Behavioural plasticity across social contexts is regulated by the directionality of inter-individual differences

Behavioural plasticity across social contexts is regulated by the directionality of inter-individual differences

Olivia L. Guayasamin

, Iain D. Couzin

, Noam Y. Miller

aDepartmentofEcologyandEvolutionaryBiology,106AGuyotHall,PrincetonUniversity,Princeton,NJ08544,USA

bDepartmentofCollectiveBehaviour,MaxPlanckInstituteforOrnithology,Konstanz,Germany

cChairofBiodiversityandCollectiveBehaviour,DepartmentofBiology,UniversityofKonstanz,Konstanz,Germany

dDepartmentofPsychology,WilfridLaurierUniversity,75UniversityAve.West,Waterloo,OntarioN2L3C5,Canada

Keywords:

Zebrafish

Collectivebehaviour Plasticity

Individualdifferences

a b s t r a c t

Anindividual’sbehaviouralphenotypeisacombinationofitsuniquebehaviouralpropensitiesandits responsivenesstoenvironmentalvariation,alsoknownasbehaviouralplasticity.Insocialspecies,we mustnotonlyexplorehowindividualsrespondtovariationsinthephysicalenvironmentbutalsohow theyreacttochangesintheirsocialenvironment.Agrowingbodyofworkhasdemonstratedthatthe behaviouralheterogeneityofagroupcanalteritsresponsiveness,decisionmaking,andfitness.Whether anindividualismoreorlessextremethanapartner–whatwetermits‘relativepersonality’–mayalso alterindividualbehaviouralresponses.Wedeterminedexploratorytendenciesofindividualzebrafish (Daniorerio)andthenconstructedpairswithvaryingdifferencesin‘relativepersonality’todetermine theeffectofdifferencesbetweenpartnersonbehaviouralplasticity.Wefindthatrelativepersonality,but notthemagnitudeofthedifferencebetweenpartners,isthemostimportantdeterminantofbehavioural plasticityacrosssocialtreatments.Despitethisoveralleffect,pairsoffishexhibitednopredictableleader- followerinteractions,suggestingthatdetailsoftheexperimentalparadigmmaybeimportantinshaping socialdynamics.

1. Introduction

Itiswellestablishedthatindividualsofmanyspeciesexhibit consistentindividualdifferencesinbehaviourandthatindividuals alsovaryinthedegreetowhichtheycanmodifytheirbehaviour inresponsetotheirphysicalenvironment(BiroandAdriaenssens, 2013;Brommer,2013;DingemanseandWolf,2013;Dingemanse etal.,2010a;McElreathetal.,2007;Nusseyetal.,2007;Sihand Bell, 2008; Sih and Del Giudice, 2012; Sih et al., 2012,2004a;

WolfandWeissing,2010),oftenreferred toas‘personality’ and

‘plasticity’,respectively.Muchoftheworktodateonanimalper- sonality hasutilized social speciesasmodel organisms,yethas beenrestrictedtostudyingthebehaviours ofisolatedindividu- alsacrossarange ofphysicalenvironments.Giventhatisolated individualsof socialspecies havebeenshown tobehavediffer- entlyfromtheirbehaviouringroupsettings(Aplinetal.,2014, 2013,2012; Magnhagen and Bunnefeld, 2009; Magnhagen and

∗Correspondingauthorat:DepartmentofPsychology,WilfridLaurierUniversity, 75UniversityAve.West,Waterloo,OntarioN2L3C5,Canada.

E-mailaddress:nmiller@wlu.ca(N.Y.Miller).

Staffan,2005;Magnhagen,2007;SchuettandDall,2009;vanOers etal.,2005;Websteretal.,2007)andthatindividualdifferences andsocialstructuremaybeexpectedtocoevolveinsocialspecies (Dingemanse and Wolf,2013;Dyer etal., 2008;Laskowski and Pruitt,2014;TannerandJackson,2012),itisimportanttoconsider inter-individualdifferencesofmembersofsocialspecieswithinand acrossdifferentsocialcontexts.

Currentmethodsofstudyingbehaviouralphenotypestypically involvecomparingtheresponsesofindividualstoarangeofphysi- calenvironments(Briffaetal.,2008;DosmannandMateo,2014;

Ordet al.,2010)and/or withinasingleenvironmentalgradient (BeckmannandBiro,2013;Dingemanseetal.,2010b;Kluenand Brommer,2013;Nusseyetal.,2007;Quinnetal.,2012;Teyssier etal.,2014),suchasdifferentlevelsofpredationrisk(Quinnetal., 2012).However,observing howindividuals respondtochanges in their physical environment is not sufficient for understand- ing the ecological and evolutionary significance of a particular behaviour in a social species. When the behavioural traits of individualsin groupsinfluencehowtheyperceive, process,and respondtotheirenvironment,thespecificcompositionofagroup mayhave fitnessconsequences for all orsome of itsmembers (Coteetal.,2008;DingemanseandWolf,2013;Dyeretal.,2008;

Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-0-381657 Erschienen in: Behavioural Processes ; 141 (2017), Pt 2. - S. 196-204

https://dx.doi.org/10.1016/j.beproc.2016.10.004

LaskowskiandPruitt,2014;WebsterandWard,2011).Forexam- ple, social context has the potential to substantially affect an individual’s fitnessby influencing the interaction betweenthat individualanditsphysicalenvironment(MagnhagenandStaffan, 2005;Magnhagen,2007;SchuettandDall,2009;Websteretal., 2007),eitherthroughinfluencinganindividual’sknowledgeofthe environmentbyprovidingaccesstosocialinformation(Aplinetal., 2012;BrownandLaland,2003;KruaseandRuxton,2002;Laland and Williams, 1997; Magnhagen and Staffan, 2003 Laland and Williams,1997;MagnhagenandStaffan,2003),orthroughalter- ation(byotherindividuals)oftheenvironmentitself(Laskowski andBell,2013;WattersandSih,2005).Theselectionpressuresthat acttomaintaininter-individualvariationmayalsodependoncol- lectivebehaviouralphenomena,suchthattheadaptivenessofany phenotypedependsontheensembleofphenotypesinaparticular group(DingemanseandWolf,2013,2010;WolfandKrause,2014;

Wolfetal.,2008,2007).

Agrowingnumberofstudieshavebeguntoaddresstheseeffects bycomparinganindividual’sasocialbehaviourtotheirbehaviourin asocialsetting(Aplinetal.,2014,2013,2012;Favreauetal.,2014;

Herbert-Readetal.,2013;Kurversetal.,2010,2009;Magnhagen and Bunnefeld,2009; Magnhagenand Staffan,2005;Nakayama et al., 2012;Webster et al., 2007).Individualsof many species tendtoconformtheirbehaviourtothatofpartnersorgroupmem- bers(PikeandLaland,2010)butindividualbehaviouraldifferences may still beexpressed tosomedegree under social conditions (Aplinet al.,2013;Herbert-Readet al.,2013;Kinget al.,2015;

Kurversetal.,2011,2009;LaskowskiandBell,2014;Magnhagen andBunnefeld,2009;MagnhagenandStaffan,2005;Nomakuchi etal.,2009;SchuettandDall,2009;vanOersetal.,2005).Forexam- ple,“shy”sticklebacks(Gasterosteusaculeatus)willbecomebolder when placed witha bolder partner,buttheirasocialbehaviour remainsasignificantpredictoroftheirsocialbehaviour(Jollesetal., 2014).Distinctbehaviouralphenotypesalsoappeartodemonstrate uniquepatternsofplasticity.Forexample,individualsthatarebold- est,mostexploratory,andmostaggressivewhenaloneshowthe smallestchangeinbehaviouruponbeingplacedinagroup,and viceversa(Coppensetal.,2010;Harcourtetal.,2009a,b;Herbert- Readetal.,2013;Hulthénetal.,2014;Kingetal.,2015;Koolhaas etal.,1999a,b;Kurversetal.,2011;Magnhagenand Bunnefeld, 2009;MagnhagenandStaffan,2005;Øverlietal.,2007;vanOers etal.,2005;WebsterandWard,2011).Similarconformityeffects havebeenfoundinstudiesofsocialbehaviourinhumangroups (Bikhchandanietal.,1998;BondandSmith,1996).

Giventhattheassessmentofinter-individualdifferencesisan inherently relative process − relying on comparisons between behaviouralmeasureswithinatestpopulation−itislikelythat the most informative measure of social effects on individual behaviour will bewhat we term the‘relative personality’:the difference in behaviour between the members of a group. For example,a “shy”individualmayrespondtoa “bold”individual bybehaving moreboldly(Jolles etal.,2014), butwhatwillthe behaviouralconsequencesbeifthis“shy”individualispairedwith anevenshyermemberofthepopulation,makingthe“shy”indi- vidualtherelativelybolderpartner?Furthermore,withingroups therearefrequentlychangingindividual-levelinteractionsamong behaviourally heterogeneous groupmembers(Bell and Stamps, 2004;Couzinetal.,2002;StampsandGroothuis,2010;Sumpter, 2006).Anindividual’s‘relativepersonality’willdependonthecom- positionof its current neighbors,and thesame individualmay behavedifferentlydependingonitsimmediatesocialcontext.Some researchershavebeguntoaddressthisquestionbytestingthesame individualinmorethanonesocialenvironment(Cornwallisand Birkhead,2008;Favatietal.,2014;Jollesetal.,2014;Kingetal., 2015;LaskowskiandBell,2013),findingthatsocialstatus,group

membership,personalitiesofpartners,andindividualtraitscanall influencebehaviouralplasticity.

Previous studies, however, have used randomlyconstructed groups,inwhichtherelativepersonalitycompositionofthegroup couldnotbeexperimentallymanipulated.Itislikelythatthepar- ticularmixofbehaviouralphenotypesinagroupwillhavealarge effectonmostaspectsofthegroup’sbehaviour,fromitscohesive- nesstothefitnessbenefitseachindividualgainsfrombeingpart ofit(e.g.,PruittandReichert,2011).Inspeciesthatformfission- fusiongroups–wheregroupcompositionchangesonquiteshort timescales(Croftetal.,2003)–behaviouralplasticitywillplaya largeroleindeterminingthesuccessofeach individualaswell asofthevariousgroupstheyparticipatein.Thus,beingableto experimentallymanipulatethesetofbehaviouralphenotypescom- prisingagroupwouldallowustoexplorethemechanismsbywhich behaviouraldifferencesbetweengroupmembersdrivethedynam- icsofcollectivebehaviour.

