Am, Midi, Nat. 160:278-288

Diel Movement Behavior of the Stripe-necked Musk Turtle(Stemotherus minor peltifer) in Middle Tennessee

JOSHUA R. ENNEN' .AND A. FLOYD SCOTTCenter jorField Biology. Aus/in Peay State University, Clarksville, Tennessee 37044

ABSTRACT.—Few studies have used radiotelemetry to focus directly on the diel behavior ofturtles. We used radiotelemetry to relocate 10 Stripe-necked Musk Turtles {Stemotlierus minorpdtifiir) every 2 h over a 24 h cycle in Middle Tennessee, Movements during the 24 b cyclewere monitored on seven occasions between 27 Jul, and 4 Nov. 2004. Stem,otherus minor peltiferwere often inactive and remained sedentary throughout a 24 h cycle; 87% of the totalrelocations revealed no movement. Overall, S. m. peltifer'^ diel movement behavior wasrestricted to evening and nocturnal hours. Frequency data suggested a uni-modal distribudonof movement during evening and midnight periods (1700-0159 h). Similarly, the meandistance traveled showed a uni-modal distrihution with a peak in the evening period of a 24 hcycle. Based on the data set witli only light and dark categories, males were found to movemore frequently in daylight relative to females. A decrease in movement (frequency anddistance) occurred with the changing of the .seasons from summer to winter. Probably due totemperature change, nocturnal and crepuscular hehaviors were highest in Jul. and Aug.,while during the autumn months, there was little preference for daylight or darkness. Thisphenomenon has been reported in several Stemotherus species; but, to our knowledge, this isthe first report for S. minor.

INTRODUCTION

Chelonian movement patterns are strongly correlated to life history and ecology, and aresubdivided into two categories: spatial (intrapopulation and extrapopnlation) and temporal(diel, seasonal and sporadic) movements (Gibbons et al, 1990). A particular diel patterncould reflect an adaptation of timing activities to coincide with temperature regimes thatwonld increase efficiency of physiological processes and locomotion (Gourley, 1979). Inturdes, this is predominantly evident in dinrnal species, which bask to regulate most of theirphysiological processes. However, this does not explain nocturnal behaviors of other ttirde.species. The evolution of nocturnal behaviors could have been influenced by compeddonrather than by physiological demands. Within a particular community, different dielbehaviors may be the mechanism temporally .separating turtles and other organismsoccupying the same guild (Gourley, 1979; Park, 1940), thereby alleviadng interspecificcompetition pressures (Gourley, 1979). Three diel patterns appear to have evolved topromote this temporal separation within a community: diurnal, crepusculai" and nocturnal.

Stemotherus minor peltifer (Smith and Glass, 1947) is a highly aquatic turde restricted to thesoutheastern United States. Previous studies have reported all categories of diel behavior(ditirnal, nocturnal and creptiscular) in the genus Slemothenis (Smith and Iverson, 2004;Bancroft et ai, 1983; Graham and Hutchinson, 1979; Lagler, 1943; Mahmoud, 1969; Ernst,1986; Dodd el ai, 1988; Dodd, 1986). However, most of these .studies used trappingtechniques restilting in the collection of diel data without the aid of radiotelemetry.Radiotelemetry is a common tool used in studying movement patterns, home-range sizesand habitat use of freshwater turtles, but to our knowledge only two studies (Dodd et al.

Corresponding author present address: University of Southern Mississippi, Hattieshurg 39406; e-mail:jo,sh 1 ia.ennen@usm .edu

278

2008 ENNEN & SCOTT: MUSK TLIRTLE BEHAVIOR 279

Legend

• Wliileoak Creek

Tennessee River

FIG. 1,—Map of study site and the counties encompassing Whiteoak Creek in Middle Tennesseewhere radio-tracked Stemotherus minor pelifer in 2004. The star indicates location of study site

1988; Dodd, 1986) have applied radiotelemetry to collect movement data on species in thegenus Stemotherus.

We used radiotelemetry to investigate basic diel movement behavior of S. m. peltifer inwestern Middle Tennessee. Our objective of tbe study was to describe tbe bi-hourlymovement behavior within a 24 h period, Becau.se others have reported a variety of dielbehaviors for the genus Stemotherus, we hypothesize that there will be no .significantdifference in tnrtle movement behavior between darkness, daylight or crepuscular period.swhen con.sidering (1) frequency of movement, (2) distance traveled, (3) gender frequencyof movement and (4) gender distance traveled.

