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1|P a g e
TheAdaptiveSignificanceofSiblicideinNazcaBoobies
AnnikkaFrostad‐Thomas
Darwin,Evolution,andGalápagos
Fall2009
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Introduction
Galapagosishometoanincrediblecollectionofseabirds,manyfoundnowhereelseinthe
world.Onesuchbird,theelegantandslightlycomicalNazcabooby,isoneofthreeboobyspeciesfound
inthearchipelago.Itistheboobyyouaresecondmostlikelytoseewhenvisiting,andthereare
between25,000and50,000breedingpairslivingonlyonEspanola,SanCristobal,andGenovesa.Nazca
boobiesfeedprimarilyonfishandplungediveoffshorefromheightsofupto328feet,gracefully
collapsingtheirwingsatthelastmomentbeforetheyenterthewater(Kricher2006:108).Unlikeitsred
andblue‐footedcousins,theNazcabooby’sfeetarenotvibrant,butratheraplainolive.Whatever
beautytheirfeetmaylackismorethanmadeupforbypristinewhitebodyfeathers,andwingsandtail
rimmedwithpureblackplumage.
AselegantastheadultNazcaboobyis,asachickitexhibitssomeratherunsightlybehavior—
murder.Withoutfail,Nazcaboobiesareobligatelysiblicidal;theirnormaltwo‐eggclutchisalways
reducedtoonebyeitherunsuccessfulhatchingofoneegg,orsiblicidalaggressionoftheolderchick
(Anderson1990b:346).ThisstrangebehaviorisnotuniquetotheNazcaboobyspecies,blue‐footed
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boobychickssometimesexhibitsiblicide,buttheconditiondoesbringupsomeinterestingquestions.In
particular,whyhassiblicideevolvedinthisspecies,andgiventhatithas,whydoNazcaboobiesstilllay
twoeggs?Fortunately,theNazcaboobyhasbeenthecentralobjectofstudyforDavidAndersonfor
morethantwodecades,andthereisdataavailableoninvestigationsintothemanyhypotheses
regardingsiblicideinthisspecies.Afewareunsupported,someseemfairlylikely,butthereisstillno
concreteevidence,anditisunlikelywecaneverknowtheexactevolutionaryhistoryofsiblicide.
Theevolutionofsiblicide
ANazcaboobynestconsistsofarelativelyflatcircularareaclearedofdebrismarkedbyasimple
guanoring,Nazcasdonotdigoutaslightbowl‐shapeddepressionlikeblue‐footedboobiesdo,nordo
theyuseanysortofextranestingmaterialtoprotectorinsulatetheeggs(Anderson1995:864).Nazca
boobieslaytheirfirsteggontheirbaregroundnestandimmediatelystartincubating,layingtheusual
secondeggfourtoninedayslater(Clifford2002:275).Thislayingasynchronyisdirectlyproportionalto
thehatchingasynchronyoftheeggs,whichis“amongthelongestofallbirds…resulting[in]sizeand
developmentaldisparities”(Clifford2002:374).Bythetimethesecondchickisborn,thefirst‐bornchick
orAchickissignificantlylargerthanthesecond‐bornbecauseithashadseveraldaysoffeedingand
growing(Kricher2006:310).Thesizedifferencethatresultsfromhatchingasynchronyisadeath
sentencefortheBchickbecauseitmakesitmucheasierfortheAchicktoexpelitby“graspinginits
beakthesiblingsneck,appendage,orskinandextendingitsnecktothrusttheB‐chickacrossthenest
scrape.”Thegraphbelowclearlyshowsthesharpdeclineindaysittakesforbroodreductionasthe
hatchingasynchronyincreases.Onceachickisoutofthenestscrapetheparentboobieswillnot
acknowledgeit,anditwilldiequicklyeitherbypredation,starvation,ortemperaturechange(Anderson
1995:861‐862).
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Anderson1988
ThebehaviorofNazcaboobychicksisconsideredsiblicidebecauseit“makesadirectand
significantcontributiontotheimmediatedeathofasiblingnestmate,”butitisimportanttodistinguish
itfromtheformofsiblicidepresentinanotherGalapagosboobyspecies.Inblue‐footedboobybroods,
chicksaresiblicidalonlyifthereisnotenoughfood,andthisiscalledfacultativesiblicidebecauseitis
“conditionalontheperpetrator’simmediateecologicalandphysiologicalsituation.”Bycontrast,Nazca
boobysiblicideisobligate,meaningthatitdoesnotdependonecologicalconditions,butis“persistent
[and]unconditional”(Anderson1990b:337‐338).
