Summary

Reproductive skew theory provides a predictive theory of the extent of reproductive sharing that is expected to occur in societies consisting, at least potentially, of multiple co-breeders. Here, we discuss some of the challenges that skew theory faces as it attempts to form the basis of a unified theory of social evolution in birds. These include the problem of distinguishing potential versus actual reproductive roles, encompassing extra-group parentage and sexual conflict, predicting the distribution of group size, and determining the appropriate null model against which to test empirical results. Despite these and other problems with skew theory as currently developed, a compilation of prior studies indicates some degree of consistency with the predictions of concession or optimal skew theory. More surprisingly, a meta-analysis indicates that interspecific patterns of sociality offer reasonably good matches to the predictions of the concession model of reproductive skew. Strong support for skew theory remains lacking, and experimental tests sufficient to reject alternative skew models have yet to be performed in birds. Nonetheless, these results offer encouragement that additional theoretical work in this field may eventually yield a useful framework for understanding the remarkable diversity of avian sociality.

Introduction

An irony of kin-selection and inclusive-fitness theory (Hamilton 1964) is that these concepts, conceived primarily as a solution to the paradox of altruism, should turn out to be so useful in providing a framework for interpreting conflict among close relatives. Indeed, as more social species have been studied in detail, it has become clear that cooperation and conflict go hand in hand in most, if not all, societies.

Cooperative breeders live in groups that usually, although not always, consist of close relatives. Following Hamilton's (1964) rule, high relatedness serves to increase fitness by facilitating kin selection and the evolution of cooperative behaviors such as are frequently observed within societies of cooperative breeders. On the other hand, if high relatedness extends to opposite-sex breeders, there is the risk of inbreeding and its attendant genetic problems leading to significantly lowered fitness. How cooperative breeders resolve these conflicting selective pressures has long been recognized as a dilemma and has served as an important focus of research in this field.

Determining how societies resolve the potential problem of inbreeding has turned out to be unexpectedly difficult. First, there are semantic problems. Inbreeding is both a relative and a hierarchical phenomenon, and it is likely that inbreeding may be avoided at some levels but not others (Dobson et al. 1997). At one extreme, mates may share no known relatives going back several generations and can be considered “outbred” in that they share fewer genes identical by descent than two random individuals in the population. At the other extreme, mates may be close relatives such as siblings or a parent and offspring. Unless all genetic variability in the population has been previously lost, such individuals will be genetically more similar to each other than two randomly chosen individuals in the population, and thus their offspring will have an elevated proportion of homozygous loci identical by descent.