Physiology and Behavior, Vol. 10, pp. 231-237. Brain Research Publications Inc., 1973. Printed in the U.S.A.

Social Isolation in the Gerbil: Its Effect on Exploratory or Agonistic Behavior

and Adrenocortical Activity'

JOHN SPENCER, 2, 3 JOYCE G R A Y AND A L L A N D A L H O U S E

Laboratories o f Developmen tal Psychobiology, Nebraska Psychiatric Institute University o f Nebraska College o f Medicine, Omaha, Nebraska 68105

(Received 10 March 1972)

SPENCER, J., J. GRAY AND A. DALHOUSE. Social &olation in the gerbil: its effect on exploratory or agonistic behavior and adrenocortieal activity. PHYSIOL. BEHAV. 10(2) 231-237, 1973 . -One hundred and eleven gerbils were either socially isolated from 28 -90 days of age or were raised in groups of 4 animals (2 males and 2 females). Ninety-two of these animals were placed in pairs in an open field and tested for 15 rain. All combinations of the same or different sex and rearing category were evaluated. Pairs of previously isolated gerbils approached and interacted with each other resulting in aggressive encounters while pairs of nonisolated gerbils interacted with novel objects and did not fight. An isolate paired with a nonisolate interacted with that animal but few aggressive responses were observed since the nonisolate was interacting with the inanimate objects. Body weights were significantly reduced while adrenal gland weights were significantly increased in female isolates but this latter measure did not correlate with number of approach interactions to the paired animal, fighting, activity or defecation. Stage in estrous cycle revealed a similar distribution for both aggressive and nonaggressive female animals. Levels of plasma corticosterone were significantly higher in paired isolates that fought than in paired nonisolates that did not fight or in isolates and nonisolates that did not receive open field testing. These results indicate that preferences for stimulus interactions are a direct result of previous rearing history. Adrenocortical processes may be modified by social isolation, but do not seem to be directly related to either emotionality or aggressive behavior in the gerbil.

Mongolian gerbil Aggressive behavior Adrenal gland Plasma corticosterone

Exploratory behavior Emotionality

SOCIAL isola t ion of Mongol ian gerbils (Meriones unguicul- a tus) at weaning p roduces a high inc idence of aggressive e n c o u n t e r s when these an imals are paired t oge the r at a d u l t h o o d [ 3 0 , 3 4 ] . F ight ing behav io r has also been re- po r t ed in the quai l [ 2 6 ] , t he dee rmouse [ 3 2 ] , t he rat [3 ,40 ] , and the mouse [ 6 - 1 0 , 19] fo l lowing per iods of social isolat ion. In o t h e r diverse species such as t he locust h o p p e r [ 2 0 ] , the ch icken [ 2 9 ] , t he guinea pig [25, the cat [ 17 ] , the dog [ 2 1 ] , and the m o n k e y [ 3 3 ] , early social i sola t ion induces changes in o t h e r behav ior p a t t e r n s such as preferences for s t imul i wi th which to in teract .

It has also been d e m o n s t r a t e d t ha t the act ivi ty of the ad renocor t i ca l sys tem in roden t s can be in f luenced by stresses such as shock, cold exposure , social i so la t ion or o the r var ia t ions in the dens i ty of g rouping [ I 0 ]. One index

of ad renocor t i ca l act ivi ty c o m m o n l y used is the weight of the adrenal gland. Adrena l weight may be modi f ied by several fac tors ; however , since the adrenal is unde r the in f luence of ACTH release f rom the p i tu i t a ry (which can cause b o t h an increase in adrenal size and in the level of g lucocor t ico ids) weight has been used as an index of adrenal func t ion . Changes in adrenal weight to stresses usually do not occur very rapidly; the re fore , adrenal weight has been used as an index of longer t e rm stresses whereas changes in levels of co r t i cos t e rone index more rapidly developing shor t - t e rm stresses.

It has been h y p o t h e s i z e d t ha t ad renocor t i ca l f u n c t i o n in roden t s may be re la ted to e m o t i o n a l i t y which is o f t en def ined as the a m o u n t of ac t iv i ty or de feca t ion in an open field test [1 ] . Earl ier s tudies have repor ted t ha t wild rats

1 This research was supported by NIH Grants No. 5 TO1-HD-00122, HD-O0370 and NS-06816. 2The authors wish to thank Dr. Larry Holcomb for providing the gerbils Dr. John Copenhaver for doing the corticosterone assays, and Dr.

