210 IRCFReptIles&AmphIbIAns•Vol17,no4•DeC2010

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further notes on Growth of Juvenile Timber Rattlesnakes in northeastern Kansas

GeorgeR.pisani1andhenrys.Fitch2

1Adjunctherpetologist,Kansasbiologicalsurvey,UniversityofKansas,lawrence,Kansas66047(gpisani@ku.edu)2(Deceasedseptember2009)FitchnaturalhistoryReservation,UniversityofKansas,lawrence,Kansas66044

Introduction

since April 2003, we have studied a small population of timberRattlesnakes(Crotalus horridus,Fig.1)residentatthefieldstationof

theUniversityofKansasenvironmentalReserves,considerablyexpand-ingonknowledgeofthespeciessummarizedinFitch(1999).ourearlierpapersinvolvedfewrecapturesoftimberRattlesnakesmarkedbyhsFfrom1990–2002,althoughonenotablerecapturewasofanadultmale(Fig.2)markedbyhsFin1978(Fitchandpisani2002).AlthougheasternKansasisthewesternlimitofthedistributionofCrotalus horridus,baseduponlifehistoryparametersforthispopulation(Fitchandpisani2006,pisaniandFitch2006),thespeciesinnortheasternKansasdoesnotappeartobeatanareaofclimaticlimitationasreviewedbymartin(2002).theareaisamosaicofwoodedlimestoneledgesandopenfieldsofmixedforbs

andgrasses,withanabundantpotentialpreybaseofsmallrodents(Fitchandpisani2006,pisaniandFitch2006). Fitch andpisani (2006)documented the expectedhigh correla-tionbetweenrattlebase-segmentwidthandsnout-to-ventlength(sVl)oftimberRattlesnakesinthispopulation.IntheirFigure-1,theypre-sentedapopulationgrowthcurve,inpartutilizingdatafromincompleterattlesplacedbyemployingamodificationofKlauber’s(1956)“treering”approach;prosandconsofthemethodologywerebrieflydiscussedbyFitch(2002).Useofacapture-processingprotocolthatattemptstominimizestresstothesnakeshasresultedinaconsiderablenumberofnewrecordsandrecaptures(foradiscussionoftheintimidation factorseebrown2008;wereferredtothisas investigator effect inFitchandpisani2006).Inthe54yearspriorto2003,hsFcollectedatotalof151timberRattlesnake

Fig. 1.FemaleCrotalus horridus,secondrecapturein14monthson11June2006,910mmsVl,506g,JeffersonCounty,Kansas.segments11+button.notetwocopperwiresonrattlesecuredtomarkstringatthiscaptureand22April2006.thisfemaleadded2segmentsand50mmsVlfrom15April2005–22April2006.shehadmoved~190meast,contained8embryos,andaddedonesegment(and30mmsVl)sinceApril2006.

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recordsfromthesesameproperties,whereasintheyears2003–2009,wehaveadded228additionalrecords.Inourpresentdatabaseof379records,123arerecaptures. W.h.martin(pers.comm.toGRp,october2009)suggestedthequalifiedapplicabilityofbase50GrowingDegreeDays(base50-GDD)toC. horridusgrowthcomparisonsthroughouttherangeofthespecies.heindicatedthat,asbase50-GDDisanagriculturalmetricandthereforeplainlynotbyitselfasoledeterminantofC. horridusactivity,amodelincor-poratingsuchdatashouldbeexplored.Correlationsexistinmanyareas,suggestingthattheabundantlocalbase50-GDDdataavailablethroughouttherangeofthespeciescouldbeusefulforapproximatingseasonalactivity. Growingdegreedayaccumulationsinvolvetheamountofaccumu-latedheatrequiredfororganismstoengageinnormalmetaboliclife-cycleevents(feedinganddigestion,courtship,etc).Calculationsofthebase50growingdegreedayfora24-hourperiodinvolvethefollowingformula:maxtemp+mintemp/2–basetemp(e.g.,50)=GDD.Foramoreexten-siveexample,see:www.wunderground.com/about/faq/degreedays.asp. thegoalsofthisstudyweretorevisitourconclusionsinFitchandpisani(2006)andtocomparethegrowthcorrelationsmentionedthereinwithsubsequentdatafromlonger-termrecapturesinthesamepopulation.

