlt 8 ~ n August 1971 25 21 16oo
September 78 12 66 1538
October 51 1O 41 1960
Novedber 148 15 133 1013 December 194i 31 163 1598
January 1972 62 237 4032
February 21 6 18 2857
Iftch 14 2 12 14I28 Az j7 i76 62 1842
2Y77 54 3807 JuIe 108 63 5000 uy 84 o a4762
Total 932 240 692 2621
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
32
months of Mrch and April The low incidence of Juveniles in Nitch and
shyeroenb~e~rco susiio -tatt there is little0r-no breeding
activity during the months of January and August
C sex atio
Small mMM trappers have often foimd a slight preponderance of
nales in their returns Among the workers with the present speciew
(Tigner 1$72 Sanchez et al 1971 Swink et al 1970 Alfonso and
Castillo 1970 Uhler 1967 Simngl and Aquino 1958) and with
Rattus argetiventer(Sunangil et al 1963 Rosell 1959 Harrisonshy
1951) Timer and Alfonso were ony those whose samples were large
enough to demonstrate that sex ratio of trapped rats vas statistically
significant from a ratio of 50 percent mles in favoT of the females
A total of 760 rats were trapped from August 1971 through July
1972 consisting of 400 rles and 360 ferles (Table 10) The over-all
sex ratio of 5183 percent mas not statistically different from the
ratio of 05 mles However the monthly sex ratios Indicated a
variable pattern There appears to be a high correlation (r a0723)
TheProportion of zia1esin -thessugae increased AppreoablyOur4Ing the
height of the breeding season and tended to decrease during the noAshy
breading months (ZLgure 5) This may be attributed to the wider home
ranges of the mules than the females Studies of Venezuelan roof rats
by Gomez (1960) indicated that sales were recaptured up to 80 meters
from the point of release while most females were recaptured from 15
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
Sln~loa c n i e sex14 Of all rats traplagura August 1971 tju6ah JMIfV I172
i
Mont s Total- Number of Number of SexaJ Chishy
rats --Males -fenes ratio s~usre August 1971 25 8 17 320 324 SepteMber 74 40 38 5128 005 October 51 30 21 5882 158 Novenber 84 50 34 5952 067 December 82 W 34 584 o86 Janwwy 1972 62 22 40 3548 522 Fmbiry 23 12 44782 0002 WrCh 14 9 5 6429 314 Aril 78 45 33 5769 185
y 71 38 33 5352 035 June 108 56 52 5185 015 u 8 41 51-9 019
Total 760 400 360 5183 21o I i f I
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
---
100
9 PREGNANT 90
O SX RATIO
z Go-110 I - -
So50--0 40
a30shy
10shy
70
A S 0 N D J F M A M J J
1971 1972
Figure 5 Graph showing relAionship between breeding and monthly 3ex ratio
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
35 to 20 meters from the original trap but not much farther Thus widely ranging males would come in contact with more traps and would be caught more often in relation to their absolute number than females IMrkes (1924a as cited by Laurie (196) however found among laboratory-bred
mice that the proportin of males was lowest in the height of the breeding season This sexual difference in iovement might also be induced by the care that females must give to their young in nests and burrows Femle rats n advanced stage of pregnancy become less and less mobile and their tendency to leave the nest is reduced
correspondingly Consequently their chances to meet with a trap will be reduced correspondingly (Table 11)
The Secondary sex ratio can be further altered by differential growth and mortality (jackson 1952) Analysis of the data by size group (Table 12) indicated that the males predominate in the youngest cass and females in the oldest class The deviation from the 50
Table 1i Number of pregnhfnt females captured in relation to stagesof pregnancy
embryos (nzn) Assumed-- (mame (days) Total Total Average parfeales erYos femle Less than 7 m 6-n 58 593 1022
718 mm 12-16 31 29 942 More than 19 mm 17-22 18 150 833
Total and averages 107 1035 967
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
36 Table 12 Sex ratio in various weight classes of rattus rattus
mindanensis
Weight Total Number of Number of Per cent glassg) - rats males -feemles- males
105 183 111 72 6o65
105-184 270 116 154 4296
185-262 388 214 174 5515
265 82 39 43 4756
Total 923 480 443 5240
percent ratio in the youngest class is suggested by Ecke (1955) to
be due to the faster growth rate among males Predominance of females
