Stable URL - Robert Trivers
Parent-Offspring ConflictAuthor(s): Robert L. TriversSource: American Zoologist, Vol. 14, No. 1 (Winter, 1974), pp. 249-264Published by: Oxford University PressStable URL: http://www.jstor.org/stable/3881986Accessed: 07/10/2010 12:05Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available rms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unlessyou have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and youmay use content in the JSTOR archive only for your personal, non-commercial use.Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained herCode oup.Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printedpage of such transmission.JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range ofcontent in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new formsof scholarship. For more information about JSTOR, please contact support@jstor.org.Oxford University Press is collaborating with JSTOR to digitize, preserve and extend access to AmericanZoologist.http://www.jstor.org
Amer. mConflictL. Triversof ComparativeZoology, Harvard University,02138Cambridge, Massachusettssynopsis. When parent-offspring relations in sexually reproducing species are viewedfrom the standpoint of the offspring as well as the parent, conflict is seen to be anexpected feature of such relations. In particular, parent and offspring are expected todisagree over how long the period of parental investment should last, over the amountof parental investment that should be given, and over the altruistic and egoistic tendencies of the offspring as these tendencies affect other relatives. In addition, undercertain conditions parents and offspring are expected to disagree over the preferred sexof the potential offspring. In general, parent-offspring conflict is expected to increaseduring the period of parental care, and offspring are expected to employ psychologicalweapons in order to compete with their parents. Detailed data on mother-offspringrelations in mammals are consistent with the arguments presented. Conflict in somespecies, including the human species, is expected to extend to the adult reproductiverole of the offspring: under certain conditions parents are expected to attempt to moldan offspring, against its better interests, into a permanent nonreproductive.Inclassicaltheory parentevolutionaryare viewedfrom rent.vestmentinanisdefinedas(PI)offspringdone by the parent for the off?anythingchancespring that increases the offspring'sof survivingwhile decreasingthe parent'sability to invest in other offspring (Trivers,assumed1972), then parents are classicallyto allocatein their younginvestmentinsuch a way as to maximizethe numberwhileare implicitlysurviving,offspringto be passive vessels into whichassumedcare. Onceparentspour the appropriateone imaginesas actors in thisoffspringthen conflict must be assumedinteraetion,to lie at the heart of sexual reproductionitself?anfrom theoffspringattemptingtomaximizeitsvery beginningreproducI thank I. DeVore for numerous conversationsand for detailed comments on the manuscript. Foradditional comments I thank W. D. Hamilton, J.Roughgarden, T. W. Schoener, J. Seger, and G. C.Williams. For help with the appendix I thank J. D.Weinrich. Finally, for help with the references Ithank my research assistant, H. Hare, and the HarryFrank Guggenheim Foundation, which provides hersalary. Part of this work was completed under anN.I.H. postdoctoral fellowship and partly supportedbv N.I.M.H. errant MH-13611 to I. DeVore.249tive success (RS) wouldwantpresumablymore investmentthan the parent is selectedto give. But unlike conflict betweenunre?lated individuals,conflictparent-offspringis expected to be circumscribedby the closeandbetweengeneticrelationshipparentFor example,if the offspringoffspring.than the parentgarners more investmenthas been selectedto give, the offspringdecreasesthe numberof its sur?therebyviving siblings, so that any gene in an off?invest?spring that leads to an additionalment decreases (to some extent) the numberof survivingcopies of itself located in sib?lings. Clearly, if the gene in the offspringexacts too great a cost from the parent,that gene will be oughTohowmuchcostspring.specify preciselyan offspringbe willingshouldto inflicton its parent in order to gain a given bene?isfit, one must specify how the offspringto weigh the survivalof siblingsexpectedagainst its own survival.The problemof specifyinghow an indi?vidual is expectedto weigh siblings againstitself (or any relative against any other) hasbeen solved in outline by Hamilton(1964),in the context of explainins:the evolution