Harold L. Williams for his advice on the preparation of this manuscript. a Present Mailing Address: Department of Behavioral Sciences, University of Oklahoma Health Sciences Center, 800 Northeast 13th Street,

Post Office Box 26901, Oklahoma City, Oklahoma 73190.

231

232 SPI{N(II!R, ( ; R A ' r <,\",1) I )AI, i lOUSt:

had heavier adrenals and were more emo t i ona l than domes t i ca t ed strains 131i , while male rats selectively bred f o r low emo t iona l i t y (i.e., loss de feca t ion in the open field) had smaller adrenals than male rats bred for high emot ion - ality [441. Nevertheless , a t t e m p t s to relate increases in adrenocor t i ca l act ivi ty, such as adrenal gland weight , wi th increases in emo t i ona l i t y in mice or rats have not estab- lished positive cor re la t ions be t w een these variables [ I , 6, 281. Levels of co r t i cos t e rone also have not shown system- atic re la t ionships wi th e i ther act ivi ty or a m o u n t ~f defeca- t ion in the open field [ 1,61.

The behavioral assessment of emo t iona l i t y should p robab ly inchide add i t iona l measures besides acl ivi ty or defecat ion. For example , it has been proposed [36] tha t aggression is one of several s u b c o m p o n e n t s of cmot iona l i ty . Thus, agonistic behavior and open field act ivi ty have been shown to be posi t ively cor re la ted in mice [6l wi th high aggression scores being associated wi th higher exp lo r a t i on and less freezing in an open field test.

Tl~ere have been n u m e r o u s studies concern ing the re la t ionship be tween aggression and various adrenocor t i ca l processes in the rat or mouse I I0 , 38, 40 ] . The s tudy descr ibed in the present paper provides compara t ive data on isolat ion induced aggression and its re la t ionship to adren- ocor t ical act ivi ty in pairs of Mongol ian gerbils. Fou r ma jo r ques t ions are examined : (1) Is there a re la t ionship be tween preferences for s t imulus in te rac t ions to an ima te (paired animal) or i nan ima te (novel objec ts ) and previous g roup rearing his tory , i.e.. isolat ion or non i so la t ion? t 2 t What is the role of sex or previous rearing ca tegory in inf luencing exp lo ra t ion or aggression wi th in pairs of isolates, noniso- lates or an isolate paired wi th a nonisola te? (3l What is the role of long- term stresses such as social isolat ion in modi fy ing adrenocor t i ca l act ivi ty (i.e., adrenal gland weight) and is this measure corre la ted wi th e i ther emo t ion - ali ty or aggression measures'? [41 What is the role of shor t - t e rm stresses such as open field tes t ing in modi fy ing adrenocor t i ca l act ivi ty (i.e., release of co r t i cos t e rone t in paired isolates or nonisolates'? A secondary goal of the present s tudy was to invest igate the repor ted reciprocal

re la t ionship be tween estrotts cycle and aggressi~m m Iemalc rodents. For example , female hamsters :He n luch le~s

aggressive in es t rous than when not in heat [ 2 4 1 Vaginal smears were t aken from all female aninqals so} HN to idcnt i iy stage of es t rous and to examine its d i s t r ibu t ion m those animals tha t were e i ther aggressive or nonaggressive.

M A T E R I A L S AND METt t ( ) I )

.I i~ima/s and ttousing ('o~zdili(u~s

O n e h u n d r e d and eleven gerbils, init ially bred in the d e p a r t m e n t of biology, Cre igh ton Universi ty, were weaned at 28 days of age. Using a split l i t ter t echn ique , all animals were e i ther placed in social isolation (one per cage) or raised in groups of 4 animals. In each group of nonisola tes , there were two males and two females. The equal pairing of males and females was to insure a balanced social develop- men t be tween all sexes. All animals were fed pellet chow and watered ad lib. The size of the plastic cages was 1 1/2 ft x l ft x 8 in. All of these cages were periodical ly cleaned. Lights in the room were left on from 8 :00 a.m. to 8 :00 p.m. Initially, there were 56 nonisola tes and 55 isolates. Two nonisola tes were discarded at tes t ing because they were pregnant . One isolate died dur ing deve lopment . Thus, the final sample used was 54 noniso la tes aml 54 isolates.

