Association, ranging and the social unit in chimpanzees of Mahale Mountains, Tanzania

In order to characterize the social unit in chimpanzees, about which several conflicting views have been proposed, the proximity matrix among 55 recognized chimpanzees and the range covered by each of them are examined, on the basis of data obtained at the Mahale Mountains during 12 months in 1978-1979. It is shown once again that chimpanzees have a bisexual social unit (unit-group). All animals belonged to one of the two unit-groups studied except a few cycling females (and a juvenile male accompanied by his cycling mother) which were seen to associate alternately with members of two neighboring unit-groups, covering a whole range of one or even two unit-groups.