250Robertact canof altruisticAn altruisticbehavior.be defined as one that harms the organismsomethe act while benefitingperformingother individual,harm and benefit beingdefinedin terms of reproductivesuccess.Since any gene that helps itself spread in ais, by definition,being selectedpopulationinaltruisticabove sensebehaviorthefor,can be selected only if there is a sufficientlythat the recipientof thelarge probabilityact also has the gene. More precisely,theratioof the act, timesthebenefit/costchance that the recipient has the gene, mustbe greater than one. If the recipientof theact is a relativeof the altruist,then thethat the recipienthas the geneprobabilityby descent from a common ancestor can beThisconditionalisspecified.probabilitycalled the degree of relatedness,Foranr0.altruisticact directed at a relative to havesurvivalvalue its benefit/costratio mustbe larger than the inverse of the altruist'sLikewisean individualr0 to the relative.is expectedto forego a selfish act if its costto a relative,times the r0 to that relative,is greater than the benefit to the actor.The rules for calculatingdegrees of re?latednessare ganisms,when inbreedingtherelevantcomplicatesin Hamilton,genealogy(see the addendumin a diploid1971). For example,speciesan individu(in the absence of inbreeding)al's r0 to his or her full-siblingsis i/2; tototochildren,cousins,half-siblings,i/4;i/2;the selectivevalue ofy8. If in calculatinga gene one not only computesits effecton the reproductivesuccess of the indi?vidual bearing it, but adds to this its effectson the reproductivesuccess of related indi?viduals,devaluedappropriatelyby therelevantof relatedness,then onedegreeshas computedwhat Hamilton(1964) callsinclusivefitness. While Hamiltonpointedout that the parent-offspringrelationshipis merely a special case of relations betweenrelatedany set of geneticallyindividuals,he did not apply his theory to such re?lations. I present here a theory ofparentrelationswhichfollowsoffspringdirectlyfrom the key conceptof inclusivefitnessand from the assumptionthat the offspringL. Triversis at all times capable of an active role inits relationshipto its parents. The form ofthe argumentapplies equally well to hapbut for simplicitythelodiploidspecies,is mostly limited to diploid spe?discussioncies.of theLikewise,althoughmanyto any sexuallyargumentsapplyreproducing species showing parental investmentthe argu?(includingmany plant species),ments presentedhere are particularlyrele?vant to understandinga species such as thehumanin whichinvest?speciesparentalment is critical to the offspring throughoutits entire prereproductivelife (and oftenlater as well) and in which an individualin a net?normallyspends life embeddedwork of near and distant kin.PARENT-OFFSPRINGCONFLICTOVERTHECONTINUATIONOF PARENTAL INVESTMENTConsidera newborn(male) caribou calffromhismother.The benefit tonursinghim of nursingin terms of his(measuredchance of surviving)is large, the cost tohis mother (measured in terms of her abilityto produce additionaloffspring) presumablysmall. As time goes on and the calf be?comes increasinglyof feedingoncapablehis own, the benefit to him of nursing de?creases while the cost to his mothermayincrease (as a function,for example,of thecalf's size). If cost and benefit are measuredin the same units, then at some point thecost to the mother will exceed the benefitto her young and the net reproductivesuc?cess of the mother decreases if she continuesto nurse. (Notethat later-bornoffspringless to the mother's eventualmay contributeRS than early-born,becausetheir repro?ductive value may be lower [Fisher,1930],but this is automaticallytaken into accountin the cost function.)The calf is not expected,so to speak,to view this situationas does his mother,for the calf is completelyrelated to himselfbut only partiallyrelatedto his futureto devaluesiblings, so that he is expectedthe cost of nursingmeasuredin terms(asof future sibs) by his r0 to his future sibs,when comparingthe cost of nursing withits benefit to himself.For example,if fu-
Parent-Offspringto be full-sibs, thenture sibs are expectedthe calf should nurse until the cost to themother is more than twice the benefit toisOnce the cost to the motherhimself.to the calf,more than twice the benefitis opposedcontinuedby naturalnursingselection acting on both the mother and thethat thecalf. As long as one imaginesbenefit /cost ratio of a parental act changestofrom some large numbercontinuouslysome very small numbernear zero, thenthere must occur a period of time duringwhich 1/2 B/C 1.This period is one ofconflictbetweenmother and n the mother favors her haltingworkingwhile natural selectionparental investmentfavors his elicitingacting on the offspringthe parentalinvestment.Theargumentin Figurehereis1.presentedgraphed(Note, as argued below, that there are spe?in whichcializedsituationsthe offspringtoselectedbeconsumeless Pl than themayparent is selected to give.)Thisto all sexuallyargumentappliesreproducingspecies that are not completelythat is, in which siblingsare notinbred,FIG. 1. The benefit/cost ratio (B/C) of a parentalact (such as nursing) toward an offspring as a func?tion of