Dispersal patterns among three species of squirrel monkeys (Saimiri oerstedii, S. boliviensis and S. sciureus): II. Within-species and local variation

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AbstractThree long-term field studies, together with numerous supplementary sources of information, demonstrate that the Neotropical squirrel monkey, genus Saimiri (Primates: Cebidae) are distinguished among mammals by the wide divergence in dispersal patterns among congeners. Both sexes of Saimiri sciureus at Raleighvallen in the Central Suriname Nature Reserve undertake dispersal on several to many occasions during their lifetime. Male dispersal and female philopatry characterize S. boliviensis studies at Manu, Peru. Among S. oerstedii, studied at Corcovado and other locations in Costa Rica, females disperse and males are philopatric. This is the second in a series of three companion reports investigating patterns and processes relevant to dispersal in these three species of squirrel monkeys. The first report, I. Divergent costs and benefits (Boinski et al., 2005), predicts the direct and inclusive fitness costs and benefits structuring species, sex and individual dispersal strategies among squirrel monkeys. III. Cognition (Boinski, 2005), the final report comprising this monograph, considers the possible cognitive mechanisms underlying dispersal among squirrel monkeys and other taxa, and suggests useful strategies to collect and interpret additional data from laboratory and field contexts.Here we evaluate the sources and potential magnitude of variation in dispersal strategies within each squirrel monkey species. For all three congeners, local edaphic and anthropogenic regimes of habitat disturbance probably represent the major source of within-species variance in the density of wild populations. Squirrel monkey population density, all else being equal, positively increases with the intensity of habitat disturbance. New evidence suggests that in addition to edaphic and recent historical disturbance regimes, in some localities in the Neotropical lowlands anthropogenic disturbance caused by pre-Columbian Amerindians remains a potent factor enhancing squirrel monkey numbers.Squirrel monkeys are predicted to exhibit density-dependent behavioural responses. In turn, these responses are expected to modulate population-level dispersal outcomes in several predictable axes. Major between-site variation in dispersal strategies, however, is unlikely for either sex among S. oerstedii or S. boliviensis. Although all natal male S. sciureus almost certainly disperse before or at the time of sexual maturity, the proportion of females emigrating from a S. sciureus troop appears more variable, dependent on local levels of within-troop competition for food. In any year or season, those mature and immature female S. sciureus with high priority access to food resources are least likely to disperse.