The proboscis monkey or long-nosed monkey (Nasalis larvatus) is an arboreal Old World monkey with an unusually large nose (or proboscis), a reddish-brown skin color and a long tail. It is endemic to the Southeast Asian island of Borneo and is found mostly in mangrove forests and on the coastal areas of the island.
This species co-exists with the Bornean orangutan and monkeys such as the silvery lutung. It belongs in the monotypic genus Nasalis.
- N. l. orientalis (Chasen, 1940), restricted to north-east Kalimantan
This animal was made known to Westerners by Baron Friedrich von Wurmb in 1781, he later sent specimens of it to Stamford Raffles in Europe.
Description
The proboscis monkey is a large species, being one of the largest monkey species native to Asia. Only the Tibetan macaque and a few of the gray langurs can rival its size.
Sexual dimorphism is pronounced in the species. Males have a head-body length of and typically weigh , with a maximum known weight of . Females measure in head-and-body length and weigh , with a maximum known mass of . The male has a red penis with a black scrotum. The underfur is light-grey, yellowish, or greyish to light-orange. Both sexes have bulging stomachs that give the monkeys what resembles a pot belly. Many of the monkeys' toes are webbed. and hangs lower than the mouth. Theories for the extensive length of their nose suggest it may be sexual selection by the females, who prefer louder vocalisations, with the size of the nose increasing the volume of the call. Nevertheless, the nose of the female is still fairly large for a primate. The skull of the proboscis monkey has specialized nasal cartilages that support the large nose. This species is restricted to lowland habitats that may experience tides. It favors dipterocarp, mangrove and riverine forests.
Behavior and ecology
Social behavior
thumb|Jumping ([[composite image, from right to left), Labuk Bay, Sabah]]
thumb|A group by the river Kinabatangan
Proboscis monkeys generally live in groups composed of one adult male, some adult females and their offspring. Some individuals are solitary, mostly males. Monkey groups live in overlapping home ranges, with little territoriality, One-male groups consist of 3 to 19 individuals, Overall, members of the same bands are fairly tolerant of each other. A linear dominance hierarchy exists between females.
Reproduction
Females become sexually mature at the age of five years. They experience sexual swelling, which involves the genitals becoming pink or reddened. Copulations tend to last for half a minute. When soliciting, both sexes will make pouted faces. In addition, males will sometimes vocalize and females will present their backsides and shake their head from side to side. The young begin to eat solid foods at six weeks and are weaned at seven months old. The nose of a young male grows slowly until reaching adulthood. The mother will allow other members of her group to hold her infant.
Communication
Proboscis monkeys are known to make various vocalizations. When communicating the status of group, males will emit honks. They have a special honk emitted towards infants, which is also used for reassurance. Males will also produce alarm calls to signal danger. Both sexes give threat calls, but each are different. In addition, females and immature individuals will emit so-called "female calls" when angry. Honks, roars and snarls are made during low-intensity agonistic encounters. Nonvocal displays include leaping-branch shaking, bare-teeth open mouth threats and erection in males, made in the same situations. Groups usually sleep in adjacent trees. Monkeys tend to sleep near rivers, if they are nearby. Proboscis monkeys will start the day foraging and then rest further inland. Their daily activities consist of resting, traveling, feeding and keeping vigilant. As night approaches, the monkeys move back near the river and forage again. Predators (potential or confirmed) of the proboscis monkey include crocodilians like false gharials and saltwater crocodiles, the Sunda clouded leopard, sun bears and reticulated pythons as well as, for probably young or sickly monkeys, large eagles (such as the crested serpent eagle or black eagle), large owls, and monitor lizards. Monkeys will cross rivers at narrows or cross arboreally if possible. This may serve as predator avoidance.
Conservation status
The proboscis monkey is assessed as endangered in the IUCN Red List of Threatened Species and listed in Appendix I of CITES. Its total population has decreased by more than 50% in the past 36–40 years to 2008 due to ongoing habitat loss because of logging and oil palm plantations, and hunting in some areas due to the species being treated as a delicacy, as well as its use in traditional Chinese medicine.