Theimmediatequestionthatfollowsishowsuchabarbaric,andseeminglydetrimental,
behaviorevolvedinthisavianspecies.ThesiblicidalnestlinglosesinclusivefitnesswhenitkillstheB
chickbecauseitsharesonaveragehalfofitsgeneswithitssibling,sotheremustbesomegainindirect
fitnessotherwisethisbehaviorwouldnothaveevolved.Onehypothesiswhichwouldseemtonicely
explainthisbehaviorisrecurrentfoodscarcity,becausethegrowingAchickwouldhaveunhindered
accesstoparentalresourcesonceitkilleditsnestmate,andwouldhaveasurvivalandreproductive
advantagebydevelopingonschedule.DavidAndersonandhisresearchteamtestedthishypothesisby
experimentallydoublingNazcaboobybroodstoseewhetherornottheparentscouldactuallyprovide
enoughfoodtofeedtwogrowingchicks.Theirprocedurewastotakeeggsfromdifferentneststhat
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werelaidatthesametimeandputtheminthesamenesttoseeiftheparentswouldbeabletofeed
both,andthesewerecomparedtocontrolgroupsofsingle‐eggnests.Itwasnecessarytoexperimentally
doublethebroodsbecausethatwaytherewouldbelittletonohatchingasynchrony,andconsequently
thechickswouldbelesslikelytosucceedinexpellingoneanotherfromthenestscrape.Theresults
werethatparentswithtwochicksinthenestincreasedtheirforagingeffortsandwereabletokeeptwo
chicksfed.Atpeakfoodintake,doubledbroodsgotonly41%morefoodthansingletons,whichledthem
tobeslightlysmallerwithage,butstillarelativelyhealthysize(Anderson1990a:2073).
Anderson1990
Eventhoughthedoubledbroodshadchicksthatwereaprettyhealthysize,theydidhavea
highermortalityratethansinglechicks,meaningthateventhoughthereisnotcurrentlyenoughfood
scarcitytonecessitatesiblicide,atsomepointinthepasttheremayhavebeenastrongenoughpressure
toevolveobligatesiblicide(Anderson1990a:2074,2077).Itiseasiesttothinkoftheevolutionof
obligatesiblicideastheinvasionofanallelewhosephenotypeissiblicidalbehavior.Duringatimeof
strongselectionpressuretoexcludenestmatesfromthelimitedfoodsource,achickbornwiththe
alleleforsiblicidalbehavior(asaresultofgeneticmutation)wouldbeatanadvantage,andthisallele
couldquicklyinvadeapopulation(Anderson1990a:2078).Oncethepopulationsis“fixedonthe
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siblicidalstrategy…thenonsiblicidalstrategycouldnotinvade[because]mostbroodscontaininga
nonsiblicidalAchickshouldalsocontainasiblicidalBchick,”thereforeeventhoughthereisapparently
nolongeranystrongselectionpressureforsiblicideitpersistsbecausethenonsiblicidalallelecannot
reinvade(Anderson1990a:2079).
OnestudydoneduringthesiblicidalagerangeofNazcaboobies(zerotosevendays)aimedto
findouthowthreehormonesgenerallyrelatedtoaggressionanddevelopmentinotheranimalsrelated
tosiblicideinNazcaboobies.Thethreehormonesstudiedwerepickedbecausetestosteroneis“often
involvedinaggressivebehaviorofvertebrateanimals,”andprogesteroneandcorticosterone“maybe
invovldedinbodymassregulations…[which]typicallyinfluencestheoutcomeofaggressivecompetition
amongnestingbirds”(Tarlow2001:14‐15).Theresearcherspredictedthattestosteronewasresponsible
forthedirectregulationofsiblicidalbehaviorandthatprogesteroneandcorticosteroneprovidedthe
developmentaladvantage(specificallyarapidincreaseinbodymassrelativetolength)thatenabledthe
successofthesiblicidalbehavior.Whattheyfoundwasthatyoungchicks“showedendocrinechanges
consistentwiththehypothesisthatsteroidhormonesmaybeinvolvedintheregulationoffatalsocial
interactions.”ThelevelsofCORTandPshowapreliminarycorrelationtotheincreaseintheAchick’s
bodymasswhenithasasecond‘challenge’eggitsharethenestwith.Animportantpartofalarger
chartisshownherethatillustratestheoveralllevelsoftestosteroneforyoungchicks.AspredictedAis
highest,buttheoneaboveBisasamplethatwastaken
duringanaggressivesiblicidalact(theonlyobservedcase
ofaBchickejectinganAchicksuccessfully)
andindicatesthat“thesecretionof[testosterone]maybe
adirect,butshortduration,responsetoasocialchallenge
(Tarlow2001:15‐19).Overall,hormonalcorrelatesneedtoberesearchedfurthertodeterminetheir
exactroleinsiblicideofNazcaboobies.
