Summary

Although several models of reproductive skew have been proposed, “transactional” models, on the one hand, and “tug-of-war” or “incomplete control” models, on the other hand, are most commonly debated. The former hold that dominants control group size by yielding “incentives” (a share of total group productivity) to subordinates, while the latter advance the idea that the dominant's control over one or more subordinates is incomplete. While high skew has been identified in several mammalian societies, most research on this topic shows that social mammals are likely to display intermediate, low, or variable reproductive skew. In an attempt to explain this pattern of results, the present chapter shows that mammals have evolved to cope with heterogeneous environmental regimes (abiotic and biotic), yielding a eutherian adaptive complex comprised of endothermy, relative brain enlargement, and behavioral flexibility. It is argued that these adaptations to environmental unpredictability favor the evolution of alternative phenotypes as well as situation and condition-dependent responses decreasing the likelihood that dominants will be able to monopolize subordinates, including females, and that high skew will be observed. Additional research is required to highlight similarities and, most important, differences in the routes to sociality between insects, birds, and mammals (see Vehrencamp 1979).

A simple mathematical model is presented linking reproductive suppression by a dominant to his/her influence on a subordinate and consequent ability to monopolize the subordinate. This treatment shows that, depending upon conditions, both types of skew models are realistic. Reproductive skew is discussed in relation to allocation decisions and the different reproductive tactics of female and male mammals.

We have collaborated on projects since 2001, and it became evident to us by 2004 that a book about reproductive skew incorporating theoretical, empirical, and review chapters might be timely. Because of our own research specializations, and because a large body of literature exists on skew in social insects, we generated a plan to prepare a volume on reproductive skew in vertebrates, approaching Cambridge University Press with a proposal. The present text represents our attempt to provide a “state of the art” overview of reproductive skew in vertebrate societies by some of the most active and highly regarded researchers in this field. It is our intention to highlight the most fundamental questions for students of reproductive skew, to assess the strengths and weaknesses of skew models, to critically evaluate skew in insect societies and skew in social vertebrates, and to identify important directions for future theoretical and empirical work.

In her foreword to our volume, Sandra Vehrencamp has provided a brief overview of the history, theory, and empirical highlights of models of reproductive skew. It is clear from her personalized account that advances in the study of reproductive skew (the within-sex partitioning of reproduction within social groups) were presaged by early work in behavioral ecology demonstrating a relationship between dispersion and quality of limiting resources, in particular, food and nesting sites, as well as variations in social behavior, social organization, and mating systems within and between populations. The relationship between these factors is analyzed in models of reproductive skew that attempt to explain the partitioning of reproduction among individuals of the same sex in animals and offer a theoretical framework for understanding the formation of social groups.