620 Animal Behaviour, 36, 2

References Graves, J. S. & Whiten, A. 1980. Adoption of strange

chicks by herring gulls. Z. Tierpsychol., 54, 267 278. Holley, A. J. F. 1981. Naturally arising adoption in the

herring gull. Anim. Behav., 29, 30~303. Holley, A. J. F. 1984. Adoption, parent-chick recogni-

tion and maladaptation in the herring gull. Z. Tierpsy- chol., 64, 9-14.

Knudsen, B. & Evans, R. M. 1986. Parent-young recog- nition in herring gulls. Anim. Behav., 34, 77-80.

Mock, D. W. 1984. Infanticide, siblicide and avian nesting mortality. In: Infanticide: Comparative and Evolutionary Perspectives (Ed. by G. Hausfater & S. B. Hrdy), pp. 3-30. New York: Aldine Press.

Noseworthy, C. M. & Lien, J. 1976. Ontogeny of nesting habitat recognition and preference in neonatal herring gull chicks. Anita. Behav., 24, 637-651.

Pierotti, R. & Murphy, E. C. 1987. Intergenerational conflict in gulls. Anim. Behav., 35, 435-444.

(Received 8 June 1987; revised 5 August 1987; MS. number: AS-479)

Parent-Offspring and Sibling-Sibling Recognition in Gulls

Theory in behavioural ecology and sociobiology predicts that parents should evolve the ability to recognize their own offspring to avoid directing parental care towards individuals that will not increase their inclusive fitness (Alexander 1974; Wilson 1975; Dawkins 1976; Wittenberger 1981; Holmes & Sherman 1983). The high incidence of adoption reported from a number of species (reviews in Pierotti 1980, in press; Riedman 1982) has indicated that this theory may be overly simplistic.

In an effort to explain how such apparently altruistic behaviour as adoption of unrelated off- spring could occur we proposed a new hypothesis (Pierotti & Murphy 1987). We argued that selec- tion should act on offspring to elicit care from unrelated adults when those offspring are separ- ated from their parents or are receiving care of low quality. This creates a conflict between adults and offspring that may be won by either contestant.

Holley (1988) has raised an interesting point in suggesting that another conflict may arise in these situations. That is, resident offspring may resist the addition of unrelated offspring to the brood of which they are members. This assumes that gull chicks are capable of distinguishing between sib- lings and non-siblings. This question has rarely been examined in gulls. Only Noseworthy & Lien (1976) have demonstrated any evidence of sibling recognition in gulls, as part of a larger study on ontogeny of habitat recognition.

We have data relevant to this point that can be

used to test the questions raised by Holley. One of us (RP) has observed chicks of western gulls, Larus occidentalis, attacking wandering chicks that were smaller than themselves, although these attacks caused no apparent damage. Chicks were excluded from territories only when scuffles between resident and wandering chicks attracted the attention of resident adults who attacked the intruder and drove it off. Other western gull chicks, however, accepted with no aggression unrelated chicks that were smaller than themselves.

In an effort to test for sibling recognition in western gulls, we offered 28 different chicks a choice of associating with familiar siblings, unfami- liar siblings, familiar non-siblings, and unfamiliar non-siblings either in pairs or in groups. In all cases, chicks preferred to associate with familiar individuals regardless of relatedness. Aggression was observed between unfamiliar individuals on only a few occasions. Until the chicks reached the age of 5 ~ weeks (and adult weight), none of this aggression was serious enough to displace the victim chick from the vicinity of the aggressor chick.

During 2 years of observing interactions among chicks of herring gulls, L. argentatus, in Newfound- land, no acts of aggression were observed between unrelated chicks. In fact cr~ching behaviour, in which unrelated chicks formed groups that were fed by several adults, was observed on three occasions (Pierotti 1980; J. Lien, personal commu- nication). No chicks were ever driven out of creches, or observed to be driven off territories by aggressive acts directed at them by other chicks, although adults were observed to attack unrelated chicks that intruded on their territories.

Similarly, in great black-backed gulls, L. mari- nus, broods of siblings tended to associate with each other more often than with non-siblings, but no aggression was ever observed between unrelated chicks, even when adults were feeding groups composed of their own and unrelated chicks.

In contrast, chicks of black-legged kittiwakes, Rissa tridactyla, showed considerable aggression directed at small intruder chicks by residents or at small residents by larger intruders (B. Roberts, personal communication). It should be pointed out, however, that in Alaska where our data were collected, this species also directs considerable aggression at smaller siblings as well, often with fatal consequences (Braun & Hunt 1983; Roberts, unpublished data). As a result, there is no special significance to aggression between unrelated chicks in this species that shows frequent siblicide.

