Low Resistance of Senior Resident Females Toward Female Immigration in Bonobos (Pan paniscus) at Wamba, Democratic Republic of the Congo

Female aggression against outgroup conspecifics is an important aspect of intergroup relationships among female primates. Intense aggression from resident females toward immigrant females suggests that the costs of intrasexual competition outweigh the benefits of group living and has been reported in some species with female transfer. In bonobos (Pan paniscus), however, immigrant females are likely to integrate smoothly into an unfamiliar group through affiliative interactions with specific older females. We hypothesized that older resident females gain an indirect benefit from female immigration by increased mating opportunities for their philopatric sons. We examined the effects of 1) age and tenure and 2) the presence of adolescent or adult sons on the probability of aggression by resident females against immigrant females. We collected 73 instances of dyadic agonistic interactions between 14 female bonobos at Wamba, Democratic Republic of the Congo, for five different periods between November 2014 and June 2018, which included four immigrant females with a group tenure of <2.5 years. The female dominance hierarchy correlated with age and tenure, the slope of the hierarchy was weak but statistically significant, and the hierarchy was not linear. We found that the rate of aggression against immigrant females decreased with age in resident females but was unrelated to the presence of mature sons. Our findings suggest that the cost imposed by female immigration varies among resident females, and that social tolerance to immigrant females may be associated with low feeding competition rather than any future benefits.