This section of Behavioural Diversity in Chimpanzees and Bonobos addresses two topics: (1) the influence of ecological and social factors on party size, party composition and intercommunity relations, and (2) male relationships. These two topics are related to one another in a number of ways. Ecological factors (mainly food abundance and dispersion) underlie the fission–fusion social organisation found in chimpanzees and bonobos. In forming and reforming parties, chimpanzees and bonobos are influenced by the presence of estrous females, as well as by food. Male relationships within and between communities are mainly shaped by the intensity of intragroup rivalries and intercommunity territoriality.

A number of hypotheses have been advanced relating ecological variables to social ones. Fission–fusion social organisation itself has been seen as a response to food abundance and dispersion, with animals seeking to adjust competition for food to the changing pattern of food available. At the same time, individuals are engaged in competition for dominance and for mating partners and, as shown in the following chapters, these factors have a determining effect on the size and composition of parties within the fission–fusion system.

Anderson et al. (Chapter 6) set out to test factors influencing fission–fusion grouping in chimpanzees with data from their research in the Taï Forest, Ivory Coast. They note a number of factors that influence party size: food patch size, habitat-wide food abundance and dispersion, presence of estrous females, location within the range (core versus periphery), and activity (hunting, foraging, travelling, nesting).