Testing Procedure

At 90 days of age, animals ( t aken from di f fe rent cages and l i t ters) were paired to provide all of the c o m b i n a t i o n s for similar or oppos i te sex and isolate and nonisola te rearing category. Table 1 i l lustrates the exper imen ta l pairing of gerbils t ha t were tes ted in the open field. For example, in the left hand co lumn, top row, a to ta l of 5 pairs of male gerbils previously raised in isolat ion were analyzed. The second co lumn represents 5 pairs of male gerbils not raised in isolat ion. In the th i rd co lumn, each of 4 isolate males was paired wi th a noniso la te male. This same p rocedure was used for the paired female groups (Row 21. There was also e i ther a male paired with a female or a

T A B L E 1

EXPERIMENTAL DESIGN t,OR PAIRED GERBILS TESTED IN TttE ()PEN F IHA) BASED ON SEX AND REARING CATEGORY

lsolales Nonisolates Isolate and Nonisotate a n d

No. of Pairs No. of Pairs No. of Pairs

Sex

Paired Males 5 5 4

Paired Females 5 5 4

Paired Male/Female 5 5 4

Paired Vemale/Male . . . . . 4

15 15 16

Total - 46 Pairs

SOCIAL I S O L A T I O N O F G E R B I L S 233

female paired w i th a male. B o t h of these animals were isolates ( C o l u m n 1) noniso la tes ( Co l um n 2) or one was an isolate and the o t h e r was a non i so la t e ( C o l u m n 3). F o r the last row of Co lum ns 1 and 2, male paired w i t h female or female wi th male c o m p l e m e n t e d each o t h e r for the pai red isolate and paired non iso la te groups.

The 92 animals (46 pairs) listed in Table 1 were tes ted in an open field c h a m b e r (3 f t x 3 ft x 3 f t ) marked off in 6 in. squares. In oppos i te corners of the o p e n field, 1 of 2 i nan ima te novel s t imul i was placed. These two objec ts consis ted of a ca rdboa rd box 3 in. square wrapped in a l u m i n u m foil and a w o o d e n spool of red thread . The two m e m b e r s of each assigned pair of animals were gent ly placed in oppos i t e corners of the o p e n field and observed for 15 rain. The n u m b e r of a p p r o a c h in t e rac t ions to b o t h novel s t imul i and the paired an imal was r ecorded for each animal. A single a p p r o a c h in t e r ac t i on was scored w h e n the an imal act ively in t e rac t ed (i.e., b i t ing, chewing and smell- ing) wi th the s t imulus objec t for 3 sec. An aggressive e n c o u n t e r was scored w h e n one (or b o t h ) animals s tood on the i r h indpaws and s lapped at the o the r animal. Vocal iza- t ions and chasing usually were t h e n observed. Genera l act ivi ty was assessed by coun t ing the n u m b e r of squares t raversed by each gerbil.

Odors f rom ur ine or s e b u m secre t ion f rom the sebaceous gland can in f luence behav io r p a t t e r n s in gerbils [ 3 0 , 3 9 ] . To con t ro l for these variables, a d i lu te so lu t ion of a lcohol fo l lowed by a warm soapy wate r and rinse was used to wipe the surface of the c h a m b e r and the s t imul i a f te r each test session. When the f loor of the test c h a m b e r was dry, ano the r pair of animals was tes ted.

Adrenal Gland Weight and Plasma Corticosterone Analysis

Fol lowing open field test ing, the an imals were re- tu rned to the i r respect ive cages. F i f t een m i n la ter all an imals were anes the t i zed w i th e the r for 1 min. The 15 rain

wait per iod was used because p lasma co r t i cos t e rone levels fol lowing stressful , novel s t imu la t i on such as open field test ing, peak and reach a s y m p t o t e at th is t ime [2 ] . Bo th adrena l glands were r emoved and wet weighed to the nearest 0.01 mg. Gland weight scores were t h e n made p r o p o r t i o n a l to the b o d y weight of the animal. In each of 32 animals (16 males, 16 females) pr ior to adrenal g land removal , 1 cc of b lood was t a k e n by cardiac p u n c t u r e and col lected in a hepar in ized tube . This p rocedure was d o n e in less t h a n 45 sec. All of these animals were r a n d o m l y chosen wi th in 4 groups: (a) eight isolate an imals previously paired toge the r in the open field; (b) eight non i so la te animals previously paired t oge the r in the open field; (c) eight unpa i red isolates, no open field exper ience; (d) eight unpa i red nonisola tes , no open field experience. The eight add i t iona l animals in G r o u p c were used to es tabl ish if long- te rm stresses such as isola t ion w i t h o u t open field exper ience would p roduce an e levat ion in co r t i cos t e rone levels. The b lood samples were f rozen for subsequen t analysis of co r t i cos t e rone using a f luo romet r i c t e c h n i q u e [23 ] . All samples were col lected b e t w e e n 3 :00 and 4 : 3 0 p.m. This per iod co r responds to the average crest of the 24 hr ad renocor t i ca l cycle in roden t s [ 1,22 ].

