Grouping of the pygmy chimpanzees

The group formation inPan paniscus is flexible as inP. troglodytes. A group consisted of 16.9 chimps on the average, and a large groups of more than 30 chimps were often observed. The most frequent groups are of the mixed group type, which occupied 80% of the observed cases. The group size varies in accordance with the seasonal changes of the abundance and distribution of food resources, but in general it is larger than that ofP. troglodytes. The socionomic sex ratio does not seem to be highly correlated with the group size, and was around 76 on the average. And neither the proportion of females with infants in a group nor that of estrous females seems to be correlated with the group size. Therefore, the group composition appears to be rather uniform. This uniformity may be attributable toP. paniscus's concentrative nature and high male/female affinity. In comparison with the other African anthropoid apes,P. paniscus shows less sexual differentiation in its social structure. This study was financed by the Scientific Research Fund of the Ministry of Education, Science, and Culture of the Japanese Government.