MATERIAI-S AND METHODS

Study .site.—The distribution of Stemotherus minor (Loggerhead Musk TurUe) is restricted toseven southeastern states: Florida, Georgia, Alabama, Mississippi, Tennessee, North Garolina,Virginia (Iverson, 1977; Ernst i-/a/., 1994; Conant and GoIIins, 1998). However, the subspecies,S. m. peltifer (Stripe-necked Musk Turtle), distribution i.s more restricted to the northern andwestern portions of the range (Ernst et al, 1994; Iverson, 1977; Conant and GoIIins, 1998),The S. m. peltifer population we sampled occurred in the Whiteoak Greek drainage of Houstonand Humphreys counties, Tennessee (Fig. 1), 120 km north of the nearest known populadonin Lauderdale County, Alabama (Mount, 1975). The site for this study (36'13'30"N,87'46'18"W) was located between Gander Branch and Grice Ford bridges on Wlñteoak Greek(Fig. 1), which flows for 42 km and drains parts of Dickson, Houston and Humphrey.scounties before emptying into Kentucky Lake (impounded lower reaches of the TennesseeRiver) at river mile 82. The area drained by Whiteoak Greek is located in the WesternHighland Rim ecoregion, which is part of the Interior Plateau (Amwine *•/ al, 2000).

280 T H E AMERICAN MIDLAND NATLÍRAI^IST 160(2)

TABLE 1.—Mass, number of days tracked, number of days inactive during tracking period, thepercentage of days that each indi\idtial was inactive, tlie longest distance traveled during a 2 h interval,lotal number of relocation points for each individual, the number of relocation points withoutmovement and the percentage of the total relocation points involving no movement for Stemotherusminoj peltifer smdied in Whiteoak Creek, Humphreys County, Tenne.ssce

11) (gender)

11-3 F1-11 F10-8 F6-0 F2-8 F2-1 MIl-IOM2 2 M8-8 M8-1 M

Mean

Mass (^)

146237162158169169110105162166

158.4

No. davstracked

7767747754

6.1

No, daysinactive

313241442I

2.5

% Daysinactive

42.914,35028.657.12557.157.14025

39.71

longestdistance

(m)

528351630333652617

30,3

No. relocations

87877487875387876553

76.7

No, relocationwithout

movemenl

83746473804274745644

66.4

% Non-movementrt'lotalion

95.485.186.583.99279.285.185.186.283

86,6

Telemetry study.—We hand-taught turtles during daylight hours (0700-1800 h) whilewading, snorkeling or canoeing. We fitted 10 individuals (5 males, 5 females) with radiotranstnitters (Wildlife Materials International model: SOPB-2190), and we monitored with atelemetry receiver (AVM Instruments Company, Ltd mode!; LA12-DS) and receivinganienna (AVM Instmments Company, Ltd model; M-Yagi). The particular mode! oftrati,smitter we tised was selected becaase of its compact size (33 mm X 13 mm X 8 mm),weight (4.6-5.0 g) and battery longevity (-248 d). In addition to tbe SOPB-2!90transmitter, we attached a Thermochron iButton® (Model DS 192K^F5) thermal sensorto the carapace of each study animal. We applied both the tbermal sensors ¡md uansmittersto the carapace with PC^uperepoxy® adhesive and allowed them to dry for at least 12 h.The total package weight (8 g) ranged from 3.2% to 7.6% of the mass of the individualturdes studied. We used these dime-sized (-17 mm diam X 6 mm height and -3.1 g)thermometers to record temperature (accurate to 0.1 G) and dme (±2 s per mo) and wereused concurrendy with another study.