The vaginal smears were t aken f rom all female animals by d a m p e n e d c o t t o n t i p p e d t o o t h picks. The smears were placed on glass slides t ha t were s ta ined w i th h e m a t o x y l i n e and eosin for microscopic evaluat ion. Stage in the es t rous cycle was measured as fol lows: Proestrous-Presence of mos t ly nuclea ted cells. Estrous--Presence of mos t ly cor- nified n o n n u c l e a t e d cells. Metaestrous--Presence of mos t ly leukocytes .

RESULTS

BEHAVIORAL MEASURES

(.4) Pairs o f Isolates Compared to Pairs o f Nonisolates in the Open Field

T A B L E 2

COMPARISON OF PAIRED ISOLATE AND NONISOLATE GERBILS ON FOUR BEHAVIORAL MEASURES

Measure:

Paired Isolates

Pairs Pairs Male Paired with Female Pairs of Males of Females of Males

Paired Nonisolates

Pairs Male Paired with Female of Females

l. Approach Interaction a. Novel

Stimuli b. Paired

Animal

2. Fighting Behavior

3. Activity

4. Bolus Count

12.2 + 2.6 l l . 0 + 3.4 14,7 + 3.6 10.4 + 2.5 22.0t± 2.6

21.0"± 2.3 22.5*5 4.1 23,0*_+ 4.6 25.0*± 4.6 6.5 ± 1.4

22.5t± 3.2 25.5?± 3.5 26.0t± 2.4

5.5 + 2.2 7.2 ± 2.6 5.7 + 2.6

12.05+ 3.0 14.75+ 2.1 11.45+ 2.1 13.65± 2.4 1.0 ± 0.5 2.0 ± 1.6 2.4 ± 2.2 2.6 ± 3.2

325.0 +21.6 311.0 + 14.7 313.0 ± 18.4 324.0 +21.6 334.0 + 34.0 344.0 ± 38.6 357.0 +_22.4 331.0 ±21.6

4.0 ± 1.0 7.0 + 2.2 4.0 _+ 1.6 5.0 + 2.4 5.0 + 1.0 3.4 ± 0.2 4.0 _+ 1.6 7.2 * 2.8

*Significantly different from other stimuli, p<0.01 tSignificantly different from other stimuli, p<0.01 $ Significantly different from Nonisolate group, p<0.01 + Standard Error of Mean

234 SPENCI.IR. (.;RAY ANI) I ) : \ L I l O I Si a

TAB l t : 3

COMPARISON OF IS()I,ATE PAIRFI) WIIH NONIS(H, klE(',EP, BII,S ()N I;OUP, BEH'XVIOR MEASI!].,,]',

Measure: Male Male I cmalc I.emalc I emale Male Male t,'cmatc lsolale Nonisolate Isolate Nonisolatc Isolate N¢misolat~.~ lsolaic NotliSOklle

I. Approach Interaction a. Novel

Stimuli b. Paired

Animal

2. Fighting Behavior

3. Activity

4. Bolus Count

14.5 ~ 2.2 22.0+~ 3.1 16.2 + 3.1 20.()+t 3.1 15.5 + 3.0 28.0+~ 4.1 16.0 :t

30.0*+ 3.6 3.0 ~ 1.2 27.2*+ 2.8 6.4 ~ 1.4 25.0*+ 3.2 6.7 ~ 1.4 28.0"~ 4.6

3.6 25 .{l! ~

4.2

2.(1 * 1.2 3.5 + 1.6 2.(1 ~_ 0.5 4.(t + 1.5 1.5 .+ 0.8 1.5 ~ 0.8 2.{) ~ 1.2 3.2

321.0 ± 21.4 340.0 ~ 28.4 327.0 .e 18.6 339.0 +_21.4 324.() + 22.2 341.0 ~ 31.2 335.0 + 14.6 352.0

