The proboscis monkey (Nasalis larvatus) or long-nosed monkey, known as the bekantan in Malay, is a reddish-brown arboreal Old World monkey that is endemic to the south-east Asian island of Borneo. It belongs in the monotypic genus Nasalis, although the pig-tailed langur has traditionally also been included in this genus - a treatment still preferred by some.
The proboscis monkey is sexuality dimorphic. Males have a head-body length averaging 75.5 cm (29.7 in) and weigh on average 20kg (44.1 lb). Females average 62 cm (24.4 in) in length and weigh half as much as the males. Further adding to the dimorphism is the large nose or proboscis of the male, which can exceed 10cm (3.9 in) in length[5], dwarfing that of the female, and hangs lower than the mouth. Nevertheless, the nose of the female is still fairly large for a primate. The proboscis monkey has a nearly long coat. Dorsally, the fur is bright orange, reddish brown, yellowish brown or brick-red. The fur is light-grey, yellowish, or greyish to light-orange ventrally. The face is orange-pink. The male has a red penis with a black scrotum. Both sexes have over-sized stomachs that protrude and give the monkeys what resembles a pot belly. Many of the monkey’s toes are webbed.
Proboscis monkeys generally live in groups consisting of one adult male, some adult females and their offspring. Other groups also exist such as all-male groups as well as less documented kinds. There are some individuals that are solitary, most of which are males.
Monkey groups leave in home ranges that overlap and there is little territorial behavior. Proboscis monkeys live in a fission-fusion society, with groups coming together at sleeping sites at the end of the day. There exist bands which are formed with the fission and fusion of groups. Groups all meet during the day and travel together, but individuals do not groom or play with those from other groups. One-male groups range from 9-19 individuals while bands can consist of as many as 60 individuals.
One-male groups usually have 3-12 individuals but can be larger. Serious aggression is uncommon among monkeys but minor aggression does commonly occur. Overall, members of the same bands are fairly tolerant of each other. A linear dominance hierarchy exists among both males and females. Male of one-male groups can stay in their groups for 6-8 years. Replacements in the resident males appear to occur without serious aggression. Upon reaching adulthood, males leave their natal groups and join all-male groups. Females also sometimes leave their natal groups, perhaps to avoid infanticide or inbreeding, reduce competition for food or increase their dominance status