Summary

The common marmoset (Callithrix jacchus) is a cooperatively breeding monkey that exhibits high reproductive skew among females. At the proximate level, this high skew is maintained, for the most part, by reproductive selfrestraint in subordinates, involving specialized behavioral and neuroendocrine responses to the presence of a dominant female. When subordinates terminate this self-restraint, however, dominant females frequently control subordinates' reproductive attempts by killing their infants. Based on data collected over 20 years from both the field and the laboratory, we propose that such infanticide constitutes not only a proximate mechanism limiting subordinate females' reproductive success, but also an ultimate mechanism favoring selection for reproductive self-restraint in subordinate females. Our hypothesis is consistent with both the commitment model of reproductive skew (Hamilton 2004), in terms of pre-conception restraint, and the discriminate infanticide model (Hager & Johnstone 2004), in terms of infanticide as a mechanism driving subordinate self-restraint. Parallel, long-term field and laboratory studies of common marmosets provide powerful interdisciplinary approaches enabling investigation of mechanisms regulating female reproductive skew at a proximate level, while providing novel insight into potential ultimate causation.

Introduction

Among primates, moderate female reproductive skew, manifest as high reproductive success among a limited number of adult females in a social group, is associated with social dominance in many species (Abbott et al. 2003). Extreme monopoly of reproduction by only one or two females, however, is restricted to most, but not all, members of a single primate subfamily, the Callitrichinae (the marmosets and tamarins). These species, especially the well-studied common marmoset (Callithrix jacchus), present an opportunity to integrate both proximate and ultimate explanations of reproductive strategies in order to better understand the evolution and mechanisms of reproductive skew.