5.(1 + 1.4 3.0 + 1.2 4.0 :~ 1.5 6.(I + 2.1 3.0 ! 1.8 2.0 ! tL9 3.0 + 1.2 6.I1

2.1

1.4

1.6

38.2

I.g

*Significantly different from other stimuli, p<0.01 +Significantly different from other stimuli, p<0.0l

Standard Error of Mean

( l / Approach-interaction behavior. The number of anima]s used in this compar ison consisted of 60 animals (15 pairs of isolates and 15 pairs of nonisolates) . The analysis o f variance for this measure consisted of nesting all animals under groups and sex pairing across st imuli [43] . Be tween t rea tment compar isons were made by the Scheffe test. The only significant in te rac t ion was be tween groups and stimuli (F = 13.5, d f = 1,54, p<0 .01) . The mean number of approach in teract ions to ei ther the paired animal or the novel stimuli are presented in "Fable 2. The various combina t ions of sex pairings for the isolate and nonisola te groups are included. Reading across rows, no differences were observed be tween any of the three sex pairings (i.e., pairs of males, females or a male paired with female) for ei ther the paired isolate or nonisola te groups. Reading down columns, paired isolates irrespective of sex in terac ted with each o the r significantly more than wi th the novel stimuli. Pairs o f nonisolates in teracted significantly more with the novel st imuli than with their paired nonisolate

partner. (2) Fighting behavior. As Table 2 illustrates, pairs of

isolates fought significantly more than pairs of nonisolates (F = 24.% d f = 1,54, p<0 .01 t . No dif ferences were observed be tween the 3 separate sex pairings regarding mean number of agressive responses.

(21 Activity and bolus count. Neither of these mea- sures revealed any differences be tween ei ther groups or sex pairing, (F = 1.2 and 1.8, d f = 1,54, p>0 .10) .

(B) Isolate Paired With Nonisolate in the Open Field

( l) Approach interaction behavior. Sixteen isolates and sixteen nonisolates were used in this compar ison. As Table 3 illustrates, reading d o w n columns, for the approach in terac t ion measure, isolate animals approached and inter- acted with their nonisola te par tner regardless of sex while the nonisolate animals in terac ted wi th the novel stimuli. The groups by stimuli in teract ion was significant (F 12.90, dr= 1,24, p<O.01).

(2) Fighting behavior. Isolate gerbils paired with

nonisolate controls did not fight regardless of the sex pairing (F = 1.4, dr= 1.24, p > 0 . 1 0 ) .

(31 Activi ty and bolus count. Neither of these mea- sures revealed any differences be tween isolate or nonisolate animals (F = 2.6, 2.1, d f = 1,24, p> 0.10).

BIOLOGICAl, MEASURES

(,4 j Body and Adrenal Gland Weights and Relationship to Behavior

Body and relative adrenal gland weights are presented in Table 4. Female isolate gerbils when compared to male isolates or female and male nonisolates demons t ra ted a significant r educ t ion in body weight (F = 7.38, d / = 1,104, p<O.Ol) and a significant increase in adrenal weight(F = 7.12, d f = 1,104, p<0 .01) . Since a reduct ion in body weight in the female group might account for the observed increase in relative adrenal weight [401, an addit ional analysis was done for absolute adrenal gland weight.

'FABLE 4

MEAN BODY AND ADRENAl. WEIGHTS FOR ISOLATE AND NONISOLATE GERBILS

Body Weight Adrenal Weight (g) (rag1

Male Isolates 62.52 -~ 2.4 62,15 * 3.2

Female Isolates 51.94'± 3.1 76.22"~ 3.8

Male Nonisolates 61.46 + 2.8 63.42 -+ 1.2

Females Nonisolates 60.00 ~- 4.1 6(}.23 ~ 2.4

*Significantly different from all other groups, p<0.O1 Standard Error of Mean

SOCIAL ISOLATION OF GERBILS

TABLE 5

CORRELATIONS BETWEEN ADRENAL GLAND WEIGHT AND BEHAVIOR IN PAIRED ISOLATE GERBILS

235

Measure Female Isolates (N = 15)

1. 2. 3. 4. 5.

1. Adrenal Weight -0.12 0.25 0.09 0.31 2. Approach Interaction (Animal) 0.35 0.42 0.25 3. Fighting Encounters 0.06 -0.24 4. Activity 0.20 5. Defecation

Measure Male Isolates (N = 15)

1. 2. 3. 4. 5.

