Abstract
Group fission in non-human primates has long been proposed to result from interactions between ecological and social factors. Several studies have documented possible causes for group fission, but its proximate causes and ultimate adaptive values are not yet fully understood. We have examined the existing hypotheses on fission from long-term demographic data of Formosan macaques inhabiting the lowland rainforest at Mt Longevity, Taiwan. Five cases of fission occurred in four social groups. We have recorded two types of fission: one involving the separation of a high-ranking adult male and multiple adult females, the other initiated by adult females from main groups. Five adult females immigrated and emigrated a few times between the main and branch groups (oscillation) in three fission events. Data presented in this study are consistent with the prediction that low-ranking females split from main groups when their fitness costs increase due to ecological pressure or population growth. However, their reproductive success may decrease after fission due to a high rate of intra-group competition. Nevertheless, it is beneficial for males to be involved in fission since this increases reproductive benefits by decreasing the sex ratio in small newly formed groups.
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