Multilevel societies facilitate infanticide avoidance through increased extrapair matings

In mammals characterized by a mating system in which a single male monopolizes reproduction, infanticide is reported to occur following a male take-over, often resulting in females returning to oestrus more rapidly than if their infant has survived. However, over the course of a 17-year study of golden snub-nosed monkeys, Rhinopithecus roxellana, a polygynous colobine species, we found no behavioural or suspected evidence of infanticide. Based on social network and genetic analyses, we found that more than half of the infants were sired as a result of extrapair matings (EPMs). Female golden snub-nosed monkeys initiated EPMs with bachelor or neighbouring resident males. These were considered ‘sneaky’ copulations because they did not occur in the presence of the leader male of the female's one-male unit (OMU). We suggest that through a process of paternity confusion, females decreased the likelihood of infanticide after a male take-over. In contrast, in a group of the same species in which the number of bachelor males was artificially reduced, EPMs were infrequent and four cases of infanticide were observed, three of them following a male take-over. Golden snub-nosed monkeys reside in a large multilevel society that develops by the fusion of several independent OMUs to form a breeding band, which is characterized by increased social cohesion and intrasexual tolerance among leader males. Females play an active role in mate choice, and engage in a diverse set of mating tactics, including counterstrategies such as EPMs to avoid infanticide. These results offer new insights into the diversity of behavioural strategies facilitated by complex social structures in nonhuman primates, as well as other social mammals.