Using a well-known model organism, the zebrafish (Danio rerio), wesystematicallymanipulatedthe‘relative personalities’

(asdefinedabove)ofpairsofindividualstodeterminetheirinflu- enceontheplasticityofexploratorybehaviour.Pairsoffishwere testedinthesameenvironmentthatwasusedtoestablishindivid- ualexploratorytendencies,sothattheeffectsofchangingsocial contextcouldbeisolatedandquantified.

Werecordedzebrafish’sexploratorybehaviourwhentestedin isolationandwhentheywerewithapartner.Alltrialsmeasured behaviourusingthesameassay,anopenfieldwithashelterthathas beenwidelyusedtomeasureexploratoryandboldnesstendencies infish(Harcourtetal.,2010a,b,2009a,b;Ioannouetal.,2008;King etal.,2013;Maximinoetal.,2010b;Nakayamaetal.,2012).Each fishparticipatedintwopairtrialsandweassignedpairingssuch thateachsubjectwasthemoreexploratorypartner(ME)duringone conditionandthelessexploratorypartner(LE)duringtheother, basedonscoresfromtheirasocialtrials,andsystematicallyvaried themagnitudeofthedifferenceinexploratorytendencybetween thepartners.

2. Methods 2.1. Subjects

Subjectswere96(63F;33M)adultzebrafish(Daniorerio)of theWIKstrain,bredintheBurdinelab atPrincetonUniversity.

Toidentifyindividualfishforthedurationofthestudy,eachfish wasinjecteddorsallyattwoseparatelocationswithVisibleImplant Elastomertags(VIE;NorthwestMarineTechnologyInc.,Washing- ton,USA;WebsterandLaland,2009).Subjectsintheexperiment wererandomlyassignedto4groupsof24uniquelymarkedfish each.Eachgroupwashousedinasingletank.Fishwereallowedto recoverfromthetaggingprocedureforatleast4daysbeforeexper- imentsbegan(Doupeetal.,2003).Allprocedureswerereviewed andapprovedbythePrincetonUniversity,NJ,InstitutionalAnimal CareandUseCommittee(IACUC;ProtocolNumber:1890).

2.2. Housingandcare

Fishwerehousedinanenvironmentallycontrolledhigh-density housingrack(PentairAquaticHabitats,FL).Lightswereona12:12 cycle(light:dark);salinitywasheldbetween900and1200micro- Siemensandtheambienttemperaturewasmaintainedbetween20 and24^◦C.Fishwereacclimatedtothehousingtanksfor4weeks beforeexperimentsbegan.Onceexperimentsbegan,fishwerefed flakefood(TetraMinTropicalFlakes)adlib.dailyafterthecomple- tionofexperimentaltrials.

Fig.1.Testingenclosureandsampletrajectories.Thephotographisastillimagefromadatavideoshowingthetestingarenausedforbothasocialandsocialtrials,containing 4identicalenclosures,eachwithashelteralongoneedge(whiteareasintheimagecenter).Trajectoriesfromasamplesetofpairtrialsareoverlaidonthepicture.Eachcolor representstheentiretrajectoryofasinglefishoverthecourseofa10mintrial.

2.3. Experimentalapparatus

Thetestingapparatusforboththeasocialandpairtrialswas a60×45cmrectangularwhitePVCenclosure(Fig.1).Onenar- rowendoftheenclosurehelda7.6cmwideplasticoverhangjust abovethewatersurfacewithplasticaquariumplantsattachedto itsunderside,toserve asashelter, andalltheinterior wallsof theenclosurewerelinedwithtexturedtransparencies.Thisdesign wasinspiredbypriorworkwithzebrafishdemonstratingthatopen fieldtasksestablishaninternalconflictbetweenapreferencefor dark,protectedareasandadrivetoexplorenovelenvironments (Maximinoetal.,2010a,2010b;Serraetal.,1999;Stephensonetal., 2011).Alltrialswerefilmedwithanoverheadcamera(SonyEX1) at1920×1080pixelsandat30framespersecond.Pairtrialswere additionallyphotographedevery2s(NikonD7000DSLR)toaidin theaccurateidentificationofeachindividual.

Fouridenticalenclosures,asdescribedabove,wereplacedin a 210×120×15cm white acrylictank surroundedby floor-to- ceiling white curtains (Fig. 1).Water depthwas maintained at 7–8cmsothatthemovementofthefishwasmostlyconstrainedto 2D.Thetankwasfilledwith‘system’wateridenticaltothatusedin thehousingtanks.Whenfishwerenotpresentinthetestingtank, bubblersandfiltersmaintainedwaterquality.Thearenawaslitby fourfluorescenttubelightsandfourChauvetLEDPAR56-24UVB Blacklights(www.chauvetlighting.com)toenhancethevisibilityof theelastomertags.