MethodsDatafromthe59records(largelyrecaptures)accumulatedsinceFitchandpisani(2006)allowamoreaccurateassessmentofgrowth,especiallythe

rapidgrowthshownbysnakesofbothsexesbearingrattlesofuptobutton+4segments.Wecombineddataonsnakeswithknowngrowthhisto-rieswithsimilardatacollectedsince1948(FitchArchiveDatabase,Kansasbiologicalsurvey).WethenplottedandcomparedgrowthratesofmaleandfemaleC. horridustoageneralizedmodelofpitvipergrowth(beaupre2002),aswellastoour2006papers. Whenpossible,weexaminedpreyandcomparedsizesofpreytakenwiththoseofsnakes.Wepalpatedfemalestodeterminethepresenceofenlargedovarianfollicles. WeexaminedthepotentialseasonalactivityofC. horridusinthissmallpopulationusingtheregionaldataforbase50-GDD(between1marchand19october,typicallythelimitsofadultC. horridusactivityatoursite),andusedrepeatedmeasuresAnoVAtocomparethattobase50-GDDoverequivalenttimeperiodsinseveraleasternportionsofthespecies’range,wheregrowthisfarmoregradual(table1).ourvaluesarefromnoAAairportdatabases(noAA—nationalClimaticDataCenter,www.airnav.com/airports/,lastaccessed31october2009)atthelocalitiesintable1.WeusedtheFAAIdentifiertoextractdatafromnoAAdatabasesthroughWeatherUnderground(www.wunderground.com.com/history/airport/,lastaccessed31october2009).selectingtheCustomtabandthenspecify-ingthedatesretrievedtheneededdata.base50-GDDdataexaminedwereforyears2003,2004,2008,and2009.Webeganourdetailedstudyofthesesnakesin2003,whichwascharacterizedbytypicalspringandfallweatherandanexceptionallywarmdrysummer.thefollowingyearwassomewhatcooler,withconsiderablymoreprecipitation(998mmin2004versus647mmin2003).years2008and2009werethemostrecentyearsofourongoingstudy.WeusedstatViewsoftware(AbacusConcepts1991)toconductstatisticaltestsandproducegraphicalrepresentations.

Results and DiscussionLocal Potential Activity Season.—Crotalus horridusatoursiteexperiencesasignificantly(RepeatedmeasuresAnoVA;p<0.01)longerpotentialactiv-ityseasonthanatfourcomparisonsites:twoeasternlocalitiesatapproxi-matelythesamelatitudeaslawrence,andtwofrommorenortherlylati-tudes(table2;Fig.3).AllpairedcomparisonsexceptmRbversusCKbdifferedsignificantly(p=0.05).Ingeneral,thebase50-GDDdecreasessignificantlyandclinallytothenortheast.WeemphasizeouragreementwithW.h.martin(pers.comm.,2009)thatthismetric,althoughusefulforgeographicallybroadcomparisons,isnotdefinitiveforsnakesthatrou-tinelythermoregulatebehaviorally(seediscussioninhuey1991). Feeding.—Fitch(1999)attributedthebulkofC. horriduspreybio-masstolargerrodentsorlagomorphs,althoughheacknowledgedthatonlythelargestsnakes(primarilymales)fedonsubadultCottontails(Sylvilagus)oradultsofgeneraof local largerrodents likeSigmodon, Neotoma,andSciurus.however,giventhesmallsamplesize(14snakes

Fig. 2.maleCrotalus horridus,firstrecaptureafter24yearson22may2002,1,200mmsVl,1,600g,JeffersonCounty,Kansas.segments10,withmanymissing.markedbyhsFon14october1978at995mmsVl,segments7+button(seeFitchandpisani2002).

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table 1.locationschosenforbase50-GDDcomparisons.

