The fission-fusion structure of animal groups may reduce direct feeding competition between group members but may require special interactions between group members to maintain social bonds. This study used behavioral observations of two habituated groups of black-handed spider monkeys (Ateles geoffroyi ) in the northeastern Yucatan peninsula to explore these two aspects of their fission-fusion social structure. Association patterns between males show signs of active companionship that is probably based on affiliative social interactions. Females are found together in subgroups at rates that are indistinguishable from a random expectation, and the absence of social interactions between them suggests that they are simply converging at feeding spots, without any preference for particular female social companions. Males and females show signs of active avoidance, associating at lower rates than would be expected by chance. These association patterns may be influenced by long distance vocal communication, as suggested by the results of simultaneous observations of two subgroups, as well as those of playback experiments using the species' most frequent vocalization, the whinny. Subgroups approach each other more often when they are within hearing range, and individuals in those subgroups vocalize more often, than when farther apart. Playback experiments showed that a close associate of the caller, who is not present in the same subgroup, is more likely to approach the speaker than another, less closely associated monkey. The size of subgroups shows no change between seasons with high fruit abundance and other seasons, and individuals do not travel more when in large subgroups compared to smaller subgroups. Together, these results suggest that social relationships in spider monkeys are maintained in part through vocal interactions between individuals in different subgroups. This in turn may influence the size and composition of subgroups in a manner that is unrelated to the abundance of food within a patch. Indirect feeding competition may still occur, probably not within subgroups but between all monkeys living within a given area.
Thesis (Ph.D. in Biology) -- University of Pennsylvania, 2001. Source: Dissertation Abstracts International, Volume: 62-02, Section: B, page: 0733. Supervisor: Dorothy Cheney.