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.
Taxonomy The proboscis monkey belongs to the subfamily Colobinae of the Old World monkeys. The two subspecies are:
N. l. larvatus (Wurmb, 1787), which occupies the whole range of the species excluding northeast Kalimantan N. l. orientalis (Chasen, 1940), restricted to north-east Kalimantan However, the difference between the subspecies is small, and not all authorities recognise N. l. orientalis. The genus name Nasalis comes from the Latin word nasus meaning "nose". 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 66 to 76.2 cm (26.0 to 30.0 in) and typically weigh 16 to 22.5 kg (35 to 50 lb), with a maximum known weight of 30 kg (66 lb). Females measure 53.3 to 62 cm (21.0 to 24.4 in) in head-and-body length and weigh 7 to 12 kg (15 to 26 lb), with a maximum known mass of 15 kg (33 lb). The male has a red penis with a black scrotum. The proboscis monkey has a long coat; the fur on the back is bright orange, reddish brown, yellowish brown or brick-red. The underfur is light-grey, yellowish, or greyish to light-orange. Infants are born with a blue coloured face that at 2.5 months darkens to grey. By 8.5 months of age, the face has become cream coloured like the adults. Both sexes have bulging stomachs that give the monkeys what resembles a pot belly. Many of the monkeys' toes are webbed.
Nose Further adding to the dimorphism is the large nose or proboscis of the male, which can exceed 10.2 cm (4.0 in) in length, 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. The nose is smaller in the female and is upturned in the young. 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.
Distribution and habitat The proboscis monkey is endemic to the island of Borneo and can be found in all three nations that divide the island: Brunei, Indonesia and Malaysia. It is most common in coastal areas and along rivers. This species is restricted to lowland habitats that may experience tides. It favors dipterocarp, mangrove and riverine forests. It can also be found in swamp forests, stunted swamp forests, rubber forests, rubber plantations, limestone hill forests, nypa swamps, nibong swamps, tall swamp forests, tropical heath forests and steep cliffs. This species usually stays within at least a kilometer from a water source. It is perhaps the most aquatic of the primates and is a fairly good swimmer, capable of swimming up to 20 m (66 ft) underwater. It is known to swim across rivers. Aside from this, the proboscis monkey is largely arboreal and moves quadrupedally and by leaps. It is known to jump off branches and descend into water.
Behavior and ecology
Social behavior
Proboscis monkeys generally live in groups composed of one adult male, some adult females and their offspring. All-male groups may also exist. Some individuals are solitary, mostly males. Monkey groups live in overlapping home ranges, with little territoriality, in a fission-fusion society, with groups gathering at sleeping sites as night falls. There exist bands which arise when groups come together and slip apart yet sometimes groups may join to mate and groom. Groups gather during the day and travel together, but individuals only groom and play with those in their own group. One-male groups consist of 3 to 19 individuals, while bands can consist of as many as 60 individuals. Serious aggression is uncommon among the monkeys but minor aggression does occur. Overall, members of the same bands are fairly tolerant of each other. A linear dominance hierarchy exists between females. Males of one-male groups can stay in their groups for six to eight 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 elevation of their social status. In Sabah, Malaysia, proboscis monkeys have been observed in mixed-species groups with silvery lutungs, and interspecific mating and a possible hybrid has been observed. Researchers believe this may be a result of the two species being confined to a small patch of riverine forest due to deforestation in order to plant oil palm trees.
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