The sample size was influenced by the natural density of this species in Wliiteoak Creek.Furthermore, the logistics of relocating individuals every otber hotir restricted the numberof ttirltes we could monitor. The limited number of days tracked was based on uansmitterfailure and the difficulty of capturing individuals to fit with a transmitter. We monitored sixindividuals for 7 d, one for 6 d, one for 5 d and two for 4 d (Table 1). This effort involvedobtaining radio fixes of each individual every 2 h over the 24 h cycle. Once we determinedthe position of a turde. the position was recorded with a Trimb!e^^' GPS receiver and aTiimble Survey Controller'^'^. Initially the purpose of this study was to determine U" there wasa movement preference for dark or light hours. If the turtle had moved, we categorized thetnovement as dark or ligbt by determining the amount (if light in a given 2 h intenal andthe time-of-day. If a 2 h interval had more than an hour of darkness or daylight, it wasconsidered a nocturnal or diurnal interval, respectively. After we analyzed the two categories(light and dark), we added a crepuscular categoiy to include 2 h intervals that encompassedperiods of dusk and dawn.

2008 ENNEN & SCOTT; MLÎSK TURTI.E BEHAVIOR 281

In the lab, we determined straight-line distance between relocation points using ESRIARGGIS 9 software and verified witli an online distance calculator. To further clarify the didbehavior, we tested for a dme-of-day preference for movement by grouping the time of eachrelocation fix into one of six categories; early morning (0200-0559 h), morning (0600-0959h), midday (1000-1359 b), afternoon (1400-1759 h), evening (1800-2L59 b) and midnight(2200-0159 h). On 27 Jul. and 12 Atig. no data were taken between 0700 and 0900 h; on 16Aug. and 2 Sept. no data were taken between 0600 and 0800 h.

Statistics.—^We used binar> logistic regression model (BLR) and general linear mode!(GLM) analyses taking into account (he repeated measures design ttsing an overall alpha of0.05. To assess the population's overall and gender-specific frequency of movement data, wetised two binary logistic regression models with the individual turtle nested witbin gender,gender and lighting period and an interaction term between gender and lighting period aspredictors. We used another BLR model with the individual turtle nested witbin gender,gender and dme-of-day as predictors and an interaction term between gender and time-of-day was run to determine the propensity to move at a certain time-of^lay. To determine ifmovement frequencies sbifted with time, we used two BLR models witb three predictors(turde ID, date and lighting period) and an interaction term between date atid liglilingperiod.

The non-normal nature of the distance data was due to the large number of times animalsexhibited no movement-s, tbus skewing the data. To account for tbese zeros and to lessenthe skevvness, we calculated mean distance of each individual turde within lighting intervals(light, dark, crepuscular) and time-of-<iay periods for a given date, and then we calculatedmean distances for each lighting interval or time-of-day. We used logiD (x + I)transformations to fit a normal distribution, and confirmed normality using a Ryan-Joinertest. To assess the overall or gender preference of turdes for a particular lighting period, wt-used two three-factor nested GLMs with turtle nested within gender as a random factor,gender and lighting period as fixed effects and an ititeraction term between gender andligliting period. We conducted another three-factor nested GLM with the individual tttrtlcnested within gender as a random factor, gender and time-of-day as fixed effects and aninteracdon term between gender and time-oiklay. Because of the Poisson distribution dtieto the numerous zeros, the data assessing the seasonality of the diel behavior (meandistance) could tiot be transfonned to fit a normal distribution. We tried reducing thenumber of dates from seven to three by combining Aug, with Sept. and Oct. with Nov. dates;however, this did not normalize the data. Altbottgh the data could not be fitted to a normaldistribution, we used logu) (x -I- Vi) transformation to reduce the severity of the non-normaldistribution and relied on the robustness of tbe GLM to assess the seasonality of the dielbehavior (mean distance). Two three-factor GLMs were used witb turtle as a random factor,date and lighting period as fixed etfects and an interaction term between date and ligbtingperiod. We used MINIT<\B® version 15 stadsdcal software for all stadsdcal analyses.

RESULTS

In total, we recorded 698 location points over the 5 mo sample period. Each turdeexperienced at least one day in wbich no movement was recorded (Table 1). These inactiveperiods varied among individuals and ranged from one to four sampling days. Whenmovement did occur within a 2 b interva!, the distance moved was highly variable andranged liom 5 to fil m (Table 1). We recorded 87% of relocation points witb no movement(Table 1). \\'hen movement occurred, 20% of individuals returned to a previously occupiedlocation in a 24 h period, displaying fidelity to a pardcular site within Uieir home range.

