| Examining variation in social systems, both across and within taxa, is essential for understanding the intricate relationships among behavior, ecology, demography, and phylogeny. By studying populations of the same taxon across an array of habitats with differing selective pressures, phylogeny essentially is held constant so that environmental determinants of social structure and organization can be elucidated.; Primates are known for their diverse, complex social systems. Our closest living relatives, chimpanzees and bonobos (Pan spp.), and a distant, neotropical relative, spider monkeys (Ateles spp.), are the nonhuman primates that best exemplify the fission-fusion social system, in which the group rarely coalesces but is divided into subgroups with variable membership.; I investigated the behavioral ecology and fission-fusion dynamics of Central American spider monkeys (Ateles geoffroyi) living in Pacific lowland wet forest. Because the fluid grouping pattern of spider monkeys has been explained as an adaptation for decreasing competition over ripe fruit, their social system in productive, wetter forests may differ from those of spider monkeys living in drier forests.; I and one field assistant conducted a 14-mo line-transect survey of the forest surrounding the study site, Sirena Biological Station, Corcovado National Park, Costa Rica. Results revealed extremely high population densities. We accrued over 1200 hr of direct observation of several radio-collared focal subjects. The main focal group had the largest group and mean subgroup sizes yet recorded. Home ranges had high densities of large fig trees that provided infructescences virtually year-round. Spider monkeys ate from around 100 different plant species, but typically just several species comprised most of their diet on a monthly basis. Although group size was very large, home ranges were relatively small, likely reflecting the high density of food resources. Correspondingly, although subgroup sizes were considerably larger than at other sites, individuals did not travel very far daily to satisfy their nutritional requirements. There was no apparent relationship between number of individuals in a subgroup and how far that subgroup traveled daily. Subgroup fissions and fusions occurred frequently, at a rate higher than predicted by chance. The fission-fusion social system of spider monkeys at Sirena was very fluid. |