in the oldest size groups suggests greater male mortality with age
D Ft Pepulation Indices and Seasonal Fluctuations
Table 13 shows the monthly population indices of rats inthe study areamp There were only two species of rats caught in the rice field
for the whole trapping period All rats caught except for four
ttus exulans were FiRttus rattus mindanensis comprising more than 99
percent of the total collection
Using the trapline catches as an index of rat population the
Population shcyed monthly fluctuations (Fig 6) The rat population
density seemed to increase with the advern of the rainy season
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
37
Table 13 Population density index of ricefield rats in Siniloan -based on m=th3y trapline- catches
onth Number of Total rats Trappin incidence trapnights caught
ea bull
August 1971 110 25 2272
September 473 7e 1649
October 295 51 1728
November 590 84 1423
December 960 82 8541
Janaryp 1972 638 62 972
Febra 258 23 89
Mrch 376 14 372
April 717 78 1088
546 71 1300
June 600 108 1800
July 290 83 2862
Total 5853 748 1295
Average percent trapping incidence per month
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
Legend 0 Trap index
70 A Rainfall 1700
7 0 Temperature
I 60 t 60Ql
50-1500
B 0 I
4 _
300404
Xi~ -1 -C_ c - c shy10 10
20o
A S 0 N D J F M A M d J 21971 I97
of fedirplinrats ishyeFigure 6 M onthly ctche s
relation to rainfall and femparcdwe
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
39
reaching the peak in iily and August During the month of August
when the field lie fallow ratu can subsist on ratooning rice succulent
grasses and insects The lowest trapping incidence occurred in
January February and N rch after the Mlagd crop has been harvested
This period corresponds to the dry months in Siniloan Since rice is
a preferred habitat its absence during these months apparently increased
conpetition for food cover and other resources resulting inthe
dupletion of the rat population through increased movement or migration
into the surrounding w -telands A vigorous wat control campaign was
also maged during the dry months and this may have aided in the abrupt
reduction inthe size of the rat population In =lRI however Uhler
(1967) observed that population peaks occurred from June to August
dFebruary
dynamics of one rat species inthe different areas points to the role
of environmental influence on rat populations
and November t This marked dissimilarity inthe population
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
4o
LI TUE CITED
AIFONSO PJ and JC CASTILLO 1970 A contribution tQ the ecology of the ricefield rat (Mttus rattus mindanensis 1earns) in Mufoz Nueva Ecija 10 pp (Typewritei
and JL LIBAY 197R Rodents of theBARPEHENN KR JP SUFANGIL Philippine Croplands 33 pp (Typewritten)
BUXTON PA 1936 Breeding rates of domestic rats trapped in I~gos Nigeria and certain other countries Jour Anim Ecol 5 53-66
CHAIANE VH 1941 A trap-removal census of smli mamalsbull J Wildl mgat 5 42-67
CRUCILLO CV FQ (FYES and JL MORALES 1954 What We Ire Doing to control field rats in Cotabato Pbilippine Islands A syrup on Rat Control Div Biol Sci Nat Res Council Phil pp 25shy45
DAVIS DAVID E 1953b The characteristics of rat populations Quart Rev Biol 28 273-401
and JT EMIN 1948 The placental scar as a
measure of fertility in rats J Wildl Mgmt 12(3) 162-166 _
and FB GOLLEY 1963 Principles in Ivmmalogy Reinhold Book Crop NY 335 P
- and 0Hall 1948 Seasonal reproductive condition
of nale brown rats in Baltimore Maryland Physiol Zool 21 272-282
_ 1951 Seasonal reproductive condition of feale brown rats in Baltimore 1ryland Physiol Zool 2926
EMLEN JT and DE DAVIS4 1948 Determination of reproductive rates in Tat population by aamiaation of carcasaes Pbysio Zoo 21(1) 59-65
ECKE DH 1955 Analysis of populations of roof xats in southwest
Georgia US Public Health Monogr 27 1-RO
EMLEN JT AW STOKES and DE DAVIS 1949 Mqthpds forestimating populations of brown rats in urban habitats Ecology 30 430-442
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
41
GOMZ JUAN C 1960 Correlation of Population of roof ratsinVenezuela with seasonal cldnges in bitat Amer Mid- i 63(1) 177-193
HAU-ORIONJL 1951 Reproduction in Mts of th6 hubgenus 1EttusBroc Zool Soc London l21(3) 673-694
