Pygmy chimpanzee social organization: Variation with party size and between study sites.

Descriptions of social organization based on interactions are difficult for fission-fusion primates, such as pygmy chimpanzees, as interactions may depend on association in parties. Frequencies of male-male and male-female affiliative and female-male and female-female aggressive interactions among Lomako pygmy chimpanzees occurred in proportion to the presence of each sex in parties. Male-male aggression and female-male affiliation, however, were more frequent than expected on the basis of party membership. Females with small swellings received more grooming and less mating than expected. Patterns of interactions at Lomako also varied with party size. Female-female affiliation predominated in small parties, while male-female affiliation predominated in larger parties. This trend parallels observed differences between the Lomako and Wamba study sites. Male-female affiliation is more frequent at Wamba where party sizes are larger. Differences between study sites may also reflect provisioning, habituation, predator threat, and habitat. Provisioning at Wamba may result in higher frequencies of aggression among males and lower levels of aggression among females. Comparison between earlier and later Lomako studies suggests that increased habituation is associated with greater differences from, rather than more similarity to, results from Wamba. Differences between Lomako and Wamba in habitat, provisioning, and human (but not non-human) predation, by affecting party size and composition, most likely account for the observed differences in social organization between the two sites.