‘Congregating’ season versus ‘dispersing’ season

The subjects of our long-term research were K- and M-groups, whose home ranges extended through the northwestern foothills of the Mahale Mountains, occupying areas of 10 km2 and 30 km2, respectively, in the middle of the Kasoje forest (Fig. 2.1). The numbers in K- and M-groups in the 1970s were about 30 and 80 individuals. Table 2.1 shows a breakdown of the groups’ composition (Hiraiwa-Hasegawa et al. 1984). The size of M-group increased to about 100 after most of K-group’s females immigrated to M-group in 1979 (see Chapter 6).

Unlike many other kinds of non-human primates, chimpanzees from the same group do not always travel together. They break up into several smaller groups when foraging, while on other occasions they meet up, interact, and form a larger group. For example, in September and October 2002, all of M-group (54 individuals at that time) travelled together for 17 of 57 (30 per cent) observation days. This congregating differs from what has been said about the unit-groups (communities) at other sites, where the entire group never gathers together (Goodall 1986). However, a subgroup typically does not remain stable in membership, and at any given time it can have almost any composition (Nishida 1968). The size of such a ‘party’, as these subgroups are called, depends upon several factors: amount and distribution pattern of staple foods, size of food patches, absence or presence of oestrous females, absence or presence of conflict among high-ranking males, etc. (Sakura 1994; Matsumoto-Oda et al.1998; Hosaka & Nishida 2002; Lehmann & Boesch 2004; Turner 2006). Moreover, Lehmann and Boesch found that party size, party duration, and male–female association increased as study group size decreased in Taï.