1952 Data on the reproduction of some mlAyanlmmls Proo Zoo Soc LU don 125(2) 4-146c
HAYNE DW 1949 Two methods for estimting population from trappingrecords J Yhmm 30 399-411
JACKSONJ WILLIAM B 1952 Poulation of wild mouse (Iroarseus leucous) subjected to the applications of DDT and parathionEcological Monographs 22 259-281 -
1952 Reproduction In Ihcific Islxnd tEcology T-I Storer (ed) Bernice P Bishop Ma-Bull 255 p 90-92
JOINSN DH 1962 Rodents and other Micronesian mnul1s col atctedIn Pacific Island M~t Ecology TI Storer (ed) Bernice P Bishop lMm Bull 225 p 21-38
iAU13 BMD 1946 Reproductlion of house ose ( ismusculus)living indifferent enviionmdnt Trans Royal Soc London
vol 133 (B) 248-281
IAVOE G FN S K and JP -UsINGIL 1970 Destructionof rice tillers by rats in relation to stages of rice develomentinLuion Fhil Agric 54(3-4)-175-8j
LESLIE PH UM VENABIES and LSV VENABIES 1952 The fertilityand population structure of the brown rat (M~ttus norvegicus) in9rn ricks and some other habitat Proc Zool So London 122(1) 187-238
RIANO WIAMA P 19(o A contribution tb the blology and bonto1of a rodent pest affecting coconut (Cocus nucifera Linn) Unpublished Undergraduate Thesis Univ Phil 36 pp
MlDINA F IS CL-WDRIACA BD]E IA CRUZ V poundAGUIGIAN r ADBEIAMNS ESLIT2 and To =~GORIO 197~2 A study on theada~ttion andreding of Xield rats (Rattus-rattu sp) in the laboratory MDCAECp- MUn Quezon City l7 -meograp d)
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
42
MLIER MILTON A 1946 Reproductive rates and cycels in the pocket gopher Jour Mmmal 27() 335-358
NELSON NM and HM EVANS 1955 Relation of Thiamine to reproshyduction in rat Jour Nuteition 55 151-163
NICHOLSON AJ and DW WAMER 1953 The rodents of 4ew Caledonia Jour Mlnm4l 34 168-179
ROSELL EON C 1959 StUdies on reproduction of the ricefielJ rat rattus argentiventer (Robinson amp Kloss) in Central Cotabato Unpublished MYsters Thesis Univ mhil 44 pp
SANCHEZ FF JP SUMINGIL FN SWINK G ATWELL A DE IA FAZ M FALL J LIBAY D SANCHEZ D TOLE INO R WEST and N KVERNO 1971 Annial Report Rodent Research Center College Lagun 80ppmimeo Unpublished
SARDMINT ARRV1953 Unpublished Annual REport to the Director of PlAntIndustry Bureau of Plant Industry Cotabato Cotabato pp
SOUTHWICK CH 1966 Reproduction mortality and growth of murid rodent populations Froc Indian Rodent Symp Calcutta Tndia pp 152-176
STICKEL LF 1948 The trap line as a measure of small mammal populations Jour Wildl Mgmt 12 -153-161
SUANuIL IJEsUS k yon Hanaooox on Tne ecopogy oT riceiield rats and their control by chemical means Pest and Dis Cont Div Bur Plant Ind Mnila 27 p (mimeographed)
0 1971 Burrows and breeding of ricefield rats in Cotabato Philippines Unpublished Masters Thesis Col State Univ Denver Colorado 38 p
SLMNGIL JP and A AQUINO 1958 Studies on rat reproduction in Namburao Occidental Mindoro Pest and Dis Cont Div Bureau of Platt Ind Yanila 14 p (-rpewitten)
and NC ROSELL 1959 Notes on the studies of Philippine species of field rats Sulamizam Bull 1(10) 41-51 (Reprint)
__ s LU SEQUI and AM MACANAS 196 Mchanism and effects of seasonal or cyclic reproduction on the outbreaks of ricefield rats in Cotabato Phil Jour Agric 28(3-4) 131-140
_xPJ ALFONSO FN SWINK and JL MORAIES 1970 More effective control of rats The Phil Recommends for Rice NFAC Quezon City 43 p (Brocahure)
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp
43
JP ~S~~SAG4 IAVOE14A~6DE Lk BkZ RR WESrDToIENTIN31J ~ n iIAYKUtRNO S~1HZ 1970
AWMZi Report Rbdent Reseearch Center College Laguna 56 pmilmeo Unpublished
TAYLOR EDARD H 1934 Philippine Land Mmlis Bui Sci MongrNO 30 Bureau of frmnting Yni3A 51 f9pp
TIGER JAMS R 197 Seasonal food habits ofR-ttusxa-ttus iindasshynensis (The Philippine Ricefleld Rt) in Central Luzon PhDThesis col State Univ Denver Colorado 66 p (Unpublished)
UHERm LD 1967 The reproduction of Rattus rattus mindanensis at I IRRI C611ege Laguna Philippines Phil AGric 51(7) 576-580
1967 The seasonal abundance of the Rattus rattus mindanensi at IRRI College Laguna Thilippines Phil Agric 51M7 5131586
VILIADOLID DV 1954 Some observations on the eoology 4nd biologyof rats in Cotabato A SyMp on Rat Cotrol Div Biol Sci Nat Res Council Phil pp 3-12
WIRTZ WIL M0 1970 Ecology of the JnesmmtRkt uexun on Kure AtollRaaii PhD Thesis Coll Unive Ann Arbor Michigan USA 132 pp