T

282 THE AMERICAN MIDLAND NATURAUST 160(2)

When analyzing only light and dark variables for categorizing a 2 h interval, turtles movedsignificantly more during darkness (Z = -2.57, P = 0.01). This significance was due toiemales moving much less during the day than at night whereas male movement was notaffected as seen in the interaction term (Z = 2.15, P = 0.034) (Fig. 2). Overall, turtles didnot move farther during daytime than night (F,, R, = 0.47, P = 0.511), and no gendereffects were found to be significantly dilferent regarding distance U aveled (F(i «, = 2.20, P^ 0.176) (Fig. 2). When a crepuscular category was added to the data, turtles had nopreferences for light, dark or crepuscular intervals when moving (Z = 0,89, P ^ 0.374; Z=-1.51, P = 0.132) (Fig. 2). Likewise, there was no gender bias with regard to frequency ofmovement during light, dark or crepusctilar intervals (Z - 0.20, P - 0.838). Overall, turtleshad no preference for any of the three lighfing categories (Fi^j«) ^ 0,40, P = 0.676), andthere was no gender bias in distance traveled (F,i,i6) = 0.61, P ^ 0.457) (Fig. 2).

Ttirtles moved significantly more often in the evening (Z = 2.66, P - 0.008) andmidnight (Z ^ 3.16, P = 0.002) periods (Fig. 3), but did not prefer the other periods.Again, this was biased by the ntimber of females moving in these periods relative to males asseen in only the one interacdon term (midnight X male) (Z ^ -2.22, P = 0.026), Likewise,mean distance differed for the time-of-day effect (F,5,4,), - 2.54, P = 0.043), but a Tukey'shonestly significant difference (HSD) test revealed that only the evening and middayperiods were significandy different (T = -3.132, P = 0,036). In botb BLR models (oneanalyzing dark and light and tbe otlier analyzing all three categories) used to determinefrequency of movement shifts with time, only the date predictor was found to be significant(Z - -5.12, P < 0.001 and Z = -2.69, P = 0.007) (Fig. 4). Congmently, the two GLMmodels used to assess the seasonality of mean distance moved bad a significant date factor(F((i.99) - 3.13, P = 0.007 and F,fi,i,„) = 3.57, P = 0.002) (Fig, 4). Creek temperaturessteadily declined from Jul. to Dec. (Fig 5).

DISCUSSION

Aithotigh our data for only light and dark categories show an inclination for nocturnalmovements by St^notherus minor peltifer, our data for the crepuscular category show nopreference for any lighting category. Consequently, we could not reject or accept our nullhypothesis that turtles would not have a preference for !ight, dark or crepuscttlar intervals.This ambiguity on preference was due to the categorization of the 2 h interval containingdawn and dusk periods as either being light or dark in the analysis with only two lightingcategories giving the categories more data points with movement. The overall preference formoving during the cover of darkness was solely due to less movement of females duringdaylight. However, the crepuscular data showed no gender bias vrith regard to fi eqttency ofmovement dtiring light, dark or crepuscular intervals, although it did appear that males hada tendency to move more often than females in the crepuscular and light periods. WiUiregard to mean distance traveled, both data sets found that mean distance moved in thepopulation was not dependent on gender or darkness {i.e., males and females had similarmean distances moved during light, dark, or crepuscular). Tbis was greatly influenced bythe sedentary nature of the species, which produced many relocation points with zeromovements. Consequently, we accepted the ntill hypothesis that no preference for any ofthe three lighting categories and gender exists in regards to distance traveled. However,overall males appear to move greater distances than females especially during light andcrepuscular intervals.

The Stemotherus minor peltifer population in Whiteoak Creek displayed disparate dielbehaviors when compared to previous studies within the genus. Ernst et ai (1994) reported

2008 ENNEN &: SHOTT: MUSK TLTRTIJ; BEHAVIOR 283

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Diel CategoriesFIG, 2,—Average percentage (bars) and mean distance (lines with SF, bars) of radio-tagged male (open

bars and solid lines) and female (shaded bars and dashed line) .Stemotherus minor peltijet moving duringlight, dark and crepuscular parts of the 24 h cycle based on checks made every 2 h on seven occasionsfrom Jul. to Nov. 2004 in Whiteoak Creek, Humphreys County, Tennessee

