Behavioral Changes in Wild Male and Female Sumatran Orangutans (Pongo pygmaeus abelii) During and Following a Resident Male Take-Over

Abstract

Research on wild orangutans has been carried out in the Ketambe research site, Sumatra, Indonesia, since 1972. This paper focuses on the last five years during which 48 individuals were observed in the research area. In 1991, the adult male who had been the resident male for almost 18 years (with only a short interruption) was challenged and defeated by a younger adult male. Three fights between the two males were observed in detail. During the interactions, only the challenging male made long calls. Only females with offspring older than 10 years associated with the new male before the take-over. The most dominant female in the area which often associated with the new male tended to lead him toward the resident male. These encounters resulted in the final defeat of the old male. This dominant female copulated with the new resident male only after the take-over and then other females with younger offspring also associated with him. The events related to this take-over are compared with the present situation in which a newly adult male, who has been in the area as a subadult since 1972, is challenging the resident male.