Identifying natural grouping structure in gelada baboons: a network approach

We use network analysis of co-herding and nearest-neighbour data for harems from several study populations to examine the natural grouping patterns of gelada baboons, Theropithecus gelada, a molecular fission-fusion primate species. We show that the association patterns of the harems reflect grouping levels of ca. two, ca. four, ca. seven and ca. 14 harems (equivalent to ca. 30, ca. 50, ca. 100 and ca. 200 individuals). These patterns are stable across both time and space. Analysis of the distribution of harem sizes across four study populations suggests that these grouping levels reflect the processes that influence harem fission, combined with a viscosity reflecting time since fission. We suggest that the natural social grouping for this species is a set of two to three harems that are the product of recent harem fission, in line with predictions from the social brain hypothesis, and that successive fission events over time cumulatively give rise to the other grouping layers. While the grouping of ca. 100 animals has previously been identified as the 'band', the ca. 50 layer was previously unidentified. We do not know what its function is, although it closely resembles typical Papio troops in size.