Proboscis Monkey Facts

Proboscis monkeys are one of Borneo’s most intriguing attractions. Here are some useful proboscis monkey facts.

Proboscis monkeys (Nasalis larvatus) might just be the most distinctive looking primates on the planet. They may be distinguished from other Bornean primate species by their large body size, big round bellies, and striking fur colors (reddish-brown torso, grey limbs, and a long white tail), but what makes them famous is the huge pendulous nose and a bright red, permanently erect penis seen in mature males.

In many parts of Borneo, the proboscis monkey is nick-named “monyet belanda” meaning “Dutch monkey”, a term coined by locals who noticed that Dutch colonizers also had long red noses and big bellies!

Quick facts:

  • Male proboscis monkeys weigh up to 24 kg (53lb) with a length of 72 cm (28 inches) and a tail of up to 75 cm in length.
  • Females are up to 60 cm long, weighing up to 12 kg (26 lb).
  • Proboscis monkeys live in groups of either one male and several females (a harem), or all-male bachelor groups
  • Their diet is mainly folivorous (leaves) and frugivorous (fruits –though these are usually non-fleshy fruits eaten unripe.) The monkeys may have to travel up to 2km per day in search of food.
  • They are incredible swimmers, preferring to use a sort of doggy paddle, and aided by partly webbed back feet. Mother with infants will plunge into the river, and after a few seconds under the water, the infant will clamber up her front and over her shoulder to breathe again
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Proboscis Monkey Ecology and Behaviour

Proboscis monkeys are found only on the island of Borneo and inhabit mainly riverine forests, mangroves, peat swamp and freshwater swamp forests. Scientists have suggested that this restriction may be due to a higher level of minerals and salts here which are an essential part of the proboscis monkey’s diet. They are thus found at low elevations, usually up to no more than about 350 m above sea level.

They are social primates and prefer to live in harem groups which consist of one male and around 8 females with young. Unlike most primate harems, proboscis monkey groups often come together, usually by riversides at night.

Another proboscis monkey behavior which is unusual in primates is the switching between harems of the adult females, which may change group several times during their lives. Young males leave the harem at around 1-2 years of age, as soon as they are able to look after themselves. At this point, they will join an all-male bachelor group containing one or two large males and a number of other juveniles.



Proboscis monkeys communicate through a bizarre range of vocal noises including roars, grunts, squeals, and a range of nasal “honks”. Most commonly during a bought of honking and squealing from a group, the large make will make a gentle “ho-hoooooong”, which seems to calm everyone else down.

Why do Male Proboscis Monkeys Have Big Noses?

The answers to this question have varied over the years. The monkeys are strong swimmers and at one point it was thought the nose acts as a snorkel to help the animal breathe. However, if this were true why is it the female does not drown?

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It has also been suggested that the nose may amplify the male’s call, or even regulate body temperature, though there is no reliable evidence to support these ideas. The most likely explanation, however, is that of Darwin’s sexual selection -females prefer males with big noses, and so these males get to have more offspring, passing along their big nose genes.

Why do Proboscis Monkeys Have Big Bellies?

Proboscis monkeys belong to the subfamily colubinae, a group of Old World monkeys easily distinguished by their enormous stomachs. These contain a vast array of bacteria which ferment the animal’s food. These bacteria provide two big advantages to the monkey:

  1. They can break down cellulose therefore enabling the animal to obtain energy from leaves (non-colubine monkeys, apes and humans cannot).
  2. The bacteria deactivate as least some of the poisons in the food, meaning that proboscis monkeys can eat types of leaves and fruit which might kill any normal animal.

There are, however, disadvantages. If colubines eat highly digestible foods such as sweet sugary fruit their stomach bacteria ferment them causing a rapid build-up of gas known as bloat. This can very quickly kill the animal. Another problem is that some plants contain chemicals that act as antibiotics. Proboscis monkeys cannot eat these either as this would kill the bacteria in their stomach.

Where to See Wild Proboscis Monkeys

The best place to observe wild proboscis monkeys is along the Kinabatangan River in Sabah, Malaysian Borneo. Daily boat tours give wildlife tourists the opportunity to watch not only primate species such as orang-utans, langurs, and macaques but also other large mammals and reptiles including Borneo pygmy elephants and estuarine crocodiles.

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Watching groups of proboscis monkeys gathering at dusk along the river in Borneo one could be forgiven for thinking these fascinating primates were common. However losses of habitat and hunting have left as few as 3000 individuals in isolated areas of Borneo, and the population trend still heads downwards at an alarming rate.

Unfortunately, without an urgent extension of forest reserves to incorporate prime proboscis monkey habitat and stricter enforcement of anti-hunting legislation, this downward trend looks to continue, and the tale of the unique long-nosed monkey of Borneo may soon become nothing more than a legend.

Sources:

  • Bennett, E.L. & Gombek, F. (1993). Proboscis Monkeys of Borneo. Natural History Publications (Borneo) &KOKTAS Sabah Berhad.
  • IUCN Redlist of endangered species
Categories Animals

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