The socioecology of fission-fusion sociality in Orangutans.

Fission-fusion systems can have the group or the individual as their primary unit. In group-based fission-fusion systems, predation risk reduction is the major benefit to grouping, in the individual-based ones the benefits are likely to be primarily social. Orangutans, like chimpanzees, are examples of an individual-based fission-fusion species. The orangutans inhabiting a Sumatran swamp forest (Suaq Balimbing) are more likely than elsewhere to form travel parties. As elsewhere, the main benefits of grouping are social: mating opportunities, protection from harassment and socialization of infants. Most animals also incur costs, but these are relatively low at Suaq Balimbing due to the high productivity of the swamp. Costs seem to be disproportionately high for females with mid-sized infants, who avoid parties.