The proboscis monkey is an iconic primate species found only on the Island of Borneo. Populations of this species are found in the Malaysian states of Sabah and Sarawak, the Indonesian province of Kalimantan, and Brunei. Proboscis monkey is classified under the family Cercopithecidae and is scientifically known as Nasalis larvatus. Throughout its range the proboscis monkey is also known by various local names such as kera belanda, monyet belanda, monyet probosis and orang belanda. In Sabah, proboscis monkey is known as bangkatan. The proboscis monkey gets its nickname – long-nosed monkey from its long pendulos nose. Proboscis monkeys typically inhabit lowland forests on water-logged acidic soils such as riverine, mangrove and peat-swamp forests, mainly along major water courses such as rivers and their tributaries not far from the coast line (Payne & Francis, 2007).
Proboscis monkeys have reddish-brown fur on the head, face, shoulders and upper back. Their fur on the lower back and stomach are grey, feet are yellowish-white while their throat and neck are white (Sjahfirdi & Noviandini, 2018). The tail is long (55-67 cm) and white or greyish in colour. The body length of adult male is about 56-72 cm with an average weight of 20 kg whereas adult female body length is about 55-62 cm and weight about 10 kg (Britannica, 2020; Sha et al., 2011). Adult males, especially dominant animals, have much larger and longer noses than females. Females have short pointy witch-like noses. The large nose in adult males is a product of natural selection and they are used in sexual displays and as amplifiers, producing loud honking calls (Covert, 2018). Infants are born with small snubby noses and dark blue faces. Their noses gradually will develop into their signature larger noses and the dark blue face slowly turns greyish and then pink, like adults when they reach maturity. Males reach sexual maturity at 60-84 months while females reach maturity at 36-60 months (Sha et al., 2011).
Both sexes have bulging stomachs that give the monkeys what resembles a pot belly. They have a specialized multi-chambered stomach (sacculated stomach), somewhat like the digestive track of cows. Their large sacculated stomach helps them to breakdown leafy material into digestible substances and also helps detoxify leaves that are otherwise poisonous to other primates (Payne & Francis, 2007; Sha et al., 2011). Generally, their diet includes leaves, fruits and shoots.
Proboscis monkeys are proficient swimmers and they swim across rivers frequently. They have partially webbed toes to aid them in swimming and allow them to walk on soft mangrove floor without sinking (Payne & Francis, 2007). Proboscis monkeys are arboreal animals where they feed and travel in the tree canopy using connecting branches. Occasionally, proboscis monkey descends to the ground to cross gaps in the forest.
The proboscis monkey has been listed as Endangered on the IUCN Red List since the year 2000 and listed under Appendix I of CITES. Habitat loss and fragmentation, forest fires, hunting and illegal wildlife trade are some of the major threats to the continued survival of the proboscis monkeys (Covert, 2018). Proboscis monkeys are an important eco-tourism attraction. Due to their association with major water courses and sleep in the trees located by the riverbanks in the evening and early morning (Bernard et al., 2011), the proboscis monkey is an easily observable animal with high encounter rates in the wild. Therefore, large numbers can be sighted by cruising along the river in a boat. Where major populations of proboscis monkey are found, such as in Sukau, Kinabatangan and Klias Peninsula in Sabah, major proliferation of tourist establishments have taken places (Sha et al., 2011). Conservation efforts are now urgently needed to safeguard the populations of proboscis monkeys.
