Common marmosets are cooperatively breeding monkeys that exhibit high reproductive skew: most subordinate females fail to reproduce, while others attempt to breed but produce very few surviving infants. An extensive dataset on the mechanisms limiting reproduction in laboratory-housed and freeliving subordinate females provides unique insights into the causes of reproductive skew. Non-breeding adult females undergo suppression of ovulation and inhibition of sexual behaviour; however, they receive little or no aggression or mating interference by dominants and do not exhibit behavioural or physiological signs of stress. Breeding subordinate females receive comparable amounts of aggression no non-breeding females but are able to conceive, gestate and lactate normally. In groups containing two breeding females, however, both dominant and subordinate breeders kill one another’s infants. These findings suggest that preconception reproductive suppression is not imposed on subordinate females by dominants, at a proximate level, but is instead self-imposed by most subordinates, consistent with restraint models of reproductive skew. In contrast to restraint models, however, this self-suppression probably evolved not in response to the threat of eviction by dominant females but in response to the threat of infanticide. Thus, reproductive skew in this species appears to be generated predominantly by subordinate self-restraint, in a proximate sense, but ultimately by dominant control over subordinates’ reproductive attempts.
At least seven cases of infanticide by females other than the mother have been observed in wild groups of common marmosets (Callithrix jacchus), with several more cases described for captive groups. Infanticide by females other than the mother has not, however, been documented for wild groups of other callitrichine species. Why might such overt aggression toward infants occur in one species and not others? In the common marmoset, a variety of social, reproductive and ecological characteristics – including short inter- birth intervals (and the resulting potential for overlapping of pregnancies and births), habitat saturation, small home ranges, and low cost of infant care (including decreased travel costs and short dependency periods compared to other callitrichines) – may contribute to an increased likelihood of two breeding females being present in a group, which in turn gives rise to the potential for competition between breeding females and ultimately to infanticide. These conditions are less common in wild groups of most other callitrichines species. All callitrichines balance the need for cooperative care of young with the reproductive competition that results from limited reproductive opportunities; however ecological and social conditions appear to tip the balance toward infanticide more frequently in common marmosets than in other callitrichine species.