time. Benefit is measured in units of repro?ductive success of the offspring and cost in compar?able units of reproductive success of the mother'sfuture offspring. Two species are plotted. In speciesI the benefit/cost ratio decays quickly; in species II,slowly. Shaded areas indicate times during whichparent and offspring are in conflict over whetherthe parental care should continue. Future sibs areassumed to be full-sibs. If future sibs were half-sibs,the shaded areas would have to be extended untilB/C - 1/4.Conflict251identicalcopies of each other. Conflict nearthe end of the period of PI over the continuationof PI is expectedin all such spe?cies. The argument applies to PI in generalor to any subcomponentof PI (such as feed?the young, carry?ing the young, guardinging the young) that can be assigned a moreor less san d by the argumentgiven here. Suchconflict is known to occur in a variety ofin the field and in the labora?mammals,forbaboons (DeVore,tory:example,1963),langurs (Jay, 1963), rhesus macaques (Hindeand Spencer-Booth,1971), other macaquesverve ts (Struhsaker,(Rosenblum,1971),et al., 1963), dogs1971), cats (Schneirlaandrats (Rosenblatt(Rheingold,1963),and Lehrman,I interpretLikewise,1963).conflict over parentalat the timefeedingof fledgingin bird species as conflict ex?for exam?plained by the present argument:ple, Herring Gulls (Drury and Smith, 1968),Red Warblers(Elliott,1969), Verreaux'sandWhitePelicansEagles (Rowe,1947),(Schaller, 1964).conflict is usuallyassumedtoWeaningoccur either because transitionsin natureare assumed always to be imperfector be?cause such conflict is assumed to serve theinterestsof both parent and offspringbyeach of the needs of the other.informingIn either case, the marked ment in favor of the view that such conflictresults from an underlyingconflict in theway in which the inclusive fitness of motherand offspring are maximized.Weaning con?flict in baboons,for example,may last forweeks or months,involvingdaily competi?tive interactionsand loud cries from theinfantin a species otherwisese?stronglylectedfor silence(DeVore,1963). Inter?actionsthat inefficientwithina multi?cellular organism would be cause for somesurprise, since, unlike mother and offspring,the somaticcells withinan organismarerelated.identicallyOne parameterthe expectedaffectingofconflictlength (and intensity)weaningis the offspring'stoitsfuturer0expected
252Robertsiblings. The lower the offspring's r0 to itsand more in?future siblings,the longerconflict.Thistense the expectedweaningOther thingssuggests a simple prediction.in whichdifferent,beingspeciesequal,a fefathermalesunrelatedcommonlytomale's successiveare expectedoffspringshow stronger weaning conflict than speciesin which a female's successive offspring arefatheredAsusuallyby the same male.conflictshown below, however,isweaningmerely a special case of conflict expectedthe period of parentalinvest?throughoutment, so that this predictionapplies to theof conflictasintensityprior to weaningwell.CONFLICTTHROUGHOUTTHE PERIODOF PlOVERTHE AMOUNT OF PlIn Figure1 it was assumedthat thefor each day (or moamount of investmentment in time) had already been established,and that mother and young were only se?lected to disagreeover when such invest?ment should be ended. But it can be shownover the amountthat, in theory, conflictof investmentthat should at each momentbe given, is expectedthe pe?throughoutriod of Pl.At any momentin the period of Pl thefemaleis selectedto invest that amountwhichmaximizesthe differencebetweenthe associated cost and benefit, where theseterms are defined as above. The infant isto induceselectedthat investmentwhichmaximizesthe difference between the bene?fit and a cost devaluedby the relevant r0.The different optima for a moment in timein a hypotheticalinspecies are graphed2. WithreasonableFigureassumptionsaboutthe shape of the benefitand costcurves, it is clear that the infant will, ateach instant in time, tend to favor greaterinvestmentthan the parent is se?parentallected to give. The period of transitiondis?cussed in the previoussection is a specialofcasethiscontinuingcompetition,the case in which parent and off?namely,over whetherspring competeany invest?ment should be given, as opposedto theirearlier competitionover how much shouldL. TriversFIG. 