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The emergence of leaders and followers is a key factor in facilitating group cohesion in animals. Individual group members have been shown to respond strongly to each other’s behavior and thereby affect the emergence and maintenance of these social roles. However, it is not known to what extent previous social experience might still affect individual’s leading and following tendencies in later social interactions. Here, by pairing three-spined sticklebacks (Gasterosteus aculeatus) with 2 different consecutive partners, we show a carryover effect of a previous partner’s personality on the behavior of focal individuals when paired with a new partner. This carryover effect depended on the relative boldness of the focal individual. Relatively bold but not shy fish spent less time out of cover and led their current partner less if they had previously been paired with a bolder partner. By contrast, following behavior was mainly influenced by the personality of the current partner. Overall, the behavior of relatively bold fish was more consistent across the stages, whereas shy fish changed their behavior more strongly depending on the current context. These findings emphasize how the history of previous social interactions can play a role in the emergence and maintenance of social roles within groups, providing an additional route for individual differences to affect collective behavior.

Intrasexual polymorphisms have evolved in a wide range of organisms. Most of them have been interpreted as the product of conditional strategies in which the tactic an individual adopts is determined by some aspect of state (e.g., age, size, condition). However, there are a few examples that appear to represent an evolutionarily stable mixture of heritable pure strategies that are maintained by frequency-dependent selection. In the present study, we produce a model of a mating system with two morphs: a territorial morph and a sneak morph. By varying the costs and limits associated with conditional strategies, mating skew, and the proportion of matings obtained by sneaking males, we examine the conditions that favor the evolution of conditional versus pure strategies. Contrary to current thinking, our results show that as long as either costs or limits are greater than zero, conditional strategists are never able to entirely replace pure strategists, and equilibrium populations may frequently consist of a mixture of conditional and pure strategists. Our results suggest that conditional strategists will be most frequent at intermediate levels of mating skew. Polymorphisms in which conditional strategists are rare or absent are most likely to evolve when mating skew is extremely high, the costs and limits of plasticity are very high, or the benefits of being conditional are very low. The limited data available suggest that high mating skew is probably the most important factor. Key words: alternative strategies, conditional strategies, costs of plasticity, mating competition, mating skew, polymorphism, tactics. [Behav Ecol 15:534–542 (2004)]

Studies of mate choice commonly ignore variation in preferences and assume that all individuals should favor the highest-quality mate available. However, individuals may differ in their mate preferences according to their own age, experience, size, or genotype. In the present study, we highlight another simple reason why preferences may differ: if there is costly competition for mates, the poorest competitors might be better off avoiding the highest-quality partners and instead targeting low-quality partners, so that they minimize the costs they incur. We present a game-theoretical model of mate choice in which males of differing quality compete for access to females and try to retain them till the time of mating. Our model predicts that high-quality males, who are better competitors, have a preference for the best females that is typically several times stronger than that of low-quality males. Early in the competitive period, the latter may even prefer low-quality females over high-quality females. Thus, variation in competitive ability generates variation in both the strength and direction of preferences. Differences in competitive ability result in assortative mating with respect to quality, which is reinforced by variation in preferences. As the time of mating draws near and there is an increased risk of ending up unpaired, all males become indifferent to the quality of potential mates. Our findings are equally applicable to female choice for males, and offer a new explanation for adaptive variation in mating preferences based on differing abilities to cope with the costs of mate choice. Copyright 2003.

We used a reproductive skew framework to consider the evolution of parental and alloparental effort in cooperatively breeding groups. The model provides the first theoretical treatment of rent payment (the ‘‘pay-to-stay’’ hypothesis) for the evolution of helping behavior of subordinates. According to this hypothesis, not all helping behavior is kin selected, but group members help in order to be allowed to stay in the group and potentially gain breeding positions later in life. We show that reproductive concessions may be replaced by complete skew and voluntary, costly alloparental effort by subordinates once future prospects are included in fitness calculations. This suggests that incomplete skew observed in long-lived species is not due to dominant control over reproduction. Rent payment is predicted to occur when relatedness between subordinate and dominant is low, survival is high, ecological constraints are at least moderately tight, and retaining nonhelping subordinates harms the dominant’s fitness. Rent may also be required from related subordinates if helping is very costly (leading to low voluntary helping effort) and ecological constraints are moderately tight. However, related subordinates do not need to have a positive net effect on the dominant’s direct fitness to be accepted as group members. We also consider compensatory responses of dominant group members as a potential threat to the stability of renting behavior. If dominants trade off parental effort against their own survival, they may selfishly reduce their own parental effort as a response to increased help. As this improves their own survival, the prospects of territorial inheritance diminish for the subordinate, and subordinates should hence be less willing to accept the rent agreement. However, we show that compensatory responses by ‘‘lazy’’ parents prevent group formation only in borderline cases. Key words: alloparental care, cooperative breeding, helping at the nest, reproductive skew. [Behav Ecol 13:291– 300 (2002)]

Variation in mitochondrial DNA is often used to trace the evolutionary history of populations and species. We here discuss the effect of infection with cytoplasmically inherited male-lethal symbionts on mitochondrial genome evolution. Male-lethal symbionts spread when killing male hosts increases the lifetime reproductive success of sibling female hosts. This increase in the survivorship of daughters from individuals bearing a male-killer will produce a concomitant increase in the frequency of mitotypes associated with the male-killer. If horizontal transmission of the microoorganism is rare and population sizes not very small, then linkage disequilibrium between microorganism and particular mitotypes will result in a reproduction of within-population mitochondrial variability both because of a selective sweep during the spread of such a micro-organism, and also at equilibrium. Male-killing symbionts may thus confound the use of mtDNA variability in estimation of population parameters. We discuss the differences between the effects of male-killers and the cytoplasmic incompatibility-inducing symbiont Wolbachia , and the possibility that estimation of gene flow between populations may also be confounded by symbiont presence.