Hormones2001
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Thepersistenceoftwo‐eggclutches
ItisclearfromthecurrentstateofsiblicideinNazcaboobiesandtheapplicationof
gamestheorydescribedabovethatNazcaboobiesarehighlyunlikelytoeverreverseto
nonsiblicidalbehavior.Whenthisfactsettlesin,itstirsupanotherveryimportantquestion:
whydoNazcaboobiesstilllaytwo‐eggclutcheseventhoughtheyonlyeverhaveonechick?
Thefirstpossibleexplanationisthattheymightactuallybeinthemiddleofevolutiontoward
one‐eggclutches,asitseemsthatparentswouldhaveahigherfitnessbynotwastingenergyon
asecondeggthatwillnotresultinafledgling.Thiswastestedbyobservinghowmanyclutches
arenaturallyoneortwoeggs.Thepercentagefluctuatesfromyeartoyear,inthechartbelow
threeconsecutiveyearsareshownandthelastshowsmoreone‐eggthantwo‐eggclutches.
However,1986wasanElNinoyear,meaningthat
foodfortheNazcaboobywasprobablyscarce.To
testthehypothesisthatfoodscarcitycausesthe
unusuallyhighamountofone‐eggclutches,mothers
weresupplementedfoodtoseeiffoodintake
affectedclutchsize.Theresultswerethat92%ofsupplementedfemaleslaidtwoeggs,andonly
70%ofcontrolfemaleslaidtwoeggs,suggestingthattheoptimumclutchsizeforNazca
boobiesremainstwo,despitetheevolutionofsiblicide(Clifford2002:278).
Iftwo‐eggclutchescontinuetobefavored,thentheremustbesomereproductive
advantagetohavingtwoeggsdespitethattheextracostofthesecondeggdoesnotresultina
secondsurvivingchick.Therearethreehypotheseshereexploredthataimtoexplainwhatthis
Anderson1990b
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advantagemightbe,theIceboxBenefitHypothesis,theProgenyChoiceBenefitHypothesis,and
theInsurance‐EggHypothesis.TheIceboxBenefitHypothesisassumesthattheBchickis
essentiallyastoreoffreshfoodfortherestofthefamilyandthatfamilieswiththisextrafood
haveareproductiveadvantageoverfamilieswithoutaBchickduringtimesoffoodshortage
(Boag2005:381).Therehasneverbeenanobservedcaseofwithin‐familycannibalisminNazca
boobies,andevenBchicksthatdiewithinthenestscrapearesimplyleftuntiltheyare
“eventuallygroundintothenestsubstrate.”Bchicksthatareejectedfromthenestusuallydie
ofpredationbyotheranimalssuchasSallyLightfootcrabs,starvation,andinonlyoneobserved
case,cannibalismbyaneighboringunrelatedadultNazcabooby(Boag2005:385‐386).Because
ofthelackofanyevidencethatsupportswithin‐familycannibalismatanytime,theIcebox
BenefitHypothesisisprettycertainlynottheexplanationforthepersistenceoftwo‐egg
clutches.
TheProgenyChoiceBenefitHypothesisassumesthattheBchickwilllivelongenoughto
competewiththeAchickforthepermanentspotinthebrood,andalsoassumesthat
sometimestheBchickwillwin.Thiscompetitionwouldresultinthestrongestchicksurviving,
meaningthattheparentshaveincreasedreproductivefitnessbyraisingthechickthatismost
likelytocontinuethegeneticline(Boag2005:381).However,thechartbelowshowsthatthe
averagecohabitationtimeisveryshort,infact,“broodreductionoccurredwithin10daysofthe
[B]chick’shatchinginallnestsexceptone,”andduringthistendayperiodtheBchickisso
smallandfrailthatitcouldhardlybeanyrealchallengetotheAchick.Onlyonecaseoutof
1,901reallysupportstheProgenyChoiceBenefitHypothesis,soitisalsonotlikelyasthe
explanationforcontinuingtwo‐eggclutches(Boag2005:387).
