Notes on the Social Behaviour of Wild Proboscis Monkeys (Nasalis larvatus)

This paper presents notes collected during 21 months of study on the social behaviour of proboscis monkeys at Samunsam Wildlife Sanctuary, Sarawak, Malaysia. All mixed-sex groups seen were harems, and average group size was nine. All-male groups also occurred. Harems were spatially cohesive, but all-male groups were more scattered. Harems changed their composition frequently with both females and males switching between groups, although infants remained with their mothers for more than a year, Each haram slept close to another one by the river on most nights. Grooming between adults was rare. Mating and births occurred throughout the year, but with pronounced peaks. Allomothering was recorded, and group movements seemed to be led by the females, not the male.