Faa airport name & location latitude/longitude elevation identifier (nad83 datum)

lWC lawrencemunicipal 39.0111111 833ft/253.9m

lawrence,Ks66044 -95.2164722 (surveyed)

mRb easternWVRegional/shepherdField 39.4019031 565ft/172m

martinsburg,WV25405 -77.9846686 (estimated)

CKb Clarksburgmunicipal 39.2966389 1217ft/370.9m

Clarksburg,WV26330 -80.2280833 (surveyed)

mGJ orangeCountyRegional 41.5099722 364ft/111m

montgomery,ny12549 -74.2646389 (estimated)

GFl FloydbennettmemorialAirport 43.3412222 328ft/100.0m

GlensFalls,ny12804 -73.6103056 (surveyed)

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ofvaryingsizesinFitch1999),biomassmaynotbethebestindicatorofpreysignificance. Althoughthenumberofidentifiablestomachsamplesinourmostrecentdatastillisnotlarge(28of379snakes[7.4%]examinedsince1948contained30identifiablepreyitems),ourdataindicatethatadultsofrodentgenerasuchasMicrotusandPeromyscusareadietarystapleofadultC. hor-ridusinthislocation.thisisconsistentwiththebroaderstudiesofClark(2002),amongothers.Weearlier(pisaniandFitch2006)indicatedthat

thestructuralcomplexityofthehabitatutilizedbyourC. horriduspopula-tionprobablycontributestoanabundantpreybase.suchcomplexityhasbeenreviewedforPeromyscus leucopusinadifferentlocalebyAndersonetal.(2003);theirhighestP. leucopusdensitieswereinsuchhabitat.ofthe30identifiablepreyitemsinourcumulativesamplethrough2009,21(70%)weremicrotinesorsoricids(Blarina, Cryptotis)foundin19recordswithsnakesVlrangeof370–1,100mm.the9“largeprey”(Sigmodon, Neotoma,andSciurus)werefoundinsnakes883–1,270mmsVl(6ofthe9withsVl>1,000mm).most(234of330)C. horridusrecordswehaveobtained(since1948)inthisareahavebeenofsnakes≤900mmsVl. Giventherelativelylongpotentialactivityseasonofourpopulation,theopportunityforneonatestofeedandassimilateamealpriortohiber-nationcannotbediscounted.norcanthisbeconclusivelyestablished,asjustoneofourneonates(female,370mmsVl)containedprey(Cryptotis).martin(2002)statedthatneonatesinhishighAlleghenyplateaupopula-tionofC. horridusdidnotfeeduntilthefollowingspring. Growth and Age at Maturity.—maleandfemalesizesascorrelatedwithrattlebasesegmentwidthchangeequivalentlywithage(slopesareinsignificantlydifferent,p=0.05;Fig.4).Averagesizesatvaryinggrowthstagesdifferfromour2006datawiththeadditionofthesenewrecords(table3).sVlsofsexesinthispopulationareneithersignificantlydiffer-entatthe“button-only”stage(Fig.5),noratsubsequentstagesofgrowth(table3,Fig.6).Comparisonofourtable3withsimilardatainmartin

Fig. 3.olddark-phasemaletimberRattlesnakefromWashingtonCounty,newyork(estimatedsVl1,200mm);therattlehas16segmentswithmanymissing(photographedinseptember2007).

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Fig. 4.maleandfemalesVlversusrattlebasesegmentwidthandregressionequa-tionsforCrotalus horridus,JeffersonCounty,Kansaswithknowngrowthhistories.

table 2.meanbase50-GDDbystationandpairedcomparisonsbetweenstations(lsD=leastsignificantDifference);*significantatp=0.05,**significantatp=0.01,ns=notsignificant.