284 T H E AMERICAN MIDLAND 160(2)

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FK,. 3,—Average perctnitage (bars) and mean distance (lines with SL bars) traveled of radio-taggedStemotherus minor peltifer mo\ing during the six tinie-oÍKiay categories within a of 24 h cycle on sevenoccasions from Jul. to Nov. 2004 in Whiteoak Creek, Humphreys County, Tennessee

a crepuscular behavior for S. mi?iorwixh a tendency to be active in the morning. In contra.st.we found S. m. peltifei did not prefer morning hours, but was more active in evening andmidnight hours. .Stemotherus odoratus has been reported to be crepuscular with t!ieconcentration of movements more restricted to dawn and morning hours (Smith and¡versen, 2004; Ernst, 1986), btit others have reported bi-modal crepuscular beha\iorsencompassing a morning and evening activity peak (Qtiay, 1971; Mahmoud, 1969).Conversely, we found S. m. peltifer^ diel frequency of movement behavior to be uni-modalfor evening and nocttirnal hours, and mean distance traveled was found to be uni-modal ioronly evening hours. This ptopensity for a more nocturnal movement behavior has beenwidely reported in the genus Stemothems. For example, Ernst et ai (1994) reported somenocturnal movements for S. minar, andjack.st)n (1988) found S. m. peltifer. S. m. minor, .S.(idomlus and S. carinatus all occitpied crevices more dtiring the day thati at night, sttggestinga nocturnal bebavior. Others bave reported nocturnal movement behaviors in S. depressus(Dodd, 1986) and S. odoratus (Bancroft et al, 1983).

In our study, we fotind tbat diel behavior of Stmiotherus minor peltifer appears to undergo aseasonal sbift. In the data set witb only dark and light categories, there appeared to be a shift(though not statistically significant) from nocturnal movement during warmer parLs of theyear to no preference for either daylight or darkness in fall. Tbe months ofjul. and Aug. sawthe highest levels of nocturnal behavior (frequency and distance). During Sept., Oct. andNov. no preference for light or dark was apparent. Similar diel shifts due to seasonality bavebeen reported ¡or several Stemotherus species, but not for 5. minor. Both S. odoratus and .S,carinctlus exhibit crepusctilar behavior during Stimmer periods, but ditirnal behaviors duringthe winter (Mahmoud, 1969), and S. defxressus displays a variety of diel behaviors during theannual cycle by being diurnal when water was coo! and nocturnal wben it was warm (Dodd elai, 1988). The shift may not be photo-related but most likely is driven by a physio!ogica!restraint linked to a decrease in water temperature in tbe fall and venter months.

2008 ENNEN & SCOTT: Mt;sK TURTLE BEHAVIOR 285

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Sampling DateFit,. 4.—Average percentage (bars) and mean distance (lines with ,SK bars) of radio-tagged Stemotherus

minor pettifer moving dnring light (open bars and diamonds), dark (shaded bars and squares) andcrepuscular (stippled bars and triangles) parLs of 24 b cycle based on checks made every 2 h on sevenoccasions from Jul. to Nov. 2004 in Wbiteoak Creek, Humphreys County, Tennessee. An asteriskidentifies sampling episodes during wbicb data are lacking for a single 2 b interval

Temperature and not the variation in photoperiod affected diel movements in Chrysem.yspicta, Clemmysguttata and S. odoratus (Graham and Hutchison, 1979). This hypothesis seemsplausible for our population as our temperature sensors indicate stibstantial seasonaldifferences in creek water temperature. Althougb tbe seasonality distance data was non-

286 THE AMERICAN MiDtjuw NATURALIST 160(2)

Fit.. 5.—Mean monthly temperatures (with range bars) in degrees Celsius of Whiteoak Creek,Humpbreys County'. Tennessee during tbe study (Jul,-Dec. 2004)

normal, we are confident in tlie robustness of the analysis and the conclusions drawnbecause both the Bl.R and GLM analyses sbowed dale to be the only significant predictor/factor.