2. The benefit, cost, and half the cost of aparental act toward an offspring at one moment intime as a function of the amount the parent investsin the act (PI). Amount of milk given during oneday of nursing in a mammal would be an exampleof PI. At p the parent's inclusive fitness (B - C) ismaximized; at y the offspring's inclusive fitness(B C/2) is maximized. Parent and offspring disagree over whether p or y should be invested. Theoffspring's future siblings are assumed to be fullsiblings. IF ? inclusive fitness.be given. Since parental investmentbeginsbefore eggs are laid or young are born, anddissince there appears to be no tside the mother (mediatedbyprimarilycon?behavioralacts) and parent-offspringflict inside the mother (mediatedprimarilyby chemicalacts), I assume that parent-off?spring conflict may in theory begin as earlyas meiosis.that the cost ofIt must be emphasizedto above (seereferredinvestmentparentalonly in terms of de?Fig. 2) is measuredcreased ability to produce future offspring(or, when the brood size is larger than one,otherto eciatespring).that early in thethis definition,imagineof PI the offspringgarners moreperiodthan the parent has been se?investmentlected to give. This added investmentmaylater investmentdecreasethe parent'sinat hand, either throughanthe offspringdur?increased chance of parental mortalitying the period of PI, or through a depletionin parentalor becauseresources,parentshave been selected to make the appropriate
Parent-Offspringadjustment(that is, to reduce later invest?wouldwhat otherwisehavement belowIn short, the offspringbeen given).maybenefit but suffer a latergain a temporarycost. This self-inflictedincost is subsumedthe benefit function (B) of Figure 2, becauseit decreases the benefit the infant receives.It is not subsumedin the cost function(C)becausethis functionrefers only to themother's future offspring.THE TIME COURSEOF PARENTOFFSPRINGCONFLICTIf one could specify a series of cost-benefitcurves (such as Fig. 2) for each day of thetime courseperiod of Pl, then the expectedof parent-offspringconflict could be speci?fied. Where the difference in the offspring'sinclusivefitness at the parent'soptimumPl (p in Fig. 2) and at the offspring's opti?mum Pl (y) is large, conflict is expectedtobe intense. Where the differenceis slight,conflictis expectedto be slight or nonexistent. In general, where there is a strongdifference in the offspring's inclusivefitnessat the two different optima (p and y), thereConflict253in the parwill also be a strong differenceent's inclusivefitness, so that both chievetheirstronglyrespective optimal values of PI. (This techniqueof comparingcost-benefitcan begraphsusedto makeotheraboutpredictionsfor examplethatconflict,parent-offspringsuch conflict shoulddecreasein intensitywith increasingage, and hence decreasingof the parent; see Fig?value,reproductiveure 3.) In the absence of such day-by-dayallgraphs three factors can be identified,of which will usually predisposeparent andto show greaterconflictas theoffspringofPIperiodprogresses.chance of self-inflictedcost.1) DecreasedAs the period of PI progresses, the offspringfaces a decreased chance of suffering a laterself-inflictedcost for garneringadditionalinvestmentat the moment.At the end ofthe period of PI any additionalinvestmentforced on the parent will only affect lateroffspring, so that at that time the interestsof parent and offspringare maximallydiThisinvergent.time-dependentchangethe offspring'schance of sufferinga self-mother's netRS atym(a)FIG. 3. The benefit and cost of a parental act (asin Fig. 2) toward (a) an offspring born to a youngfemale and (b) an offspring born to an old female.One assumes that the benefit to the offspring of agiven amount of Pl does not change with birthorder but that the cost declines as a function of thedeclining reproductive value (Fisher, 1930) of themother: she will produce fewer future offspringanyway. The difference between the mother's in?clusive fitness at m and y is greater for (a) than for(b). The same is true for the offspring. Conflictshould be correspondingly more intense betweenearly born young and their mothers than betweenlate born young and their mothers.
254inflictedRobertcost will, other things being equal,predisposeparent and offspring to increas?ing conflict during the period of Pl.of parentalreplenishment2) Imperfectresources. If the parent is unable on a dailybasis to replenishresources invested in thethe parent will suffer increasingoffspring,of its resources, and, as time goesdepletionshould riseon, the cost of such depletioneven if the amountofdisproportionately,resourcesForinvestedper day declines.a female may give less milk perexample,inthefirst half of the nursing perioddaythan in the second half (as in pigs: Gill andif she is failingbutThomson,1956),to replenishher energy losses,throughoutthen she is constantlyher deficitincr
Robert L. Trivers Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138 synopsis. When parent-offspring relations in sexually reproducing species are viewed from the standpoint of the offspring as well as the parent, conflict is seen to
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ROBERT TRIVERS Department of Applied Sciences, University of California, Santa Cruz, California 95064 USA A REVIEW OF THE MASTERPIECE OF NATURE: THE EVOLUTION AND GENETICS OF SEXUALITY. By Graham Bell. University of California Press, Berkeley. 45.00. 635 p.; il
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