Location Mean Standard Standard Base50-GDD Deviation Error

lWC 3684.25 233.9606 116.9803

mRb 3158.00 152.0855 76.0428

CKb 3026.50 169.3960 84.6980

mGJ 2579.25 58.6025 29.3012

GFl 2085.25 88.3794 44.1897

Paired Mean Fisher F-test Comparison Difference LSD test

mRbvs.mGJ 578.75 ** **

mRbvs.GFl 1072.75 ** **

mRbvs.lWC -526.25 ** **

mRbvs.CKb 131.50 ns ns

mGJvs.GFl 494.00 ** **

mGJvs.lWC -1105.00 ** **

mGJvs.CKb -447.25 ** *

GFlvs.lWC -1599.00 ** **

GFlvs.CKb -941.25 ** **

lWCvs.CKb 657.75 ** **

Fig. 5.sVls(mm)comparedforbutton-onlystagefemale(n=52)andmale(n=25)Crotalus horridus,JeffersonCounty,Kansas.

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(2002:table5)indicatesthatsnakesinthisKansaspopulationgenerallygrowfasterthroughrattlestages5+button(Fig.7).however,notallsnakesareabletoattainmaximalgrowthdespitegeneralizedpreyabundanceandagenerallyfavorablethermalregime.Fiveofoursnakeswereatstage1+button(alltakenindifferentyears),andfourofthosewerecapturedinJune–July;thefifth(female,495mmsVl)wasanoctobercaptureandlikelywascompletingherfirstfullseason. ourfewsnakeswithknowngrowthhistoriessincetheirbirthyearsconfirmtherapidpotentialgrowthofneonatesasreportedinpisaniandFitch(2006).thosedata,combinedwithournewrecordsforsnakesbear-ingcompleterattles,allowamoreaccuratepictureofrattlesegmentnum-berandgrowthinthispopulation(table3),revisingourresultsinFitchandpisani(2006:table3).Forexample,afemale(646mmsVlatfirstcapture)added72mmsVlandonerattlesegmentfromApril–september2003.Anotherfemale,bornfall2002andmarked(buttononly,403mmsVl)inApril2003,gained487mmsVlandhadadded9segments(aver-age54mmpersegment)whenkilledbyavehicleatthefieldstationinseptember2006.throughthattime,shehadaddedanaverageofthreerattlesegmentsperyearandanaverageannualsVlincreaseof162mm.Athirdfemale(605mmsVlinApril2006,segments3+button)hadgrownto810mmsVlbymay2009andaddedfivesegments(average41mmpersegment).thesethreefemalesaveragedanincreaseinsVlof56mm perrattlesegment.twootheryoungsnakescollectedinmay2009(bothat

rattlestage2+button)hadsVlsof550mm(male)and520mm(female);anApril2006femaleinthesame2+buttonclassmeasured560mmsVl. ourfoursmallestfemalesknowntocontainenlargedovarianfolliclesaveraged822mmsVl(range803–834mm).thesmallestofthese(fiveenlargedfollicles)hadeightrattlesegments,withanindeterminatenumbermissing.Anothermeasuring820mmsVlhada12-segmentrattlelackingjustthebutton.thefivefemaleshavingbothenlargedovarianfolliclesandcompleterattlesaveraged868mmsVl(range820–890mm)withrattlesaveraging7.6(+button)segments. Cumulatively,theseobservationsindicatethatfemalesinthispopula-tionmayreadilyaddthreesegmentsperyearthroughtheirthirdyearandwillhavethepotentialtomatethatyearandbearafirstlitterduringtheirfourthyear,consistentwithourearlierinference(Fitchandpisani2006).taylorandDenardo(2008)andbeaupre(2008)reviewedtherelationamongforagingsuccess,growth(increasedsVl),andreproductiveeffortin(respectively)Crotalus atroxandC. horridus.evaluatingourdatainlightofthesestudiessupportsourconclusionthatfemaleC. horridusinourpopula-tiongenerallyexperiencehighforagingsuccess. Intable4,wehaveaddedpooledbutton-only(indicatedsimplyas“b”)records(FitchArchiveDatabase,Kansasbiologicalsurvey)toourmostrecentrecordsofsnakeswithknowngrowthhistories.notunex-pectedly,thegreatpreponderanceofbrecordsareconcentratedinthefall(september–november).however,18.4%(9of49)femalebrecordsare

Fig. 7.RattleofyoungmaleCrotalus horridus(715mmsVl),DoR,JeffersonCounty,Kansas,8september2006.Rattlehas3+buttonsegments.

