Life-history traits as causes or consequences of social behaviour: why do cooperative breeders lay small clutches?

Cooperatively breeding birds tend to exhibit high adult survival and relatively small clutch sizes. According to the life-history hypothesis for cooperative breeding, high survival creates conditions for philopatry based on difficulties that dispersers face when competing for territories in a landscape with slow territory turnover. However, this hypothesis evokes a puzzle because high fecundity should also lead to problems in territory acquisition because of the large number of competitors for each vacancy. We suggest two reasons for the observed association between small clutch size, high survival rate and cooperative breeding in birds. The first reason is that when survival rate is a better predictor of cooperative breeding than fecundity, a general life-history trade-off between clutch size and survival rate will create the observed association between cooperative breeding and the two life-history characters. Theoretically, a high survival rate is expected to predict cooperative breeding better than fecundity, because a high survival rate increases both habitat saturation and the direct benefits of staying at home. The second reason is that the reproductive value of the first offspring each year is higher than that of subsequent offspring for cooperative breeders (the offspring depreciation hypothesis). This is because these offspring will be able to delay dispersal and gain indirect benefits by helping at home. We show that this. under very general conditions, decreases the optimum clutch size of cooperative breeders below that of non-cooperative breeders

Male mating constraints affect mutual mate choice: Prudent male courting and sperm limited females

Costs of sperm production may lead to prudence in male spermallocation and also to male mate choice. Here, we develop a life history–based mutual mate choice model that takes into account the lost-opportunity costs for males from time out in sperm recovery and lets mate competition be determined by the prevailing mate choice strategies. We assume that high mating rate may potentially lead to sperm depletion in males, and that as a result, female reproduction may be limited by the availability of sperm. Increasing variation in male quality leads, in general, to increased selective mate choice by females, and vice versa. Lower-quality males may, however, gain access to more fecund higher-quality females by lowering their courting rate, thus increasing their sperm reserves. When faced with strong male competition for mates, low-quality males become less choosy, which leads to assortative mating for quality and an increased mating rate across all males. With assortative mating, the frequency of antagonistic interactions (sexual conflict) is reduced, allowing males to lower the time spent replenishing sperm reserves in order to increase mating rate. This in turn leads to lower sperm levels at mating and therefore could lead to negative effects on female fitness via sperm limitation

Fairness evolution in the ultimatum game is a function of reward size

I formulate a simple model of the ultimatum game, in which a proposer and a responder can receive a reward if they agree on how to divide this reward between them. The model is easy to analyse and shows that strong tendencies to fair division are expected when evolution of strategy frequencies follow the traditional gradient dynamics assumed in evolutionary models. The mean stable offer is typically around 20-40% although this depends on the maximum payoff and if rejection thresholds can evolve independently from proposals. The stable proportion offered at evolutionary equilibrium increases with the maximum payoff, if proposal and acceptance thresholds are dictated by the same strategy and cannot evolve independently. If proposal and acceptance evolve independently, the stable proportion instead decreases with the maximum payoff. The stable outcome may also show substantial variation. (C) 2007 Elsevier Ltd. All rights reserved

Evolutionary Resolutions of Conflicts with Mates and Offspring

In sexual conflicts and conflicts between parents and offspring, natural selection acts in different directions on the two sides. For example, males may be selected to achieve many mates, but female fitness may be maximised by monogamy. In this thesis I study the evolutionary outcome of these situations mainly by theoretical modelling. I present an empirical investigation of mating interactions in the marine isopod Idothea baltica L., where males are aggressive to overcome female reluctance to mate, and make a game-theoretical analysis of the system to explain the temporal patterns of male and female aggression from the changing payoffs in the evolutionary conflict. I show that evolutionary arms races of quantitative traits that increase power in conflicts will be halted if their costs increase fast enough. It is also shown that if the traits involved in the arms race are behaviours, the arms race will concomitantly decrease the extent of conflict, so that the conflict is automatically resolved. It is shown that the solution of the sexual conflict over provisioning of young may influence the subsequent evolution of clutch size. A specific insect case is analysed where the conflict is resolved so that females may exploit males, and where the conflict influences social behaviour. It is further shown that in plants, the parent-offspring conflict over seed provisioning is influenced by endosperm ploidy and multiple paternity

A model of triploid endosperm evolution driven by parent-offspring conflict

The parental investment in angiosperms comprises the endosperm, a nutrient reserve that is used during seed development. The endosperm contains genes from both parents. The most common endosperm form is the 3n Polygonum-type with more maternal genetic influence than paternal, i.e. with two maternal nuclei and one paternal nucleus. The evolutionary original state is thought to be a diploid endosperm with equal influence of the parents. We focus on the evolution of the triploid endosperm and show that a gene for triploid endosperm would have an initial advantage in a population of diploid endosperm type plants, and increase to fixation. We assume that endosperm amount is controlled by endosperm genes. Then a gene causing triploid endosperm will increase the influence of the mother plant on parental investment. The production of endosperm with two copies of the maternal genes will modify the inheritance of endosperm amount and cause an increased production of seeds

The dynamics of sexually antagonistic coevolution and the complex influences of mating system and genetic correlation.

Sexual conflict has been proposed to be a mediator of speciation but recent theoretical work, as well as empirical studies, suggests that sexual conflict may also be able to prevent speciation and to preserve genetic polymorphism within a species. Here, we develop a population genetic model and study the effects of sexual conflict in a polymorphic population. The morphs mate assortatively based on different sexually antagonistic traits and females are assumed to suffer a cost when the proportion of matching males is high. We consider the model in two different mating systems; promiscuity and polygyny. Our results show that genetic polymorphism may be maintained through negative frequency dependent selection established by assortative mating and female conflict costs. However, the outcome significantly differs between mating systems. Furthermore, we show that indirect selection may have profound effects on the evolutionary dynamics of a sexual conflict

The evolution of prudent choice

Question: What mate choice strategies are evolutionarily stable when individuals vary in quality and there is competition over mates? Mathematical method: We analyse a life-history-based game theoretical model of a mating system. The evolutionarily stable strategy (ESS) may be random mating, or any kind of assortative mating dependent on the numerical values of a set of probabilities. We solve for the evolutionarily stable probability values. Key assumptions: Males of high or poor competitive ability pair with highly or poorly fecund females, and can also attempt to take over a female if she has already paired. Only one sex plays an active role in mate choice. Conclusions: We show that increased opportunities for a successful takeover favours the evolution of 'prudent' mate choice, where low-quality males reject high-quality females, and high-quality males reject low-quality females. This solution may also evolve because of high fighting costs. Alternative parameter settings lead to solutions where only one class of males discriminates between females, or when all males mate indiscriminately. We calculate the correlation between male and female quality in pairs, and show that this correlation may be positive, zero or even negative, depending on the ESS mate choice strategy

Nonrandom mating preserves intrasexual polymorphism and stops population differentiation in sexual conflict

Evolutionary conflict between the sexes is predicted to lead to sexual arms races in which male adaptations for acquiring mates (" offense" traits) are met by female counteradaptations - for example, to reduce mating rate (" defense" traits). Such coevolutionary chases may be perpetual. However, we show here that the coevolutionary process may also lead to a stable state in which multiple offense- defense trait pairs are maintained. This type of polymorphism below the species level is a result of sexual conflict in combination with nonrandom mating. Our results show that if nonrandom mating occurs with respect to male and female conflict traits, genetic correlations will act to stabilize the trait frequencies so that all morphs are maintained. We discuss the results in special relation to the evolution of female polymorphism in diving beetles and argue that the process we describe may be a general force that maintains polymorphism in other taxa as well