Residential status and seasonal movements of wild orang-utans in the Gunung Leuser Reserve (Sumatera, Indonesia)

Abstract

Data on the presence of wild orang-utans, Pongo pygmaeus, in the Ketambe area (Sumatera, Indonesia), collected over 12 consecutive years, were analysed to study population structure and residential status. Forty-three individuals could be identified during the study period. The socionomic sex ratio was close to unity, at least when subadault and adult males were considered as one category. The adult sex ratio was skewed towards females. A clear distinction between residents and non-residents could be made, but there was apparently no transient sex and no positive correlation between the densities of non-resident males and local females capable of conceiving. Hence, the hypothesis that non-resident males wander to locate receptive females was not supported. Instead, the data reveal seasonal movements and show that non-residents of both sexes are equally attracted to the study area when food is abundant. Two (not mutually exclusive) mechanisms that may explain how ranging of non-residents is tuned to food available are proposed.