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table 3. RattlesegmentsandmeansVl(mm)formaleandfemaleCrotalus horridusinJeffersonCounty,Kansas.

Males Females Segments Mean (n; range; SE) Segments Mean (n; range; SE)

Button only 351.04 (25; 283–449; 9.58) Button only 367.23 (52; 298–470; 5.34)1 + button 492 (3; 464–530; 19.70) 1+ button 438.5 (2; 382–495; 56.5)2 + button 587.17 (12; 525–632; 10.46) 2+ button 569.7 (10; 520–610; 8.18)3 + button 643.75 (11; 570–712; 14.91) 3+ button 631.71 (7; 570–718; 19.19)4 + button 723.25 (8; 647–816; 19.43) 4+ button 706 (4; 504–807; 69.23)5 + button 769.25 (4; 658–845; 41.03) 5+ button 810 (2; 810; 0)6 + button 753.25 (4; 716–800; 17.99) 6+ button 789.0 (11; 644–900; 27.69)7 + button 894.75 (4; 820–999; 37.52) 7–11+ button 914.25 (4; 890–957; 14.82)8 + button 864.6 (5; 790–921; 21.18)9–15 + button 1,000.75 (4; 910–1170; 60.14)

Fig. 6.AbsenceofsignificantsexualsizedimorphisminCrotalus horridusfromJeffersonCounty,Kansas.Females:mean±se664.93±18.86mm(n=163;r=298–1065);males:mean±se791.42±20.41mm(n=167;r=283–1318).

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inApril,with10%(5of49)morerecordedinthemonthsofmay,June,andAugust.Aselaboratedabove,notallsnakesareabletoattainmaximalrateofgrowth.Although100%ofmalebrecords(n=19)arefall-only,wepresentlyfeelthatthisisanartifactofthegreaternumberoffemalesinoursample. Growth Rate and Models.—beaupre(2002)presentedageneralizedmodelofpitvipergrowth.Underlyingassumptionsofthemodelare:(1)large,infrequentmeals,and(2)abruptasymptoticreallocationofresourcesfromgrowthtoreproductioninfemales.themodelwasnotintendedtobeall-inclusive,anddidnotconsidersuchfactorsasanenergyvalueforfemaleannualmaintenanceactivities(i.e.,foragingtoaccumulatesufficientenergyreservesfordailyactivityorforsuccessfulhibernation).beaupre(2002)statedthat:“Itseemsclearthatfoodavailabilityandtemperatureinteractincomplexwaystoaffectgrowth.”

olderfemalesinourpopulation,aspredictedbybeaupre’s(2002)model,seemtoceasetheirrapidearlysVlincrease(table3),althoughecdysismayremainfrequent.oneexampleisan840-mmsVlfemale(rat-tlestage8+button,April2005)thathadaddedjust70mmsVlbyJune2006,butfourrattlesegments(average17.5mmpersegment). Usingourdatafrom2006–2009(pooledsexes)ofsnakeswithcom-plete(buttonpresent)rattlestringsorsnakeswithknowngrowthhistories,weplottedincreasesinsVlandwidthofbaserattlesegment(Figs.8&9).thebest-fitsecond-orderpolynomialregressionofthosedata(Fig.9)usingmeanvaluesateachrattlesegmentisfavorablycomparabletothe“treerings”plotofourdatainFitchandpisani(2006,Fig.8),givingusincreasedconfidenceinour2006conclusions.pooled(male+female)growthtrajectorydatafromoursnakes(Figs.8&9)seemtoustobemostsimilartothatofmalesinbeaupre’s(2002)model,withhighermeanForagingsuccessof0.050–0.075anda+4to+8°Cthermalregime.

table 4.DistributionofrattlesegmentclassesbysexandmonthofC. horridus,JeffersonCounty,Kansas.b=button.