Stemotherus minorpellifer could have adapted a more noctttmal behavior to avoid prédationpressure from diurnal predators. In general, it has been suggested that nocturnalmovements of amphibians and repti!es are adaptive behaviors that reduce dittrnalprédation by birds (Zug et al, 2001). However, to otir knowledge, no literature exists thatreports avian prédation on S. minar. Ernst et ai (1994) listed only alligators and humans aspotendal predators for adult .V. minor, but reported herons and crows as predators of .S.odorcttus. Avian prédation has been documented in other turde species: S. odoralus andKinostemon bauri by Snail Kites {Roslrhamus sociabilis) (Beissinger, 1990), S. odoratus by Boat-tailed Grackles {Quiscalus Tnajor) (Bancroft et al, 1983), and Chrysemys picta bellii by Red-shouldered Hawks (Buteo lineatus) (Welch, 1987). Among amphibians, diurnal prédation,not necessarily avian, has heen suggested as a selecdon pressure leading to nocturnalmovement behaviors in frogs (Pechmann and Semlitsch, 1986) and salamanders (Semlitschand Pechinann, !985).

Gompetition for food resources might also infitience diel behavior. Temporal separationbetween two species with comparable diets may alleviate competition, thus causing thespecies to select for differeni die! beha\iors (Gourley, 1979). Tbe evening crepuscular andnocturnal nature of Slemothenis minor peltifer may be adaptive to minimize resourcecompetition with other species of turdes (e.g., Graptemys spp.) and other non-turtle species.Excltiding the peninsula of Florida and eastem Georgia, S. minor i^, sympatric with at leastone species of Graptemys in the megacephalic or "broad-headed" group throughout itsrange. Interestingly, adults in both S, minor and Cniplemys species in the "broad-beaded"group have a largely molluscivorous diet while smaller adults and juveniles feed on insects(S. minor—Folkerts, 1968; Tinkle, 1958; Graptemys—reviewed by Moll and Moll, 2004). While

2008 ENNEN & SCOTT: MUSK TURTLE BEHAVIOR 287

S. minor and several "broad-headed" Graptemys species have similar diets, they havedissimilar diel behaviors. The genus Graptemys is considered diurnal and is well known as abasking turtle (Ernst el ai, 1994). This dispatate diel behavior between Graptemys and .V. m.peltifei-could be tbe product of interspecific competition promoting temporal separadon.Altliottgh tbis temporal separation hypothesis has not been ftil!y examined betweenGraptemys spp. and S. minor, Lindeman (2000) has hypothesized tbat competitiveinteraction.s between molluscivorous turde species, including S. minor und "broad-headed"C^aptemys species allopatric to "narrow-headed" Graptemys species, as the mechanism forretaining the large head width.

In general, Stemotherus minor peltifer v/as very sedentary throttghout a 24 h cycle. Becauseseveral contradicdons between tbe two data sets occurred, we suggest that, overall, S. tn.peltifer preferred to move during evening and niidnigbt hours (1700-0159 h). Althoughturdes moved more frequently during the cover of darkness, this was solely influenced bythe female bias for movements in the dark relative to light. No overall diel behavioralpattern was supported for mean distance traveled during any ligbting period. However,males appear to move more frequently and farther distances than females during mostligbdng periods, but tbis was not statistically supported. The diel behavior bad a uni-modaldistribution in frequency of movement during the evening and midnight periods and a uni-modal distribution in mean distance traveled in the evening period. A shift also occurredwith the changing of the seasons in frequency and mean distance of movement. This wasprobably due to temperature change because most movement was highest in Jul. and Aug.and there was little movement dtiring the atitumn months. The evolutionaiy significance of.S\ m. peltifet's diel behavior is sti!! ambiguous without additional research.

Acknowli'dgments.—^We would like to thank the private landownci-s for allowing the ii.se of tbeir land fortbis project. Tbese individuals included Mr. John Cook, Mr. Raiidy Norfleet, Mr, Fred Payne. Ms, SusanGilmore and Mr. Tom John,son and family. For assistance in tbe field, we are indebted to Josb Maloney,[on Davenport and Nathan Parker. For GIS mapping, we tbank Andy Barrass, Micky Paiks and KentKirk. Foi-gtiidance in matters of .statistical analysis, we tbank Carol Baskauf, Jeff Bay and Carl Qtialls. Wetbank the following people for reviewing early versions of tbis mantiscript: Jake Scbaefer, Brian Kreiser,Danna Baxley, Nathan Parker and Ben Cash. Tbis research project was funded by Austin Peay StateUniversity's Center for Field Biology, and all tbe necessary permits and animal care policies of AustinPeay State Univenity were followed.

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288 THE AMERICAN MIDLAND NATURAUST 160(2)

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Ei) 9 NOVEMBER 2007 ACCEPTÏD 2 APRIL 2008