1. Adrenal Weight -0.07 0.03 -0.60* 0.02 2. Approach Interaction (Animal) -0.06 0.43 -0.09 3. Fighting Encounters -0.26 -0.29 4. Activity 0.34 5. Defecation

*p<0.01

Adrenal glands were still heavier in the female isolate group (F = 6.91, df 1,104, p<0 .01) . The o ther three groups, male isolates and male and female nonisolates were not signifi- cantly different for either body or adrenal gland weight measures.

Spearman rank correlat ions be tween adrenal gland weight in male and female gerbils that demonst ra ted aggressive responses and three addit ional behavioral mea- sures (approach, activity, defecat ion) are presented in Table 5. None of these correlat ions were significant except the negative correlat ion be tween adrenal weight and activity in the male isolate group.

(B) Plasma Corticosterone

In Figure 1, plasma cor t icos terone levels are presented for isolate and nonisolate gerbils. These plasma levels were significantly higher in paired isolates that had received prior open field s t imulat ion than paired nonisolates or unpaired isolates and nonisolates that had not received open field testing (F = 17.02, df = 3,28, p<0.01) . Each animal in the paired isolate group that was analyzed for cor t icos terone levels had demons t ra ted at least 10 aggressive responses within the 15 rain period of open field testing.

(C) Stage of Estrous For Agressive and Nonaggressive Gerbils

For ty % of the female animals that fought were in proestrous, 32% were in metaest rous and 28% were in estrous. Of those females that were paired with a male and fought, 2 were in estrous and 3 were not. The stage in estrous cycle for the remaining female animals that did not fight was distr ibuted as follows: 33% were in proestrous, 35% were in metaest rous and 32% were in estrous.

g

8 g 8

50

40 B

30 B

g 2 o -

10 - -

PAIRED PAIRED UNPAIRED ISOLATES CONTROLS ISOLATES

:1:

INPAIRED ONTROLS

FIG. 1. Mean plasma corticosterone levels for paired isolates (N = 8), and paired controls (N = 8), open field experience and unpaired isolates (N = 8) and unpaired controls (N = 8) no open field

experience. Vertical bars represent standard error of mean.

DISCUSSION

The results of the present s tudy demonst ra te that social isolation of mongol ian gerbils at weaning produces approach and interact ion and then fighting when these animals are paired together in an open field. Isolate gerbils interact much less f requent ly wi th inanimate novel objects than with their parners in the open field. Gerbils that have

23~ SPIiN(' t R. ( ;RAh' , \XI) IJAI.IItII SI

been raised in groups prefer to interact with novel stinruli rather than a paired animal and these animals do not fight. The variable of sex within pairs of gerbils had no systematic effect on approach interact ion or aggressive behavior. An earlier investigation [301 has reported a higher f requency of aggressive responses in paired female gerbils than in paired males: however, it is not clear what role learnhrg or habi tuat ion played since a round robin testing sequence was used and the reported sex differences were evident only afler several test trials.

The finding that nonisolate gerbils prefer to interact with novel, inanimate objects has also been reported for other species J4,171. For example, both rats and cats will approach and investigate unfamiliar objects. One of the salient aspects of explora tory behavior according to Berlyne I5] is inspective locomotor responses where the key stimulus properties consist of novelty and complexi ty . Differences in approach behavior between isolate and nonisolate gerbils may depend on these stimulus properties. The critical factor in predict ing the amount of explorat ion of an object seems to be the degree of novelty. The novelty of an object is thought to be inversely related to the amount of previous exposure to it. Since the nonisolate gerbils have been exposed to o ther animals of b~th sexes during developinent , it seeuls plausible ttmt tile choice of st imulus objects for them to explore would be tile novel inanimate stimuli. While both the inanimate objects and the paired animal are novel stimuli to the isolate gerbil, the more important cue that could produce preferences for stimulus interact ion may depend on complexi ty . The paired animal might represent a broader range of stimuli character- istics inchlding a visually moving target, which releases audi tory and olfactory signals.

