Abstract

1 The fission-fusion nature of pygmy, or bonobo chimpanzee (Pan paniscus) social organization allows an examination of the influence of ecological and social factors on party size and composition. 2 Party size varies with the size of the food patch. The estimated amount of food removed proved a useful measure for smaller patches. Party size increased with the size of the food patch and large food patches appeared to contain superabundant food. 3 Party composition changed with party size. The male proportion of the party increased with party size. Parties of pygmy chimpanzees were based on cores of females that regularly associated; males appeared to be attracted to larger parties in order to maintain proximity to females. 4 The major difference between the female-based social structure of Pan paniscus and the male-bonded P. troglodytes may be related to differences in patch size distributions. 5 The affiliative female homosexual behaviour of genito-genital rubbing observed in this species of chimpanzee was also correlated with food abundance.