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Humphries2005
ThethirdhypothesisinvestigatedhereistheInsurance‐EggHypothesis,whichassumes
thatthereproductivefitnesscostoflayingtheBeggislessthanitsininsurancevalueinthe
eventthattheAchickshouldeitherfailtohatchordiebeforetheBchickhatches(Evolution
1990b:337).Also,theInsuranceEggHypothesis“predictsthattwo‐eggclutchesshouldalways
yeiedhigherreproductivesuccessthandoone‐eggclutches”(Clifford2001:341).Researchers
recordedthehatchingandfledgingsuccessesofnaturaloneandtwo‐eggclutchestoseeif
therewasindeedareproductiveadvantagetolayingasecondegg.Theyfoundthattherewas
aninsignificantdifferenceinhatchingsuccessofindividualeggsbetweentheclutchsizes,but
thatthetwoeggclutcheshada“higherprobabilityofhatchingatleastonechick”(Anderson
1990b:343).Thishigherprobabilityofhatchingatleastonechickfromatwo‐eggclutchshows
areproductiveadvantage,butfortheInsuranceEggHypothesis,thecostoflayingtheeggmust
belessthanthisgain.ThecostoflayinganegghasnotbeendeterminedinNazcaboobies,but
fortherelatedred‐footedboobyithasbeendeterminedtobeanaveragedailyinvestmentof
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1%‐2.4%ofthebooby’sdailyenergyexpenditure(Anderson1990b:344).ThefactthatBeggs
“contributedthesurvivinghatchlingin19.2%of…two‐eggclutches”showsthatthereisa
significantadvantagetotwoeggsthatoutweighstheparentalinvestmentoflayingtheegg(itis
importanttonotethattheBegghatchlingsurvivorsdidnotkilltheAchick,theysurvivedafter
eithertheAchickfailedtohatchordiedbeforetheBchickhatched)(Anderson1990b:342).
Thechartbelowillustratesthatnotonlywastheprobabilityoffledginghigherinnaturaltwo‐
eggclutches,butwasalsohigherinexperimentallydoubledclutchesthanreducedclutches,
consistentwiththeInsuranceEggHypothesis’predictionthattwoeggsshouldalwaysbe
advantageous(Clifford2001:340).
Clifford2001
Conclusions
Wewillprobablyneverbeabletodetermineexactlywhatcausedobligatesiblicideto
evolveinNazcaboobies,butthemostpromisingexplanationseemstobethatsomepast
selectionpressure,probablyfoodscarcity,causedasiblicidalalleletoinvadethepopulation
thatcannotrevertevennowthattheselectionpressureisgoneandparentsareabletoforage
fortwochicks.Throughtheinvestigationofseveralhypotheses,itappearsmostlikelythatthe
Nazcaboobiesoptimumclutchsizecontinuestobetwodespitetheeffectsofsiblicidebecause
theBeggprovidesasignificantinsurancevalueagainsttheprematurefailureoftheAeggor
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chick.Somefurtherresearchisneededbothtounderstandthemechanismsthatcontrol
siblicideandwhytheBeggisneededasinsurance.Hormonalcorrelateshavebeen
preliminarilyinvestigated,butitisstillunclearwhyboobieshavesuchalowhatchingsuccess
thattheyneedtwoeggs.Nazcaboobies“hatch51%‐61%oftheireggs,whereassingle‐egg
[obligatelysiblicidal]speciesallhatchatleast85%oftheireggs.”Thisisonepossibleareaof
furtherresearchbecause“themajorityofunhatchedeggscontainnovisibleembryo,indicating
thatembryosdiedshortlyafterfertilizationorwereneverfertilized”;eithertheground
temperatureofthenestscrapeisabovethelethaltemperatureforavianembryos,orNazca
boobyinfertilityis“exceptionallyhigh,”eitherofwhichwouldbefascinatingareasforfurther
research(Anderson1990b:343‐345).
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