Ecological sex differences in wild orangutans

Abstract

Sex differences in diet, ranging, and activity budgets (‘ecology’) can have two plausible, non-exclusive causes: differential needs due to reproduction in females and differences in body size, as well as sex differences in sociosexual strategies, usually because males are forced to travel more widely or minimize feeding time relative to females. The authors of this chapter evaluated these two hypotheses by examining sex differences in the ecology of orangutans inhabiting a Sumatran swamp forest, using two different methods. The greater reproductive burden on females is reflected in their spending more time per day feeding overall, more time foraging on insects, and less time resting, but females did not engage more in tool-assisted foraging or less in acquiring vertebrate meat. Despite the large range of body sizes, the influence of body size on time budgets, diet and the toughness and elasticity of food items was minor. However, larger males spent more time feeding on fruit than smaller ones. The other differences between unflanged males and flanged males were more compatible with different sociosexual strategies: unflanged males moved more and travelled faster than flanged males, and had shorter feeding bouts. Thus, the overall pattern of differences largely reflects sex differences in requirements due to reproduction and male sociosexual strategies. The effects of body size on diet may be so small because tooth morphology rather than body strength determine food choice.