2.4. Procedure

Eachfishfirstcompletedthreeasocialtrials,inwhichitwas aloneintheenclosure.Asocialtrialswerespaced48hapart.Fish

werenettedfromtheirhometanksandplacedintobeakerscon- taining300mLofsystemwater.Asinglefishwasgentlyplaced intoeachofthefourenclosures,allowingfourindividualtrialsto berunsimultaneously.After2minofacclimationtime,themove- mentofeachisolatedfishwasvideo-recordedfor9min.Allfish werethenassignedexploratoryscores(seebelow)onthebasisof whichpairingsweredetermined.24haftertheendofthelastaso- cialtrial,eachfishparticipatedintwopairtrials−inwhichtwo fishwereplacedintoeachenclosure–spaced24hapart,witha differentassignedpartnerforeachtrail.Pairtrialswereotherwise identicaltotheasocialtrials.Attheendoftheexperiment,allfish weresexed(estimatedfrombodydimensionsandcoloration)and measured(bodylength:distancefromsnouttotipoftailfin;body depth:distancefromfrontofdorsalfintobelly).

2.5. Determiningexploratorytendency

Individualexploratorytendencieswerequantifiedusingthetra- jectorydatafromallthreetrials,extractedfromthevideosusing customsoftwaredesignedin-house(Rosenthaletal.,2015).From thesetrajectoriesweextractedthefivefollowingbehaviours:total distancetravelledduringthetrial,mediandistancefromtheclosest enclosurewall,mediandistancefromtheshelter,totaltimespent outoftheshelter,andmediandurationofeach‘visit’totheshelter.

Weusedmedianvaluesfordistancesfromthewallsandshelterand forsheltervisitdurationsbecausethedistributionsofthesemea- sureswereallhighlyskewed.Allmeasureswerenormalizedby dividingallrecordedvaluesbythemaximalpossiblevalueofeach measure(andsoaredimensionless).Allrawmeasureswereentered intoaPrincipalComponentsAnalysis(PCA)usingMathematica(v.7,

WolframResearch)togiveuncorrelatedfinalscoresrepresenting theexploratorytendencyofeachfish.

Onlythefirstcomponent(PC1)resultingfromthePCAwasfound tobesignificantanditdescribed57.7%ofthevariance(Supplemen- tarymaterials,TableS.1).TherawmeasurementloadingsonPC1 confirmedourintuitionthatthiscomponentsuccessfullycaptured exploratorytendency(TableS.2; Dahlbometal.,2011a;Moretz etal.,2007;TomsandEchevarria,2014;Tomsetal.,2010).Foreach ofthethreeasocialtrialscompletedbyeachfish,wemultiplied allfiverawmeasurementsbytheeigenvectorofthisfirstprinci- palcomponent,resultinginthreeexplorationtendencyscoresfor eachfish(oneforeachasocialtrial).Thesethreescoreswerethen averagedtoarriveatasingleasocialexploratorytendencyscorefor eachfish,denotedA_iforindividuali.Thisprocedureforassigning exploratoryscoreswasestablishedandvalidatedwithaseparate populationofzebrafish,whosebehavioursyieldedalmostidentical PCAresults(seeAppendixAfordetails).

Toallowfordirectcomparisonsofbehaviouracrossbothsocial treatments,individualexploratorybehaviourduringthepairtrials wasmeasuredexactlyasin theasocialtrials,i.e., forthis anal- ysisthemovement of eachmember of thepairwasquantified independently, as ifit were alone. Ifindividual identitieswere unclearat anypoint inthe video,we combinedvisualanalysis ofthehigher-resolutionDSLRimageswithacustomMATLAB(v.

R2012B,Mathworks) scriptand manuallyassigned identitiesto eachtrajectoryfragment.Thesamefivemeasuresasabovewere extractedfromthetrajectoriesofeachmemberofthepairsepa- ratelyandthedataweretransformedusingthePCAdimensions determinedfortheasocialdata,ensuringthatindividual’sscores onpairtrialsaredirectlycomparabletotheirasocialscores.The firstprincipalcomponentoftheresultingscorewasassignedasthe new,social,exploratorytendencyscoreforthatindividualonthat trial,S_i.Everyindividualthereforereceivedtwonewexploratory scores(inadditiontotheirasocialscore),oneforeachoftheirtwo socialtreatments.

Forpairtrials,weadditionallyanalyzedthecoordinatedmove- mentofmembersofeachpairduringtheirexcursionsoutfromthe shelter.Wequantifiedhowofteneachpartnerinitiatedtheexcur- sion(lefttheshelterfirst)orreturnedtotheshelterfirst.Wealso measuredthemeandurationsofexcursionsandthemeandistance betweenthetwofish.Finally,wealsocountedanysoloexcursions

−whenasinglepartnerleftandreturnedtotheshelterwhilethe otherfishremainedundertheshelter.

2.6. Pairingfish

Eachindividualcompletedtwosocialtreatments:oneinwhich theywerethemoreexploratory(ME)partnerandonewhenthey werethelessexploratory(LE)partner,adeterminationbasedon theirasocialexploratorytendencyscores(A_i).Inaddition,wesys- tematicallyvariedthemagnitudeofthedifferenceinexploratory tendencybetweenthemembersofapair,ameasurewecallthe Intra-PairExploratoryDifference(IPED).TheorderoftheMEand LEsocialtreatmentswasrandomizedbetweenfish.Thethreeleast (most)exploratoryfishineachtankcompletedbothpairingswith more(less)exploratoryindividuals.

