The proboscis monkey (Nasalis larvatus) of Borneo belongs to the primate family Cercopithecidae. It is grouped with the langurs, leaf monkeys, and colobus monkeys in the subfamily Colobinae. The feature that gives this odd-looking monkey its common name is the large, tongue-shaped nose of the adult male. This nose can be as much as 4 in (10 cm) long. It sometimes hangs down over the mouth, but extends when the male makes a loud honking noise. In the female, the nose is slightly enlarged but not as pendulous as in the male; in young proboscis monkeys, the nostrils are upturned.
The color of the proboscis monkey's coat ranges from light to reddish brown, with underparts that are gray or cream. The facial skin is reddish in adults, and blue in infants. The average head and body length is 21-30 in (53-76 cm), the weight is 16-53 lb (7-24 kg), and the tail is 21-30 in (53-76 cm). The male can be up to twice as large as the female. The preferred habitat of this species is mangrove or peat swamps and riverine forests. Proboscis monkeys move easily through the branches of trees and, because of their partially webbed hind feet, are good swimmers in or below the water. They feed during the day on fruit, flowers, leaves, seeds, and aquatic vegetation.
Groups range in size from three to 30 individuals, usually based on one adult male and a number of adult females. These groups occupy a home range of less than 1 sq mi (2 sq km). Large troops often feed together, but individuals usually sleep alone in a tree in fairly close proximity to other troop members. Mating is probably possible at any time during the year, and a single young is born after a gestation period of about 166 days.
The proboscis monkey is endemic to the island of Borneo. Because of its relatively inaccessible habitat, the species was safe for many years from human intrusion. Today, however, even mangrove swamps are being cleared and suitable monkey habitat is being reduced. As the species becomes more accessible, it is vulnerable to hunting by local people who consider its meat a delicacy. A 1986 study estimated the total population of proboscis monkeys at approximately 250,000 individuals. The current population may be considerably smaller; one researcher recently estimated the total population in all protected areas combined at less than 5,000. International conservation organizations consider this species to be vulnerable (IUCN; International Union for the Conservation of Nature) or endangered (U.S. Fish and Wildlife Service).