Genetic investigation of an unhabituated, savanna-woodland chimpanzee (Pan troglodytes schweinfurthii) population in Ugalla, western Tanzania
Socioecological models derive from the field of behavioral ecology, wherein social strategies are seen as adaptive responses to evolutionary and environmental forces. Chimpanzee social structure is largely explained by their dietary preference, however, little is known of chimpanzee adaptations to savanna-woodland environments. This dissertation tests the explanatory power of socioecological models on a chimpanzee population occupying the savanna-woodlands of Ugalla, Tanzania. By conducting a genetic survey across 624 km2, the following predictions are tested: (1) chimpanzees will occur at lower population densities than at forested sites, reflective of more diffusely distributed resources; (2) lower population densities, resulting in theoretically indefensible larger home ranges, will decrease benefits gained from male philopatry, resulting in a more continuous distribution of Y-chromosome haplotypes than is characteristic of populations studied in forested environments; and (3) genetic diversity will be lower, compared to other eastern chimpanzee populations, due to potential isolation. The population density of Ugalla is 0.25 (CI 0.16-0.38) individuals/km2, which is ten times less than the lowest forested site density. The Ugalla males appear to remain in their natal community, as geographic clusters of rare Y-chromosome haplotypes were found, suggesting the maintenance of male-philopatric communities. Relative genetic diversity of the Y-chromosome was exceptionally low among the males, and autosomal diversity was comparable to other eastern chimpanzee populations. These results indicate that although chimpanzees in Ugalla occur at much lower densities in response to a savanna-woodland habitat, male philopatry is maintained, suggesting this social structure is a highly conservative adaptation, and one which may have been shared with our earliest ancestors as they shifted to a similar environment.