April May June July August September October–November

Males3+b(n=5) 2+b(n=2) 1+b(n=1) 1+b(n=2) 2+b(n=1) b(n=7) b(n=12)

4+b(n=1) 3+b(n=1) 2+b(n=1) 3+b(n=1) 4+b(n=1) 2+b(n=3) 2+b(n=4)

5+b(n=1) 4+b(n=2) 3+b(n=2) 6+b(n=1) 8+b(n=1) 3+b(n=1) 3+b(n=1)

6+b(n=1) 5+b(n=1) 7+b(n=2) 7+b(n=1) 5+b(n=1) 4+b(n=4)

8+b(n=2) 10+b(n=1) 8+b(n=1) 6+b(n=1) 5+b(n=1)

12+b(n=1) 7+b(n=1) 6+b(n=1)

8+b(n=1)

9+b(n=1)

15+b(n=1)

Femalesb(n=9) b(n=3) b(n=1) 6+b(n=1) b(n=1) b(n=15) b(n=20)

2+b(n=2) 2+b(n=2) 1+b(n=1) 2+b(n=3) 5+b(n=1) 1+b(n=1)

3+b(n=1) 3+b(n=2) 3+b(n=1) 4+b(n=1) 7+b(n=1) 2+b(n=3)

6+b(n=3) 6+b(n=2) 4+b(n=1) 5+b(n=1) 9+b(n=1) 3+b(n=3)

8+b(n=1) 6+b(n=3) 6+b(n=1) 4+b(n=2)

11+b(n=1)

Fig. 9.Growthtrajectoryand2nd-orderpolynomialequationforCrotalus horridus,JeffersonCounty,Kansas(circlesaremeanbasesegmentwidthversussVl,paren-theticalnumbersaresamplesizes).

Fig. 8.meansand95%confidencebarsofrattlesegmentsfromCrotalus horridus,JeffersonCounty,Kansaswithknowngrowthhistories(samplesizesasinFig.4).

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ConclusionsourmostrecentdataconfirmthepatternofgrowthinthispopulationofC. horridusnotedinour2006papers(veryrapidgrowththroughthefourthpost-buttonshed;Fig.10).bythatstage,maleshaveattainedanaveragesVlof723mmandfemalesanaveragesVlof706mm.thiswouldseemtobearesultoftheabundantpreybaseinahabitatmosaic,combinedwithasignificantlylongeraveragepotentialgrowingseasonthanateasternlocalitiesofthesamelatitudeandsimilaraltitudes,oratmorenortherlylatitudes(Fig.11). RapidgrowthfacilitatesearlierfemalereproductioninourpopulationthanC. horridusshowsatpartsofitsrangecharacterizedbyshortergrowingseasons(brown1993,martin2002).Apotentiallylongeractivityseasonandarichresourcebasealsonodoubtfacilitatefemaleaccumulationofsuf-ficientenergyreservestoreproducemorefrequentlythaninlocalitiesmorelimitedinenergyand/orthermalresources. thegrowthcurvepresentedbyFitchandpisani(2006),althoughbasedlargelyonaveragingrattlesegmentdataandfittingsnakestopre-sumptivesize-ageclasses,accuratelycomparestothepresentmorerestrictedcalculationsusingonlysnakeswithknowngrowthhistoriesand/orcom-pleterattlestrings.Continuedstudieswillrefineapplicabilityofmathemati-calmodelstoobservedgrowthinthispopulation. Wewillcontinuetoexploreweathervariablesthatarecollectedbothlocallyandnationallyandthat,whencombinedwithbase50-GDD,willallowbetterapproximationofactualmicroclimatesavailabletoC. horridusinthispopulation.theabilitytousewidelyavailablehistoricalweatherdatarelevanttoC. horriduslifehistorywouldallowrefinementofmodelingthroughtherangeofthespecies.

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Fig. 10.maleCrotalus horridus(DouglasCounty,Kansas),August2006.estimatedsVl1,000mm,rattlehas6segmentswithmanymissing.

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Fig. 11.Femaleyellow-phaseCrotalus horridus(WashingtonCounty,newyork),september2007.estimatedsVl600mm,rattlehas7+buttonsegments.thissnakeisconsiderablyolderthanafemalefromJeffersonCounty,Kansasatacom-parablerattlestage.

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