Male orang-utan bimaturism and reproductive success at Camp Leakey in Tanjung Puting National Park, Indonesia

Unlike most mammals, male orang-utans exhibit bimaturism, in that mature individuals express one of two distinct morphological forms. Socially subordinate, ‘unflanged’ males are comparable to females in their size and facial morphology, while socially dominant ‘flanged’ males exhibit extreme sexual dimorphism and secondary sexual characteristics, primarily in the form of cheek pads (‘flanges’). Although male ornaments in other species are often phenotypically plastic, such characteristics in orang-utans are irreversible—and, given that both morphs are sexually mature and can father offspring—their adaptive significance remains unclear. We determined paternity of orang-utans at Camp Leakey in Tanjung Puting National Park, within the home range of one long-term dominant male, Kusasi, before, during and after his period of dominance, in comparison with subordinate male conspecifics. We found that Kusasi fathered substantially more offspring conceived during his dominant period than any other male and that socially subordinate, unflanged males only fathered offspring during periods of rank instability. We conclude that orang-utan male bimaturism is consistent with an evolutionarily stable reproductive strategy and that reproduction within the range of a dominant, flanged male is highly skewed in his favour, while unflanged males may largely wait for reproductive opportunities.