The Social Organisation of a Population of Sumatran Orang-Utans

Female orang-utans in a Sumatran swamp forest live in large, but stable, and widely overlapping home ranges. They preferentially associate with some of their female neighbours, possibly relatives, to form socially distinct clusters that also experience reproductive synchrony. Sexually mature males range more widely than females, but among them the dominant adult male has a relatively more limited range. His ranging and that of the subadult males reflect the local abundance of sexually attractive females. The other adult males tend to avoid these concentrations and focus on areas away from the dominant male. Females show philopatric tendencies. Male-biased sex ratios at birth give way to heavily female-biased sex ratios among adults. This suggests a net loss of males as they mature, due either to excess male mortality (e.g. by male mating competition), excess male dispersal from the population or a combination of both. We conclude that the orang-utan social organisation is best described as a loose community, showing neither spatial nor social exclusivity, consisting of one or more female clusters and the adult male they all prefer as mate.