Observations on the sexual behavior and birth seasonality of proboscis monkey (Nasalis Larvatus) along the lower Kinabatangan, Northern Borneo

Abstract

Sexual behavior of Nasalis larvatus was observed along the Lower Kinabatangan River in eastern Sabah, Bornean Malaysia, during a two-year field study. Eight sexual mounts and two non-sexual mounts in one-male groups were recorded using scan sampling and event sampling. All observed copulations comprised single mounts, with copulatory bouts averaging 25 seconds. The adult female was the solicitor when solicitations were observed with certainty. Sexual harassment by older infants and young juveniles of copulating pairs was also observed. Nasalis larvatus infants were observed almost throughout the study. No correlation could be established between birth dates and environmental parameters, although there appeared to be more births during the wet season.