The Social Unit of Wild Chimpanzees

A Reconsideration Based on the Diachronic Data Accumulated at Kasoje in the Mahale Mountains, Tanzania

Abstract

A reconsideration on the social structure of wild chimpanzees was made on the basis of ranging and association patterns of discriminated individuals inhabiting Kasoje in the Mahale Mountains, Tanzania. Records of roll-call for each chimpanzee at the feeding station and in the bush have been maintained continuously since 1966. The bulk of the data analyzed in this paper was collected at the two permanent feeding stations, Myako Camp and Kansyana H. Q., as well as in the bush between October 1973 and March 1974 and between December 1976 and November 1978.
As for the males two unit-groups (K- and M-groups) were easily identified by their ranging and association patterns: the home range of a unit-group was accordingly defined as the area utilized by all its males and each unit-group was found to occupy a partially separate range, which was entirely consistent with the data reported by Nishida and Kawanaka (1972).
On the other hand, the following five categories for females were perceived according to their ranging patterns and formation of associations with the male (s) of different unit-groups:
(1) Females whose ranges lie within the home range of K-group and whose formation of associations is restricted to those with the male (s) of this group.
(2) Females whose ranges lie within the home range of K-group but who associate alternately with the male (s) of K- and M-groups at the overlapping area utilized by both groups.
(3) Females who use the home ranges of both K- and M-groups and form associations alternately with the male (s) of respective unit-groups.
(4) Females whose ranges lie within the home range of M-group but who associate alternately with the male (s) of M- and K-groups at the overlapping area utilized by both groups.
(5) Females whose ranges lie within the home range of M-group and whose formation of associations is restricted to those with the male (s) of this group.
Diachronic data of Kasoje indicate that some females have shifted their ranges of their own volitions from within K-group's range to within M-group's and that no female has been continuously recorded for long period to remain in categories (2), (3), and (4) while females in categories (1) and (5) are relatively stable. These facts strongly suggest that the aforementioned five categories can be regarded as female stages, not idiosyncrasies, and that categories (2), (3), and (4) can be considered unstable transitional stages of female transfer between unit-groups.
Thus chimpanzee unit-groups are better described as bisexual units. The theory proposed by Wrangham (1975, 1979) that the community (=unit-group) concept should be limited to males is discarded here because it only explains phenomena observed in a relatively short-term study.