The proboscis monkey prefers to consume immature leaves over older ones. Group sizes range from 3 to 32 individuals. This a diurnal and semi-terrestrial species. At Gunung Palung Nature Reserve the favorite trees used for sleeping were the dungun, Heritiera sp. There are two levels to the social system of the proboscis monkey.
Females do most of the grooming in the group.
Males leave their natal group, but some females also leave. These calls are made by adult males and this functions to calm another member of group down. These calls are made by adult males of the group. This functions as an aggressive call which threats other members of the group, and is also made in the presence of predators. These calls are given by juveniles of both sexes and adult females.