W Ratnieks and Tom Wenseleers Laboratory of Apiculture and Social Insects Department of Animal and Plant Sciences University of Shef64257eld Shef64257eld S10 2TN UK Laboratory of Entomology Zoological Institute Catholic University of Leuven 3000 Leuv ID: 35321
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Altruismininsectsocietiesandbeyond:voluntaryorenforced?FrancisL.W.RatnieksandTomWenseleersLaboratoryofApicultureandSocialInsects,DepartmentofAnimalandPlantSciences,UniversityofShefÞeld,ShefÞeld,S102TN,UKLaboratoryofEntomology,ZoologicalInstitute,CatholicUniversityofLeuven,3000Leuven,BelgiumThealtruismofinsectworkershaspuzzledresearchers :asituationwherebycoercionfavoursindividualstocomplywiththesocialnormsofthegroup,evenifthisisnottheirfirstchoice[7].:anactionthat,onaverage,decreasesthelifetimedirectfitnessofanactorandbenefitsoneormorerecipients[4].:socialpressureintheformofthepunishmentorpolicingofgroupmembers,usuallytopreventthemfromactingselfishlyandtherebyharmingthegrouporcolonyasawhole;thewordsenforcementandsanctionhavesimilarmeanings[5].Enforcedaltruism:altruism,orthelevelofaltruism,favouredbynaturalselectionactingonindividualsinsocialgroupswithcoercion[14].:socialgroupsinwhichsomeindividualsspecialiseinworkorhelpingtoenhancethedirectreproductionofothers.HamiltonÕsrule:theinequalityarisingfromW.D.HamiltonÕsinclusivefitnesstheory[4]thatpredictswhenasocialaction,suchasaltruism,is Correspondingauthor:Ratnieks,F.L.W.(F.Ratnieks@Sheffield.ac.uk0169-5347/$Ðseefrontmatter2007ElsevierLtd.Allrightsreserved.doi:10.1016/j.tree.2007.09.013 societies,includingvertebrateandhumansocieties,andinterspeciÞcmutualisms.FamilylevelsofrelatednessareinsufÞcientforcompletevoluntaryaltruismHamiltonÕsruleshowsthathigherrelatednessfavoursgreateraltruismaltruism.Inhymenopteraninsectsocietiesheadedbyasingle,once-matedqueen,sistersarehighlyrelated,by0.750.75.However,aretheobservedlevelsofrelatednesssufÞcienttoexplaintheextremeworkeraltru-ismobservedinmanyinsectsocieties?Forexample,intheApismellifera,queensmatewithabouttenmales,andworkersarerelatedbyonly0.30.3.Howcanthislowlevelofrelatednessresultinsocietiesinwhichfewerthanonein1000workersattemptstolayeggseggsoronein10000femalelarvaedevelopsintoaqueenqueen?Recenttheorysuggeststhat,intheabsenceofcoercion,itit.Thereasonbecomesclearwhenweconsideregglayingbyworkersworkers.InthesocialHymenoptera,workerscanlayhaploid,male-destinedeggs(Figure2).Theyareselectedtodosobecausetheyaremorerelatedtotheirownsonsthantosonsofeithertheirmotherqueenorsistersister.Nevertheless,notallworkersareselectedtoreproduce.Ifallworkerslaideggs,therebyleavingnonetowork,thecolonywoulddieandtheinclusiveÞtnessofeveryindividual,includingtheegglayers,wouldbezerozero.Theequilibriumproportionofworkersthatshouldreproduce,intheabsenceofcoercion,canbedeterminedfromHamiltonÕsrulerule(Box1)andis14%forspeciessuchasstinglessbeesorbumblebees,inwhichqueensmatewithasinglemalemale,and54%forthehoneybee.Importantly,theseÞguressuggestthattheintermediaterelatednesslevelsfoundininsectsocietiescannotcausetheextremealtruismfoundinthehoneybeeandmanyotherspecies,inwhichveryfewworkersreproduce.Theory,however,alsoshowsthatthesehighlevelsofworkeraltruismcanoccurifthepotentialaltruistsarecoerced,thatisifaltruismisenforced.Inthehoneybee,workersselectivelykillorpoliceeggslaidbyotherworkersworkers.Theoryshowsthatsuchpolicingcanreducetheincen-tiveforworkerstolayeggstosuchanextentthatfew,orevennone,shouldevenattempttodososo.Evidenceforenforcedaltruism:egg-layingbyworkersAlthoughworkerpolicingwasÞrstdiscoveredinthehon-[6],ithassincebeenfoundinseveralotherant,beeandwaspspecies[5,18].Thekillingofworker-laideggsbythequeen(queenpolicing)isalsowidespreadandisparticularlycommoninspecieswithsmallcoloniesolonies.Ifpolicingdoescoercealtruismbyreducingtheopportunityforworkerstoreproduce,wepredictthatfewerworkersshouldlayeggsinspecieswithmoreeffec-tiveeggpolicingg.Recently,acomparativestudyoftenspeciesfoundstrongsupportforthisprediction:speciesinwhichworker-laideggshadagreaterchanceofbeingkilledhadsmallerproportionsofegg-layingng(Figure3Thedeterrenteffectofcoercionhasalsobeenshownbycomparingworkerreproductionincolonieswithandwith-outaqueenqueen(Figure3).Inqueenlesscolonies,policingisabsent,andalastbatchofworker-derivedmalesisrearedbeforethecolonydiesout.Inqueenlesshoneybee40%oftheworkerslayeggseggs.ThisproportionisclosetotheinclusiveÞtnesstheorypredictionbasedonvoluntaryaltruismalone(54%,Box1)andgreaterthanthe0.01Ð0.1%thatreproduceincolonieswithaqueenqueen.Queenlesscoloniesgiveanideaofwhatinsectsocietieswouldbelikeifenforcementwereabsentandaltruism Figure1.EnforcedcooperationininsectandvertebratesocietiesandinterspecificWorkerpolicingpreventsworkersfromreproducingintheApismelliferamellifera;(b)queenpolicing(inset)preventsmostworkersfromreproducingsuccessfullyinthebumblebeeBombusterrestristerrestris;(c)aworkerwithactiveovariesisaggressedinthequeenlessantHarpegnathossaltatorsaltator;(d)inthehoneybee,differentialfeedingforcesmostlarvaetodevelopasworkers;onlylarvaerearedinroyalcells(arrow)candevelopasqueensqueens;(e)subordinateworkersfromtheallodapinebeeExoneurabicolorareevictedfromthenestwhentheyhavebeenincontactwithaforeignmalemale;(f)aworkerthatattemptedtooverthrowthebreederfemaleispunishedinthequeenlessantDinoponeraquadricepsquadriceps;(g)inthecooperativelybreedingcichlidNeolamprologuspulcher,subordinatesthatdonothelplookaftertheeggsandfryareevictedevicted;(h)rhesusmonkeys(Macacamulatta)whodonotsharefoodarepunishedpunished;(i)inmeerkats(Suricatasuricata)dominantfemalespreventsuccessfulbreedingbysubordinatessubordinates;(j)soybeanplants(Glycinemaxsanctionrootnodulebacteriathatdonotfixnitrogen,asshownbythissplitrootexperimentexperiment.Reproduced,withpermission,fromF.L.W.Ratnieks(a,d),theBBCNaturalHistoryUnit(LifeintheUndergrowthseries)(b),J.Liebig(c),M.Schwarz(e),T.Monnin(f),J.Desjardins(g),M.Hauser(h),A.Young(i)andB.Rousseau(j). TRENDSinEcologyandEvolutionVol.23No.1