Notes on the sexual behavior of the proboscis monkey (Nasalis larvatus)

Proboscis monkey (Nasalis larvatus) sexual behavior was studied at the Tanjung Puting National Park in Kalimantan Tengah, Indonesia, for 12 months. Data were collected on one-male groups and all-male groups using event sampling for copulations, presents, and same-sex mounts. Proboscis monkeys follow typical colobine patterns of sexual behavior with females soliciting copulations through head shaking, pout face, and the presentation of the hindquarters. Males appear to be single-mount ejaculators. Harassment of copulating pairs and female-female competition for a mate were observed. Same-sex mounts (female-female and male-male) were also observed. Additional data are needed to evaluate their function.