The Hidden Costs of Sociality: Intra-Group Variation in Feeding Strategies in Sumatran Long-Tailed Macaques (Macaca Fascicularis)

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Abstract
Groups of long-tailed macaques (Macaca fascicularis) are often fissioned into a large main party and one or more small parties. We studied the size and composition of the main party in relation to fruit supply and the ecology of the various age-sex categories in order to establish the cause of this fissioning. The size of the main party varied not only because the size of available fruit trees varied but also because the various age-sex categories were so different in their feeding strategies that they sometimes went separate ways. Our observations suggest that body size and sex profoundly affected food requirements and the risks attached to leaving the main party. We concluded that adult females left the main party mainly to escape from feeding competition and attempted to reduce travel time when they had infants so as to minimise the costs of carrying infants. Males, by contrast, left above all because of their different food requirements; subadult males left to eat more fruit than the others, adult males mainly to eat bigger fruit than the others. This intra-group variation in feeding strategies constitutes a usually hidden cost to sociality.

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