INTRODUCTION

Most diurnal primates live in permanent social groups. Chimpanzees, however, exhibit a fission–fusion social structure in which individuals of the social group or community spend some time alone and the rest of the time in parties of varying composition. These, in turn, comprise subsets of the total community. In this regard, chimpanzees resemble several other species of large-bodied, frugivorous primates. A common explanation is that in such species, predation risk, a major factor in producing group living, is relaxed because of large body size. Group size then adjusts to the distribution and abundance of food sources, which vary over time (van Schaik & van Hooff 1983; Terborgh & Janson 1986; Dunbar 1988). These explanations regard plentiful food as a permissive factor for group formation. The possibility that individuals actually gain more food by joining groups through information transfer or better exploitation of resources has received less attention in primate than in other animal studies (Wilson 1975; Rodman 1988) and deserves further study.

A number of researchers have investigated variables important in controlling party formation in chimpanzees. Besides grouping at plentiful food sources, the benefits of grouping they have considered include mating prospects, measured by the presence of sexually receptive females (Goodall 1986; Sakura 1994; Boesch 1996; Wrangham 2000; Anderson et al., Chapter 6; Mitani et al., Chapter 7; Wrangham, Chapter 15), cooperative hunting (Boesch 1996) and safety in numbers, either from predation (Boesch 1991), conspecific threat (Bauer 1980), or human disturbance (Sakura 1994). Research investigating party size in chimpanzees has focused on limits to party size, assuming that benefits of grouping are always high enough that observed patterns are a product of the costs (Wrangham 1979, 2000).