2.7. Measuringplasticity

Individualplasticitywasdefinedasthedifferencebetweenaso- cialandsocialexplorationscores(S_i−A_i),andwascalculatedfor eachindividualforboth itsMEandLEsocial trials.Thischange in behaviour is a known proxy for behavioural plasticity (Sih etal.,2004b).Anegativeplasticityvalueindicates thatanindi- vidualbecame lessexploratorywheninapairrelativetowhen alone,whileapositivevalueindicatesanincreaseinexploratory

behaviour.Mostimportantly,thetrajectoriesforeachmemberof thepairwereanalyzedindependently,asifthatfishwerealone.

In this way,pairtrial behaviourcouldbedirectly compared to behaviourduringthesolotrials.

2.8. Dataanalysis

AllstatisticalanalyseswereconductedinR(v.3.0.2.RDevel- opmentCoreTeam)usingthepsych,quantpsych,car,lme4,and ppcorpackages.Results withP<0.01 arereportedas significant duetocorrectingformultiplestatisticaltests(Bonferonnicorrec- tion␣=0.01).Wetestedforindividualbehaviouralchangewithin andacrosstheLEandMEsocialtreatmentsusingWilcoxonSigned Ranktests.Todetermineifthischangewassignificantlydifferent fromzero,Mann-WhitneyUtestswereused.Toseeifpartners behaviourallyconformedtoeachother,weusedWilcoxonSigned Rankteststocompareasocialexploratoryscoredifferences(A_i−A_j) betweenpartners,totheirdifferencesinexploratoryscoreduring thetwosocialtreatments(IPED=S_i−S_j).Wefurthercomparedthe changeinscoredifferencesbetweentheasocialandsocialtreat- ments[wedenotethisIPED=(S_i−S_j)−(A_i−A_j)].Weconstructed 100lists ofshuffled(randomized)pairingsofallfishand calcu- latedIPEDfortheserandompairs.Realandshuffleddistributions werecomparedusinga2-sampleKStest.Weexaminedindivid- ualplasticityfortheLEandMEsocialtreatmentsusingseparate linearmixedmodels(LMMs).Becausepairingswerewithinhome tankandoccurredovertwodays,wefittedhometanknumberand experimentdayasrandomeffects.ForbothmodelsweenteredA_i andIPEDascontinuousfixedeffects.ThefixedeffectsfortheLEfish modelhadavarianceinflationfactor(VIF)of4.039,andthefixed effectsfortheMEfishhadaVIFof1.059.Step-wisemodelselec- tionwasusedtodeterminethebestfittingmodels.Inallcaseswe presentthebestfittingmodelsasdeterminedbytheAkaikeInfor- mationCriterion(AIC)andF-tests,asthesignificanceofallterms areunchangedcompared tothefullmodels.Finally,correlation analysiswithKendall’s wasused todetermine whetherindi- vidualsshowedsimilarplasticityinexploratorytendencyacross theMEandLEsocialtreatments.Becausethisanalysisexplored intra-individualplasticityacrosssocialtreatments,onlyindividu- alsthatparticipatedinbothtreatments(MEandLE;N=72)were included.Todeterminewhetherthecorrelationinplasticityacross socialtreatmentscouldbeexplainedbyA_i,partialandsemi-partial correlationanalyseswereemployed(Aronetal.,2012).

3. Results

Therewasnoeffectofsex(t(94)=0.60,P=0.53),body length (r=0.03,t(94)=0.25,P=0.80),orbodydepth(r=0.08,t(94)=0.81, P=0.42)onasocialexploratorytendency.

3.1. Asocialexplorationisapoorpredictorofsocialbehaviour, butaccountsforcorrelatedplasticityacrosssocialtreatments

Asocial exploratory tendency (A_i) was a poor predictor of exploratory behaviour during the LE or ME social treatments (S_i). Though asocial exploratory tendency was correlated with explorationscoresduringtheLEtreatment(␶=0.268,P=0.008), it was not correlated with exploration scoresin theME treat- ment(␶=0.099,P=0.34).Therewasalsonocorrelationbetweenan individual’ssocialexplorationacrosssocialtreatments(␶=0.139, P=0.18).Inaddition,anindividual’sscoreduringpairtrials(inboth theLEand MEconditions)wasnotpredictedbytheirpartner’s asocialexplorationscore(allAdj.R²<0.10,allp>0.10).

Plasticity was defined as the change in an individual’s exploratorybehaviourbetweentheasocialandsocial trials(i.e.,

Fig.2.Individualplasticitycorrelatesacrosssocialtreatmentsanddependsonaso- cialexploratoryscore.Thefigureshowsindividuals’plasticity(changeinexploratory behaviourfromasocialtosocialtreatment)whentheywerethemoreexploratory (ME:x-axis)andlessexploratory(LE:y-axis)partner.Thethickblacklineisthe best-fitlinearmodelofthedata(R²=0.09,F=7.3,P=0.009,␤=0.346[95%CI=0.09, 0.60]).Thecolorofeachdotrepresentsthatindividual’sasocialexploratoryscore (Ai;seecolorlegendatright).

