The context of female dispersal in Kanyawara chimpanzees

In most social mammals, members of either one sex or both leave their natal group at sexual maturity. In catarrhine primates, male emigration is the predominant pattern. Female philopatry facilitates cooperation among kin, and female reproductive success is influenced by these relationships. Chimpanzees (Pan troglodytes) are unusual in that dispersal is almost exclusively by females. While plausible ultimate hypotheses can explain this dispersal pattern, the proximate causes of female dispersal are unknown and warrant examination due to variation in dispersal behaviour and the associated high costs of immigration. In this study, we examine the behavioural and hormonal context of female dispersal in chimpanzees of Kibale National Park, Uganda, in order to understand variation in dispersal patterns and gain insight into functional explanations. Using over 10 years of behavioural, endocrinological, and demographic records, we examined the significance of 5 potential predictors of the timing of dispersal: (1) maturational state; (2) association patterns; (3) mating patterns; (4) physiological stress; and (5) feeding ecology. Female dispersal was not strictly predicted by chronological or gynaecological age, and dispersal did not correlate with shifts in glucocorticoid levels. We found no evidence that females avoided mating in their natal group, even with known relatives, suggesting that inbreeding avoidance is not a sufficient proximate explanation for dispersal in chimpanzees. Instead, variation in social development and the availability of energy for dispersal were implicated and necessitate more intensive examination along with subtler variation in maturational trajectories.