Evidence for dominant wild female chimpanzees investing more in sons

Parents are expected to invest more resources in the offspring gender that promises more grandchildren. In a variety of vertebrate species skewed sex ratio at birth and differential parental investment in sons and daughters have been documented. Wild chimpanzees, Pan troglodytes, living in the Tai. National Park, Cote d'Ivoire, were followed for 15 years. This community followed the typical species pattern in that males showed natal philopatry and the sex ratio at birth was almost 1:1. An analysis of 33 inter-birth intervals revealed that dominant females invested about 2 years more in sons, whereas subdominant females invested about 11 months more in daughters. The first difference is significant. Sons of dominant females had higher survival than other youngsters. The benefit of such a facultative investment is discussed. The absence of such a differential investment by mothers in other chimpanzee populations is compatible with an explanation based on variations within the female-biased dispersal pattern in this species and the possible role of maternal condition. (C) 1997 The Association for the Study of Animal Behaviour.