S_i−A_i)andcalculatedseparatelyfortrialsinwhichthefocalindi- vidual was the more (ME) or less (LE) exploratory partner. A bivariatecorrelationconductedwithKendall’srevealedasignif- icantcorrelationinindividualplasticitybetweentheLEandME socialtreatments(Fig.2),suchthatindividualsthatdemonstrateda largeincreaseinexploratorybehaviourduringonesocialtreatment tendedtoalsodosointheother(␶=0.245,P=0.007).However, there were also significant bivariate correlations between aso- cial exploratoryscore and individual plasticityfor both the LE (␶=−0.302,P=0.004) and ME (␶=−0.268,P=0.007) conditions.

Thus,todeterminewhetherasocialexploratoryscore(A_i)could account for this relationship, we used partial correlation anal- ysisand re-calculated thecorrelation in plasticitybetweenthe LEandMEconditionswhilecontrollingfortheeffectsofasocial exploratoryscore.Thiscausedtheobservedcorrelationinplastic- itybetweentheLEandMEtreatmentstolosesignificance(partial

␶=0.178,P=0.03), demonstrating that an individual’s plasticity acrossdifferentsocialtreatmentsis partiallyexplainedbytheir asocialexploratorytendency(A_i),suchthatlessexploratoryindi- vidualsaremoreplastic.

3.2. Individualplasticityislargelydeterminedbyrelativesocial condition,andleadstobehaviouralconvergencebetweenpartners

To determine whether partners became more alike in exploratorybehaviourduringtheirinteraction,wecomparedthe differenceinpartners’asocialscorestotheirIPED,thedifference betweentheirscoresduringthesocialtrials(Fig.3A).Thediffer- encebetweenpartners’scoresdecreasedfromasocialtosocialtrials (medianasocialdifference=0.307;medianIPED=0.045;Wilcoxon signed-rank test, W=3669, P<0.001). This change in distance betweenindividualscoresofpartnersfromtheasocialtothesocial condition, which we denoteIPED, wassignificantly negative, indicatingthatpartners’exploratorybehaviourconverged(Fig.3B;

median change=−0.38;KStest, compared to randomizeddata, D=0.26,P<0.00001).Theconvergenceinpartners’behaviourwas largely drivenby increasedexploration by theless exploratory (LE) individual.In nearly halfof all social trials(45 of 96),the individualthatwasidentifiedaslessexploratory(LE)–basedon asocialscores(A_i)–exhibited moreexploratorybehaviourdur- ing thesocial trials than themore exploratory (ME)individual (Fig.3A,thepartofthesocialdistributionthatis<0).Furthermore, during theLE social treatment, individuals displayed increased exploratorybehaviour(comparedtotheirexploratorybehaviour

in isolation),but this effectwasnot seen duringtheME social treatment(Fig.3C).Whenindividualswerelativelylessexploratory thantheirpartner(LEsocialtreatment),theincreaseinexploratory behaviourwassignificantlygreaterthanzero(U=3497,P<0.001;

medianchangeinscore=0.299).However,whenindividualswere themoreexploratorypartner(MEsocialtreatment),theydidnot exhibitsignificantplasticity(U=2315,P=0.96;medianchangein score=−0.075).Comparingplasticityacrosssocialtreatmentscon- firmedthatindividualsexpressedsignificantlydifferentpatterns ofplasticitydependingonwhethertheyweretherelativelyless exploratoryormoreexploratorymemberofapair(Fig.3C;KStest, D=0.25,P=0.005).

3.3. Asocialexploratorytendenciesdonotpredictthe

coordinationofmovementbetweenindividualsduringpairtrials

Themoreexploratory(ME)individualwasnotmorelikelytoini- tiateexcursionsfromtheshelter(t(190)=0.176,P=0.392),bethe firsttoreturntotheshelter(t(190)=0.011,P=0.398),orperform moresoloexcursionsoutfromtheshelter(t(190)=0.009,P=0.398).

Higherasocialscores(A_i)werealsonotsignificantlycorrelatedwith initiatingexcursions(r=0.137,t(94)=1.336,P=0.092),beingthe firsttoreturntotheshelter(r=0.021,t(94)=0.203,P=0.420),or performingmoresoloexcursions(r=0.124,t(94)=1.216,P=0.114).

Thedifferenceinasocialscoresbetweenpartners(IPED)didnot correlate withmeanexcursion duration (r=0.180,t(94)=1.771, P=0.080)orthemeandistancebetweenthefish(r<−8.6×10⁻¹⁸, t(94)<−8.4×10⁻¹⁷,P=1).Insummary,wedidnotfindevidence thatdifferencesinasocialexploratorytendencydirectlydetermine thecoordinationofmovementbetweenpartners.

3.4. Asocialexploratorytendencypredictsthemagnitudeand directionofbehaviouralchange

Whilerelativeexploratorytendency–whetheranindividual was more or less exploratory – determined whether individ- uals exhibited significantplasticity during the social trials, we attemptedtounderstandwhatfactorspredictedindividualplas- ticityinexploratorybehaviourwithineachsocialtreatment.