To summarize, although Holley's suggestion that resident chicks exclude wandering unrelated chicks is reasonable, there is little evidence that this

Short Communications 621

could be a primary mechanism leading to preven- tion of adoption. Gull chicks do appear to discrimi- nate between familiar and unfamiliar chicks, but they are rarely aggressive towards unfamiliar chicks, and what aggression is observed is only effective at displacing strangers when the resident chicks are near adult size, or when scuffles between chicks attract the attention of adults that attack one of the combatants.

Holley has chosen to comment on the final two of our four predictions, and has argued that our results that test these predictions are equivocal. To place these predictions in a proper context, it is appropriate to cite the second of our four predic- tions as well: adult gulls should recognize their own offspring and reject unrelated chicks, but this can be offset by the risk of rejecting their own offspring (Pierotti & Murphy 1987, page 435). There is considerable evidence supporting this prediction, i.e. nearly as many chicks are killed by unrelated adults as are adopted, and many more are rejected without being killed. This suggests rather strongly that (1) some adults are capable of recognizing unrelated chicks, and (2) the major problem facing wandering gull chicks seeking adoption is unre- lated adults rather than the response they receive from potential foster siblings, as suggested by Holley.

RAYMOND PIEROTTI* DIANE BRUNTON t

EDWARD C. MURPHY~ *Department o f Zoology,

University o f Wisconsin, Madison, WI 53706, U.S.A.

t Department o f Biology, Yale University, New Haven, CT06511, U.S.A.

~Institute o f Arctic Biology, University o f Alaska, Fairbanks, AK 99701, U.S.A.

References Alexander, R. D. 1974. The evolution of social behavior.

A. Rev. Ecol. Syst., 5, 324-383. Braun, B. M. & Hunt G. L., Jr. 1983. Brood reduction in

black-legged kittiwakes. Auk, 100, 469-476. Dawkins, R. 1976. The Selfish Gene. Oxford: Oxford

University Press. Holley, A. J. F. 1988. Intergenerational conflict in gulls.

Anim. Behav., 36, 619 620. Holmes, W. G. & Sherman, P. W. 1983. Kin recognition

in animals. Am. Scient., 71, 46-55. Noseworthy, C. M. & Lien, J. 1976. Ontogeny of nesting

habitat recognition and preference in neonatal herring gull chicks. Anita. Behav., 24, 637~51.

Pierotti, R. 1980. Spite and altruism in gulls. Am. Nat., 115, 290-300.

Pierotti, R. In press. Intergenerational conflicts in species of birds with precocial offspring. Proc. X1X Int. Ornithol. Congr.

Pierotti, R. & Murphy, E. C. 1987. Intergenerational conflicts in gulls. Anita. Behav., 35, 435-444.

Riedman, M. 1982. The evolution of alloparental care and adoption in mammals and birds. Q. Rev. Biol., 57, 405-435.

Wilson, E. O. 1975. Sociobiology: the New Synthesis. Cambridge, Massachusetts: Harvard University Press.

Wittenberger, J. F. 1981. AnimalSocialBehavior. Boston. Duxbury Press.

(Received 20 July 1987; revised 9 September 1987; MS. number: A8-486)

The Relative Importance of Body Size and Aggressive Experience as Determinants of

Dominance in Pumpkinseed Sunfish, Lepomis gibbosus

Most studies on the determinants of dominance investigate the effect of one determinant while controlling other potential determinants. Investi- gations of the relative strengths of these factors are few. That is, when more than one type of domi- nance-determining asymmetry exists between two contestants, we often lack the knowledge that would allow us to predict winners and losers (but see Braddock 1949; Frey & Miller 1972; Henderson & Chiszar 1977; Beaugrand & Zayan 1985; Ewald 1985). This study investigates the relative contribu- tions of prior aggressive experience and body size asymmetries to the determination of dominance in pumpkinseed sunfish, Lepomis gibosus.

When pumpkinseed sunfish are similar in size, asymmetries in prior social experience affect the outcome of future contests that establish domi- nance relationships. Prior losers are defeated by prior winners and by individuals that have had no recent social experience (isolates). However, prior winning confers no advantage in contests with isolates (Beacham & Newman 1987). To quantify the relative strengths of asymmetries in prior experience and size, I staged dyadic contests between small individuals that had recently won a contest and large individuals that had recently lost a contest. I expected the prior experience effect to override the size effect when the asymmetry in size was small. When the asymmetry was large, I expected the size effect to override the prior experience effect.

Test subjects ranged from 48 to 100 mm total length and from 1-4 to 14.4 g. The sexes were not determined. However, no individual exhibited reproductive behaviour, and I have shown (unpub- lished data) that sex does not affect dominance