While the official Indonesian name for this monkey is Bekantan, an Indonesian nickname is 'monyet belanda', meaning 'Dutch monkey' or 'Orang Belanda', the Indonesian word for 'Dutchman', as Indonesians noticed the Dutch colonisers often also had a large belly and nose.A distinctive trait of this monkey is the male's large protruding nose, from which it takes its name. The big nose is thought to be used to attract females and is a characteristic of the males, reaching up to 7 inches in length. The females also have big noses compared to other monkey species, but not as big as the males. Besides attracting mates, the nose serves as a resonating chamber, amplifying their warning calls. When the animal becomes agitated its nose swells with blood, making warning calls louder and more intense.
Proboscis Monkey belong to the order of Primates, from the family Cercopithecidae and subfamily Colobinae (Bennett & Gomber, 1993). According to Bennett & Gomber (1993), in the Old World, these monkeys are divided into two groups known as cercopithecines and colobines. Proboscis Monkey are colobines. Males are much larger than females, weighing up to 24 kg (53 pounds) and reaching 72 cm (28 inches) in length, with a tail of up to 75 cm in length. Females are up to 60 cm long, weighing up to 12 kg (26 lb). This large sexual dimorphic difference is greater than in any other primate.
The adult Proboscis Monkey is mainly reddish-brown, with grayish limbs (Bennett & Gombek, 1993). According to Burnie (2001), young Proboscis Monkeys have a blue face, blackish fur and a relatively normal sized nose at birth. As they grow older, fur coloration changes and the nose grows. Adult males have a large and fleshy nose which overhangs its mouth, but the female Proboscis Monkey does not have a large nose in comparison to the male.
The Proboscis Monkey is endemic to Borneo's low elevation mangrove forests, swamps, and lowland riparian forests. One of the largest populations is found in the Danau Sentarum National Park. It lives in small groups of 10 to 32 animals. Group membership is very flexible, and animals are known to move from group to group quite often.
The Proboscis Monkey's lifestyle is both arboreal and amphibious, with its mangrove swamp and riverine environment containing forest, dry land, shallow water allowing wading, and deep water requiring swimming. Like other similar monkeys, the Proboscis Monkey climbs well. It is also a proficient swimmer, often swimming from island to island, and has been picked up by fishing boats in open ocean a mile from shore. While wading, the monkey uses an upright posture, with the females carrying infants on their hip. Troops have been filmed continuing to walk upright, in single file, along forest trails when they emerge on land, the only non-human mammal, with the exception of gibbons and giant pangolins, known to use this form of locomotion for any length of time.
Proboscis Monkeys usually lives in a harem which comprises one adult male, several females, and their offspring, but sometimes the male and female Proboscis Monkeys move between social group. The Proboscis Monkey is mostly arboreal, but sometimes the animals migrate downriver into the mangrove forest to feed.
The monkey also has a large belly, a result of its diet. Its digestive system is divided into compartments, with bacteria that digest cellulose and neutralize toxins from certain leaves. This lets the monkey eat leaves and remain in the forest canopy. The contents of their stomach weigh in at about a quarter of their whole body. The diet consists mainly of seeds, leaves, fruits, and mangrove shoots.
Due to ongoing habitat loss and hunting in some areas, only about 1000 are known to still exist in the wild. In Sarawak, the population of this species has declined from 6500 in 1977 to only 1000 in 2006. The Proboscis Monkey is evaluated as Endangered on the IUCN Red List of Threatened Species. It is listed on Appendix I of CITES. The Proboscis Monkey is protected by law in all regions of Borneo. In Malaysia, this species are protected by a number of laws including: Wildlife Protection Act (Federal Law), Wildlife Protection Ordinance 1998 (Chapter 26) and Wildlife Conservation Enactment 1997 (Sabah State Law).