It would appear from our results that mutual prefer- ences for st inmlus interact ion are primarily responsible for increasing or decreasing aggressive responses in gerbils. In the isolate paired with nonisolate group, little fighting was observed because the animals were not mutual ly responding to eacll other. Thus, one critical variable for the occurrence of agonistic behavior in gerbils seems to be reciprocal body interaction. Since this occurred only between paired iso- lates, previous rearing history is inrportant in determining if agonistic behavior is to occur. In agreelnent with our restilts, it has been reported that whereas pairs of isolate mice will fight, an isolate mouse paired with a nonisolate mouse will not [ 16 ]. Addit ional ly, there may be more than one componen t that signals aggressive behavior in rodents. For example, the present study demonst ra ted no significant relationship be tween number of approach interact ions to a paired isolate and number of fighting encounters {see Table 5). Thus approach interact ion behavior does not invariably cuhninate with fighting behavior. It seems probable that agonistic behavior, subsequent to approach, may depend on several cues, all of which may need to be present for initiation of fighting. Urine or defecat ion odors, marking behavior or o ther posture displays, defense of terr i tory and certain endogenous n~echanisms all [lave been postulated to regulate aggressive responses within pairs of rodents I10, lfl, 18, 30, 39 I. One endogenous factor that does not seem to influence f requency of aggression in female gerbils is stage of estrous. Female animals in this investigation fought as of ten regardless of which stage of estrous they were in.

The longer-term stress of social isolation produced an increase in adrenal gland weight in female gerbils. Similar findings have been reported in isolate female rats and mice

bred for e inot ional i ty I It), 12, if l, ] 1 5 , 4 4 l () thor iuvesl iga- tors have reported that adrena] weight is reduced I(>llowing social isolation 113,401. These conflicting results may haw been dtic to differences in rearing cage silo ¢~I itt il~ construct ion, different methods nf testing ¢,i ~u ihc H~ec:ies used for testing I 10 ]

The increase in adrenal gland weighi and reduclion in body weight in only female isolates is difficull to interrupt bccause several n-mchanisms lnay be involved. While earlier work 137, 4 l , 421 has suggested that isolation can act asa long-term general stress which is responsible for enhanced adrenocort ical activity, other investigators i l0 12. 141 have proposed that both adrenocort ical and pituitary gonadal systems may bc inw)lved. I-:emale mice have been reported to have had both heavier adrenals and heavier sex organs following social isolation [10I . This finding suggests that there may be all increase in circuhlting sex horlnone'~ e.g. estrogen during isolation that could involve various adrenocort ical processes. There is evidence tllat neonatal injections of estradiol can influence both adrenal gland size 1271 and reduce food intake /451. Poss ib ly tlle in',olvc- c e n t of certain hormonal mechanisms c a ) be partially responsible for the reported differences ~! body and adrenal weights in the female isolate gerbils

The finding that adrenal gland weigtit does not correlate with the behavioral measure of emot ional i ty in isolate gerbils is consistent with results from other rodent studies I1, (,, 10, 2Sl . Although general aggression also does not correlate with adrenal gland weight, separate sub-components of aggression namely, dominance, escape or defeat may deserve more careful analysis I10, 15, lt~].

The shorter stress of open field st imulat ion caused an increase in cor t icos terone levels in pairs of isolate gerbils that fought. Pairs of nonisolates or isolate animals that did not receive open field st imulation did not show similar elevations in plasma levels. Thus, previous social isolation and then stressful open field st imulation are both necessary to produce an increase in plasma cor t icosterone levels. It is not possible, however, in this s tudy to ascertain whether tile elevated plasma levels were due Io only tile open field experience, only the agonistic behavior or both of these factors since there are no data for isolate gerbils that received open field st imulai ion hilt did n<~t fight, e.g., isolates paired with nonisolalc,',.

File indication front lhc.se Iindings is that social isolation of gerbils at weaning influences both behavioral and adrenocort ical processes. Differences witllin pairs of gerbils on behavioral measures of explorat ion or aggression are not dependent on the sex of the animal but on the previous rearing category. Nonisolates explore one chlss of stimuli t inanimate) and do not fight while isohltes explore another class of stimuli (animate) and i1 mutual body interact ion occurs do fight. The only measure timt provided a clear sex different iat ion in the groups was adrenal gland weight and body weight. ~l though female isolates had heavier adrenals and reduced body weights, il is unclear in what ways sex steroid levels might also influence these findings. It is possible that tile ineasure of adrenocort ical activity may only partially provide an mdex of any stress induced by long-term social isolation II(IJ. It seems plausible that a series of complex interact ions occur between endogenous neuroendocr ine systenls, e.g., adren- ocortical or pituitary, gonadal and exogenous environ- mental manipulations, e.g., isolation tu then influence behavior patterns such as aggression.

SOCIAL ISOLATION OF GERBILS 237

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