FaunaHub

Proboscis Monkey (Nasalis larvatus)

MammalPrimateBorneo

Male proboscis monkey (Nasalis larvatus) with a large pendulous nose, Borneo.

Male proboscis monkey (Nasalis larvatus), Borneo.

Image: Charles J. Sharp, CC BY-SA 4.0, via Wikimedia Commons.

Overview

The proboscis monkey (Nasalis larvatus) is an unmistakable primate found only on the island of Borneo, famous for the male's huge, drooping nose and conspicuous pot belly. Reddish-brown above and paler below, these are large, heavily built leaf-eating monkeys (colobines) that live in the mangrove, swamp, and riverside forests of the island.

They are also among the most aquatic of all monkeys — strong swimmers with partly webbed feet that readily cross rivers and even dive to escape danger.

Conservation note: the proboscis monkey is Endangered, threatened by the loss of its specialised riverine and mangrove forests. Verify current status at iucnredlist.org.

Habitat & Range

Proboscis monkeys are endemic to Borneo, where they keep close to water in coastal mangroves, peat swamp forest, and riverine forest. They rarely stray far from rivers and tend to gather in riverside trees to sleep, which makes them dependent on these particular, increasingly threatened habitats.

Diet

Proboscis monkeys are mainly folivores (leaf-eaters), also taking unripe fruit, seeds, flowers, and shoots. Like other colobine monkeys they have a large, chambered stomach with microbes that ferment tough leaves — which is part of why they look pot-bellied. They tend to avoid sweet ripe fruit, which can ferment dangerously in their specialised gut.

Behavior

Proboscis monkeys live in groups, typically a dominant male with several females and young, plus separate bachelor groups. The male's oversized nose is thought to amplify his calls and to signal quality to females, growing larger with status. These monkeys are excellent swimmers: they cross rivers by swimming (sometimes after a running belly-flop leap from a branch) and can swim underwater, an unusual skill among primates that helps them evade predators such as crocodiles.

Human Interaction & Conservation

Proboscis monkeys are a flagship species for Borneo's wildlife and a draw for ecotourism, but they are declining because the riverside and mangrove forests they depend on are being cleared for timber, oil-palm plantations, and settlement, and they are sometimes hunted. Protecting connected riverine forest is essential. Consult the IUCN Red List for current status.

More photos of the proboscis monkey

A female proboscis monkey with a baby, showing the smaller female nose.

Proboscis monkey (Nasalis larvatus), female and young.

Image: Charles J. Sharp, CC BY-SA 4.0, via Wikimedia Commons.

Frequently Asked Questions — Proboscis Monkey

Why does the male proboscis monkey have such a big nose?
The male's large, pendulous nose is thought to serve as a signal and a sound amplifier. A bigger nose can make his calls louder and deeper, advertising his size and status to females and rivals, and nose size is linked to dominance — so it likely evolved through female preference and male competition.
Can proboscis monkeys really swim?
Yes — they are among the most aquatic of all monkeys. With partially webbed feet, proboscis monkeys swim well, cross rivers regularly, and can even swim underwater, sometimes leaping from branches into the water. This helps them move around their riverine habitat and escape predators.
Why do proboscis monkeys look pot-bellied?
Their large, rounded bellies house a big, chambered stomach full of microbes that ferment the tough leaves they eat, much like a cow's. This specialised digestive system lets them extract nutrients from foliage but gives them their distinctive pot-bellied appearance.
Are proboscis monkeys endangered?
Yes — they are assessed as Endangered. The main threat is the destruction of their specialised mangrove and riverside forests for logging, oil-palm plantations, and development, along with hunting in places. Because they depend so closely on these habitats, protecting them is crucial; current status should be checked against the IUCN Red List.

Sources and further reading

Authoritative wildlife references used for general educational context. Conservation status should always be verified against current IUCN Red List data. External links open in a new tab.