Rank & Paternity in Chimpanzees

a young chimpanzeeThe Effect of Rank on Paternity among Male Chimpanzees

Principal Investigator: Nicholas Newton-Fisher
Co-investigators:
M. Emery Thompson (Harvard)
V. Reynolds (Oxford)
C. Boesch (Max Planck)
L. Vigilant (Max Planck)

Project dates: 2003-2007

Paternity studies of wild chimpanzees can provide particular insights into two aspects of chimpanzee behaviour and social demography, examining (1) the effectiveness of alternative male mating strategies to investigate the fitness benefits of male competition for social rank, and (2) the extent of extra-group paternity and thus the effectiveness of male efforts to exclude rivals from their territory. Observational studies across populations and subspecies of chimpanzees (Pan troglodytes) have described a strongly male-philopatric society with linear dominance hierarchy of males. One of the biggest advantages to high social rank is expected to be increased access to high-quality mating opportunities.

The priority of access model, a kind of “ideal free” distribution for obtaining paternity, predicts that males’ probability of paternity is determined by their rank and the number of females simultaneously in oestrus. Boesch et al. (2006, AJPA) found that paternity observed among the chimpanzees of the Taï forest was close to that predicted by the model. However, while the model’s predictions that the number of competitors and the monopolisability of females influence a high-ranking male’s mating success are quite reasonable for chimpanzees, the reliability of this model is challenged by complexities of chimpanzee socioecology.

In this study we analyzed the distribution of paternity in the Sonso community of wild East African chimpanzees (P. t. schweinfurthii) in the Budongo Forest, Uganda. The Sonso community had a larger number of competing males than other communities in which paternity has been investigated, providing an opportunity for a strong direct test of the priority-of-access model. We assessed the paternities of 24 offspring and examined the characteristics of sire versus non-sires. We used these results to investigate the relationship between male social rank and paternity and test predictions of the priority of access model, together with alternative models. The study community occupies a relatively small home range, and is surrounded by other community, offering potential for extra-group fertilisations. The study community is surrounded by other communities in a small home range, and so we also examined the extent of extra-group paternities as an indicator of the success of cooperative territoriality as a male reproductive strategy.

Publications

Newton-Fisher, N. E., Emery Thompson, M., Reynolds, V., Boesch, C., & Vigilant L. (2010). Paternity and social rank in Budongo forest chimpanzees. American Journal of Physical Anthropology 142: 417-428.

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