An agent-based model of coercive female transfer in a multilevel society

Abstract

Hamadryas baboons, Papio hamadryas , differ from almost all other primate species in that males separate female kin by aggressively coercing females into joining their one-male units. Male behaviour thus appears to be the sole determinant of social structure, shaping patterns of relatedness. However, recent genetic studies have found that female relatedness within one-male units (OMUs) is higher than across OMUs, contrary to expected results of male behaviour. This has caused some researchers to speculate that female hamadryas baboons have some influence over dispersal. In this study, we used an agent-based model to parse how the hamadryas social system influences OMU female relatedness. We hypothesized that (1) the multilevel social structure in hamadryas baboons leads to greater-than-random relatedness in OMUs and that (2) subtle female preference leads to even higher relatedness, similar to the pattern observed in genetic data. Our agent-based model of hamadryas baboon society and dispersal was based on available field data and ran for 150 years in discrete 6-month intervals. To test hypothesis 2, we implemented manipulations mimicking female kin attraction, which increased females’ chances of being transferred into a unit containing closely related females. As output of every simulation, pedigree relatedness of adult female dyads within and across OMUs was calculated. We validated our model by comparing patterns of group size, population size, adult sex ratio and dispersal rates with available field data. The results of our simulations supported both hypotheses. Relatedness within OMUs was slightly higher than across OMUs when females had no influence over dispersal. When we implemented our scenarios of female influence on dispersal, the relatedness within OMUs increased even more, approaching the effect size of genetic data from the field. We conclude that, despite conflicting interests, a combination of male and female behaviour create a complex pattern of gene flow in this unique multilevel social system. • We used an agent-based model to examine genetic relatedness in hamadryas baboons. • Multilevel social structure with male coercion slightly increased unit relatedness. • Female kin attraction produced the empirically observed pattern of relatedness. • Hamadryas females appear to disperse in parallel, despite strong male coercion.