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Sexual Conflict About Parental Care: the Role of Reserves View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by University of Debrecen Electronic Archive vol. 159, no. 6 the american naturalist june 2002 Sexual Conflict about Parental Care: The Role of Reserves Zolta´n Barta,1,* Alasdair I. Houston,2,† John M. McNamara,1,‡ and Tama´s Sze´kely 3,§ 1. Centre for Behavioural Biology, School of Mathematics, and thus, parents should have sufficient reserves to raise University of Bristol, University Walk, Bristol BS8 1TW, United their young to independence. Parents in poor condition Kingdom; may terminate care and abandon their nest or young (Sny- 2. Centre for Behavioural Biology, School of Biological Sciences, University of Bristol, Woodland Road, Bristol BS8 1UG, United der et al. 1989; Olsson 1997; reviewed by Clutton-Brock Kingdom; 1991; Sze´kely et al. 1996). Given that energy reserves play 3. Department of Biology and Biochemistry, University of Bath, an important role in determining patterns of care, an evo- Bath BA2 7AY, United Kingdom lutionary account of care should adopt a state-dependent approach (Houston et al. 1988; Houston and McNamara Submitted February 5, 2001; Accepted November 30, 2001 1999; Clark and Mangel 2000). State-dependent models have been developed to investigate the decision of a single sex by Kelly and Kennedy (1993) and Webb et al. (in press). For instance, Kelly and Kennedy (1993) showed that fe- abstract: Parental care often increases the survival of offspring, male Cooper’s hawks Accipiter cooperi should desert their but it is costly to parents. Because of this trade-off, a sexual conflict young when their reserves fall below a critical level. In over care arises. The solution to this conflict depends on the inter- their model, however, the behavior of deserting females actions between the male and female parents, the behavior of other was constrained by their not allowing the females to re- animals in the population, and the individual differences within a sex. We take an integrated approach and develop a state-dependent mate. In a more general model by Webb et al. (in press), dynamic game model of parental care. The model investigates a single a deserting female can remate and renest within the same breeding season in which the animals can breed several times. Each breeding season. Interestingly, their model found that a parent’s decision about whether to care for the brood or desert female deserts not only when her reserves are low (and depends on its own energy reserves, its mate’s reserves, and the time thus she is threatened by starvation) but also when her in the season. We develop a fully consistent solution in which the reserves are high. In the latter case, the deserting female behavior of an animal is the best given the behavior of its mate and immediately starts to search for a new mate and thus in- of all other animals in the population. The model predicts that fe- males may strategically reduce their own reserves so as to “force” creases her reproductive success by remating. their mate to provide care. We investigate how the energy costs of In many birds, fish, and mammals, however, the payoffs caring and searching for a mate, values of care (how the probability from caring and deserting not only depend on the envi- of offspring survival depends on the pattern of care), and population ronment and the energy reserves of the parents but also sex ratio influence the pattern of care over the breeding season. on the behavior of other animals in the population. First, the success of the current breeding attempt depends on Keywords: sexual conflict, parental care, offspring desertion, dynamic whether the focal animal’s mate cares or deserts. Second, game, reserves, body mass regulation. the operational sex ratio (Emlen and Oring 1977) can determine the time to remate and hence influence the payoff from desertion. There is evidence that animals re- Parental care is an energetically demanding behavior spond to the operational sex ratio. For example, male fish (Ricklefs 1974; Golet and Irons 1999; Ho˜rak et al. 1999), more often terminate care if the sex ratio is biased toward * Corresponding author. Present address: Behavioural Ecology Research females (Keenleyside 1983; Balshine-Earn and Earn 1998), Group, Department of Evolutionary Zoology, University of Debrecen, Deb- and female rock sparrows Petronia petronia desert their recen H-4010, Hungary; e-mail: zbarta@delfin.klte.hu. brood in years when many unmated males are in the pop- † E-mail: [email protected]. ulation (Pilastro et al. 2001). These aspects of parental care ‡ E-mail: [email protected]. require a game-theoretic analysis (Maynard-Smith 1982; § E-mail: [email protected]. Hammerstein and Parker 1987) in which the behavior of Am. Nat. 2002. Vol. 159, pp. 687–705. ᭧ 2002 by The University of Chicago. the focal pair as well as other members of the population 0003-0147/2002/15906/0008$15.00. All rights reserved. is considered. The importance of including the behavior 688 The American Naturalist of other members of the population is illustrated by Webb (i.e., the net energy gained in a day). Searching for a mate et al. (1999). They show that modeling the behavior of a decreases the reserves of the male, but he may find a mate. single pair in isolation from the rest of the population The mate-finding probability of a male depends on the results in different evolutionarily stable strategies (ESSs) number of mate-searching individuals of both sexes (Webb from ones in which future expectations, including remat- et al. 1999). To illustrate this, let us assume that only a ing probabilities, depend on the behavior of all members few females are searching for a mate. If only the focal male of the population. Taken together, a full understanding of searches for females, he will find one with high probability parental care requires an analysis that is both state de- despite their low numbers. However, if many males search, pendent and game theoretic. For a related discussion in the focal male’s probability of finding a mate will be low. the context of alternative reproductive behaviors, see The probability of finding a mate also depends on the Alonzo and Warner (2000). mate-search efficiency k, which may reflect population Here, we develop a state-dependent game model in density (McNamara et al. 2000). If the focal male finds a which the parents’ decisions depend on their own energy mate, he becomes mated. We assume that mate-finding is reserves, their mate’s reserves, and time in the season. The random (i.e., there is no mate choice). As a consequence, payoffs for performing each action are not specified in the level of energetic reserves of the mate is drawn from advance, since they depend on future expectations, which, the distribution of the reserves of the unmated mate- in turn, depend on the future behavior of the focal animal searching females (i.e., at the time of pair formation, one and the behavior (both past and future) of other popu- member of the pair has no information about the energy lation members. We use the model to show that state- reserves of its partner). dependent models can produce very different predictions Once a pair has formed, they then produce some off- from state-independent ones because they allow the ani- spring after a fixed period. For example, in birds, this mals to optimize simultaneously their behavior and re- period may represent the time required to prepare a nest serves. We then investigate how the energetic costs of var- or to produce eggs. Producing offspring reduces the re- ious activities (e.g., parental care, reproduction, mate serves. This could represent the energetic cost of nest search), care parameters, and the population sex ratio in- building, territory defense, or egg laying. We assume that fluence parental decisions. if the reserves of the male’s mate are not high enough to cover this cost, then she dies and no offspring are pro- duced. The male then becomes unmated after the time The Model period needed for offspring production. We consider the behavior of a focal male and a focal female Having produced the offspring, both members of the p in a large population of NM males and NF females (N pair decide whether to care for the offspring until they ϩ NMFN ) during a breeding season of T d. For simplicity, become independent or to desert (i.e., we assume that the we only describe the behavior of a focal male. If we do animals decide once for each breeding attempt). We as- not explicitly specify the female’s behavior, then similar sume that by this time, each parent has been able to ob- reasoning applies to her too. For a technical account of serve the energy reserves of its partner. Thus, when making the model, see appendices A and B. decisions, both parents know each others’ reserves exactly. The state of a focal male is represented by his marital The male decides first, and the female then decides on the status (unmated or mated), his energy reserves, and, if he basis of her partner’s decision. This is a reasonable as- is mated, the energy reserves of his mate. The male dies sumption in internally fertilizing animals such as birds, of starvation if his reserves fall to zero. However, his re- mammals, and some fish because the male can leave the serves cannot increase above some maximal level. All family immediately after fertilization, whereas the female changes in reserves are stochastic.
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