Effects of sexual dimorphism in body size on feeding postural behavior of Sumatran orangutans (Pongo pygmaeus)



Male orangutans (Pongo pygmaeus) weigh about twice as much as females. Observations in northern Sumatra of adults of both sexes (one male, three females) feeding in the same trees reveal the effects of body size dimorphism on feeding behavior: The male tended to use larger branches than the females, and to employ above-branch postures (sitting and standing) with greater frequency. The females employed suspensory under-branch postures more often. When feeding techniques were variable, the male tended to pull in branches to detach food with the mouth, whereas the female plucked more fruit by hand. By controlling for postcranial morphology and habitat structure, these results provide the first rigorous quantitative test of predictions about the effects of body size on primate positional behavior, and raise further questions about sexual diethism in feeding postural behavior of primates of varying absolute size.