For both social treatments, the best fitting linear model of exploratory score had only one significant predictor variable:

asocialexploratoryscore(A_i;Table1,bottom).Individualswith the lowest asocial exploratory scores demonstrated the great- estplasticity:alargeincrease inexploratoryscorefromasocial tosocialtrials. However,thebestfittingmodeldidnot include themagnitudeofinter-individualdifferencesinexploratoryten- dencybetweenpartners asapredictorvariable.In otherwords, inourdata,theprimarydeterminantofplasticitywasindividual exploratoryscores.Themagnitudeofthedifferencesinexploratory tendencybetweenpartnersdidnotsignificantlypredictplasticity.

4. Discussion

Weexaminedtheexploratorybehaviourofzebrafishbothalone and in pairs specifically constructed to explore the effects of differencesinexploratorytendencies.By measuringexploratory behaviourintheexactsameenvironmentduringboththeasocial andsocial trials,(i.e.,byanalyzingindividualmovementduring pairtrialsasifthefocalfishwerealone),wewereabletoisolate andquantifytheeffectsofchangingsocialtreatmentonindividual behaviour.Wefoundthatthedegreeofplasticityinexploratory behaviouranindividualdisplaysisprimarilydeterminedbytheir relativeexploratorytendency.Individualsonlydisplayedsignifi- cantplasticity–defined asthechangein theirbehaviourfrom theasocialtothe socialtreatments – whentheywere theless

Fig.3.Distributionsofdifferencesinpartnerexploratorybehaviourandplasticity.A.Densitydistributionsofdifferences(betweenpairedindividuals)inasocial(blue)and social(red)exploratoryscores(IPED).Onlypositiveasocialvaluesareshown(i.e.,foreachpair,thescoreforthelessexploratory(LE)individualissubtractedfromthatof themoreexploratory(ME)individual),andthesamecomparisonsareusedforthesocialdistribution.B.Thechangeindifferencesbetweenpartnersfromtheasocialto thesocialtrials(IPED).ThefigureshowsdensitydistributionsofIPEDforreal(orange)andrandomized(grey)pairings;seetextfordetails.C.Densitydistributionsof plasticity(changeinexploratorytendencybetweentheasocialandsocialtreatments;S_i−A_i)whenindividualswerethemoreexploratory(ME;black)andlessexploratory (LE;green)partner.Seetextforstatisticalcomparisons.AlldistributionswerecreatedusingtheSmoothHistogramfunctioninMathematica(v.9,WolframResearch).

Table1

Multipleregressionanalysisofindividualplasticityforbothsocialtreatments.Differentmodelswererunforeachtreatment(MEandLE).Inbothtreatments,individuals withlowerasocialexplorationscores(A_i)demonstratedgreaterplasticityinthedirectionofincreasingexploratorybehaviour.

B SEB ␤ t P

Moreexploratorytreatment(Adj.R²=0.23,vs.nullmodelF(2,93)=15.26,P<0.001)

Constant 0.145 0.111 1.305 0.195

A_i −0.740 0.165 −0.416 −4.497 <0.001

Lessexploratorytreatment(Adj.R²=0.50,vs.nullmodelF(1,94)=97.82,P<0.001)

Constant 0.308 0.083 3.72 <0.001

A_i −0.911 0.092 −0.714 −9.89 <0.001

Note:N=96.

exploratorypartner(LEtreatment).Thisplasticitywasinthedirec- tionofincreasingexploratorybehaviour,suchthatinalmosthalf ofourpairings,thelessexploratory(LE)individualsactuallydis- playedstrongerexploratorytendenciesduringthepairtrialsthan theirmoreexploratory(ME)conditionpartners.Withineachsocial treatment,thebestpredictorofplasticitywasanindividual’saso- cialexploratorytendency(A_i),withless exploratoryindividuals exhibitinggreaterplasticity,inlinewithpredictionsfromtheliter- ature(Coppensetal.,2010;Harcourtetal.,2009a,b;Herbert-Read etal.,2013;Hulthénetal.,2014;Jollesetal.,2015,2014;Kingetal., 2015;Koolhaasetal.,1999a,b;Kurversetal.,2011;Magnhagenand Bunnefeld,2009;MagnhagenandStaffan,2005;Øverlietal.,2007;

Ruiz-Gomezetal.,2008;Sneddon,2003;vanOersetal.,2005;Ward etal.,2004;WebsterandWard,2011).

Theincreaseinexploratorybehaviourbythelessexploratory individuals resultedinaconvergence ofexploratorybehaviours betweenpartnersduringthepairtrials,aneffectthatisinlinewith the‘conformityhypothesis’(Kingetal.,2015;Magnhagen,2007;

WebsterandWard,2011).Resultsfrompreviousworksuggesta mechanismforthisconformityeffect:lessexploratoryindividuals areknowntobemoreresponsivetotheirsocialenvironmentand topaymoreattentiontosocialcues(Coppensetal.,2010;Harcourt etal.,2009a,b;Jollesetal.,2015,2014;Koolhaas,2008;Koolhaas etal.,1999a;Kurversetal.,2010;MagnhagenandBunnefeld,2009;

MagnhagenandStaffan,2005;Ruiz-Gomezetal.,2008;Sneddon, 2003;vanOersetal.,2005;Wardetal.,2004).Bycontrast,more exploratoryindividualsdonotattendorreactasstronglytotheir socialsituation.Conformityeffectsarethereforetheresultofless exploratoryindividualsadaptingtheirbehaviourtomatchthatof theirmoreexploratorypartners.

