Genetic Analyses Suggest Male Philopatry and Territoriality in Savanna-Woodland Chimpanzees (Pan troglodytes schweinfurthii) of Ugalla, Tanzania

Socioecological models predict that as resources become more sparsely distributed, primate populations will occur at lower densities and this demographic shift may have some effect on social structure. In savanna-woodland habitats, chimpanzees live at lower densities and in larger home ranges compared to forested habitats, presumably because of more widely dispersed food availability. These factors may result in chimpanzee home ranges being economically undefendable, leading to a reduction in male philopatry and territoriality. To test this hypothesis, we genotyped 237 fecal samples collected from Ugalla at 12 autosomal and 13 Y-chromosome microsatellite loci. We considered individuals that were sampled together at the same place and time to have been associating in the same party; with repeated sampling of an individual in different places and with different associates, we established which individuals belonged to the same community and the community locations. We identified 44 females and 69 males, carrying four different Y-chromosome haplotypes. One Y-chromosome haplotype was prevalent and found throughout the study site. The three rarer haplotypes occurred in spatially discrete clusters, which corresponded with the locations of communities identified through analysis of autosomal genotypes. Together with an observation of an aggressive interaction, these results suggest that, like chimpanzees living in species-typical forest habitats, the chimpanzees of Ugalla are organized into male philopatric, territorial communities, and that this social structure is reliably expressed under a variety of ecological conditions. This study reminds us that primate social structures may be adaptive across a range of habitats, and/or subject to phylogenetic constraint.