Althoughtheleastexploratoryindividualswerethemostplastic acrosssocialtreatments,ouranalysiscontradictsthishypothesized mechanismforconformity.Wefindthattheamountofincreased exploration displayed by the less exploratory partner − their

‘adjustment’tothebehaviouroftheirmoreexploratorypartner− doesnotdependonthemagnitudeofthedifferenceinexploratory

tendency betweenthepartners. In ourdata,the appearanceof aconformity effectwasdrivenbytheleastexploratoryindivid- ualsshowingthelargestresponsetobeinginasocial situation, but themagnitude ofthis responsewasnotdeterminedbythe exploratorytendencyoftheirpartner.Ifconformity werebeing drivenbythelessexploratoryindividualinapairattemptingto matchthebehaviourofthemoreexploratorypartner,wewould have expected to seegreater adjustments where the disparity betweenthepartnerswasgreater.

Unlike many published findings (Harcourt et al., 2010a,b, 2009a,b; Jolles et al., 2015, 2014; Leblond and Reebs, 2006;

Nakayama et al., 2012), differences in asocial exploratory ten- dency were not found to directly determine the coordination ofmovementorleadershipbehaviours betweenpartnersin our experiment.Severalstudiesofpersonalityinasocialcontexthave shownthatbolderindividuals(whichmaysharetraitswithour moreexploratoryindividuals)become‘leaders’whenpairedwith ashyerpartner:theyaremorelikelytoleaveashelterfirstand toleadthepair’sexcursionoutfromtheshelter(Harcourtetal., 2010a,b,2009a,b;Jollesetal.,2015,2014;LeblondandReebs,2006;

Nakayamaetal.,2012).Weobservednosucheffects,andspeculate thatthereareseveralpotentialreasonswhy.First,thisstudyused zebrafishasamodelorganism,anditispossiblethatzebrafishcoor- dinatedmovementisdifferentfromthatofspeciesusedinprior work(primarilyGasterosteusaculeatus).Second,weusedapurely exploratorytaskduringtheasocialandsocialtrials.Unlikeinother publishedstudies,ourexploratorytaskwasnotcombinedwitha foragingtask(i.e.,therewasnofoodintheenclosureandthefish werenotfood-deprived).

Perhapsmostimportantly,thesetupofourpairtrialsdiffersin akeyrespectfromthatofmostpriorwork.Pairsinourstudywere abletointeractfreelythroughouttheirpairsessions,whereasin mostpreviousworkabarrierwaskeptbetweenthefishtoallow forindividualidentification(Harcourtetal.,2010a,b,2009a,b;Jolles etal.,2016,2015,2014;Nakayamaetal.,2012).Thepresenceof abarrierduringsocialinteractionsmayconfoundsocialinterac-

tionswithwallfollowingbehaviour,whichboththecurrentwork andothers(Dahlbometal.,2011b;Ferrarietal.,2014;Jollesetal., 2015,2014;Maximinoetal.,2010b)haveshowntobegreaterinless exploratoryindividuals.Thefactthatwedidnotobserveanyeffects ofexploratorytendencyonleadershipinourfree-swimmingpairs suggeststhatthepresenceofabarrierduringsocialtrialsmayfun- damentallyalterthedynamicsofsocialinteractions.Despitethe lackofclearleader-followerdynamics,ourdatadoindicatethat individualbehaviourwasaffectedbythepresenceofapartner.

5. Conclusions

Consistentinter-individualdifferences,oftentermed‘person- ality’, limit an individual’s flexibility in reacting to changing environmentalconditions–bothphysicalandsocial.Butthedegree ofplasticityacrossconditionsthatanindividualdisplayscanitself beconsideredapersonalitytrait.Ourresultsdemonstratethatplas- ticityacrosssocialcontextsinzebrafishvariesbetweenindividuals inaconsistentmanner,iscorrelatedtotheirasocialexploratory tendencies,anddependsonthesign–butnotthemagnitude–of therelativepersonalityoftheirpartner.Plasticityislikelytoalso dependonmanyotherfactorsthatremaintobeexploredsuchas groupsize,thepersonalitycompositionofthegroup,andthetypes ofenvironmentalchallengesthegroupfaces.

Fundingsources

This work was supported by the National Science Founda- tion(GRFPtoO.L.G.),NationalScienceFoundation(PHY-0848755, IOS-1355061, EAGER-IOS-1251585 to I.D.C), ONR (N00014-09- 1-1074,N00014-14-1-0635toI.D.C),ARO(W911NG-11-1-0385, W911NF-14-1-0431 toI.D.C), Human Frontier Science Program (RGP0065/2012toI.D.C),andanNSERCPost-DoctoralFellowship toN.Y.M.

Acknowledgements

WethankA.Strandburg-Peshkin,C.Twomey,andS.Fogartyfor insightfuldiscussions,C.Twomeyforwritingourcustomtracking software,thelabofR.Burdineforprovidingthesubjectsusedin theseexperiments,andC.Hastyforassistancewithanimalcare.

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