Proboscis Monkey Facts: The Big-Nosed Swimmer of Borneo

The proboscis monkey is the best swimmer in the monkey world and the owner of its most extraordinary nose. Endemic to Borneo, males carry a pendulous nose up to 17 cm long, a permanently distended belly full of fermenting leaves, and partially webbed feet that let them cross rivers infested with crocodiles.

At a glance

Scientific name	Nasalis larvatus
Family	Cercopithecidae, subfamily Colobinae
Male weight	16-24 kg
Female weight	7-12 kg
Lifespan	About 20 years
Diet	Young leaves, unripe fruit, seeds, flowers
Habitat	Mangrove, peat swamp and riverine forest
Range	Borneo only
Status	Endangered

Key takeaways

• The male’s nose amplifies vocal calls and acts as a visual and acoustic advertisement of size and status.
• Partially webbed toes make proboscis monkeys the strongest swimmers among primates; they can swim 20 m underwater.
• A chambered, bacteria-filled stomach ferments leaves — and makes ripe, sugary fruit potentially fatal by causing bloat.
• They live in harems of one male with several females, which gather into larger sleeping aggregations along rivers at dusk.
• Habitat loss from oil palm expansion and shrimp farming has driven them to endangered status.

The nose

Only adult males grow the full pendulous nose, and it keeps growing with age. Research links nose size to body mass, testis size and harem size, and the nose serves as a resonance chamber that deepens the male’s honking call. In effect it is a billboard: females and rival males can assess a male’s quality from a distance across dense swamp forest. Compare with other extreme facial signals in Monkeys With Colorful Faces and Mandrill Facts.

Swimming and river life

Proboscis monkeys live along rivers and in mangroves, and crossing water is a daily necessity. Their partially webbed hands and feet, streamlined belly-first entry and ability to swim submerged make them unique among monkeys. They leap from overhanging branches in a spectacular belly-flop that tourists on Kinabatangan river cruises regularly witness. See Can Monkeys Swim?

Digestion and diet

As a colobine, the proboscis monkey ferments young leaves and unripe fruit in a multi-chambered stomach where bacteria break down cellulose and detoxify plant compounds. The permanently swollen abdomen is the visible result. The system is finely tuned: sugary ripe fruit ferments too fast, producing gas and potentially lethal bloat, so proboscis monkeys deliberately avoid it. This is a striking example of why feeding wild monkeys human food can kill them. See also Monkey Diet by Species.

Social structure

The core unit is a one-male harem with two to eight females and their offspring, plus separate bachelor groups. At dusk, several units converge on riverside trees to sleep together, which reduces predation risk from clouded leopards and crocodiles and makes evening boat trips the reliable way to see them. See How Monkeys Sleep and Why Do Monkeys Live in Groups?

Conservation

Populations have fallen by more than half in recent decades. Mangrove clearance for aquaculture, peat swamp drainage, and oil palm plantations have destroyed the riverine forest they depend on, and because they will not cross large open areas, remaining groups become marooned in fragments. The Kinabatangan corridor in Sabah, Bako National Park in Sarawak and Tanjung Puting in Kalimantan hold important populations. See Threats to Monkey Habitats, Monkeys in National Parks and Best Places to See Monkeys.

Frequently asked questions

Why do proboscis monkeys have big noses?

The nose amplifies male calls and signals body size and status; larger-nosed males hold larger harems.

Can proboscis monkeys swim?

Yes — they are the best swimmers of all primates, with partially webbed feet, and can swim considerable distances underwater.

Where do proboscis monkeys live?

Only on Borneo, in mangrove, peat swamp and riverine forest across Malaysia, Indonesia and Brunei.

Are proboscis monkeys endangered?

Yes. They are listed as endangered, mainly due to loss of coastal and riverine forest.

Do female proboscis monkeys have big noses?

No. Females have a small upturned nose; the exaggerated nose is a male-only trait that grows with age.

Related: Asian Monkeys, Weird and Wonderful Monkey Facts, Ultimate Guide to Monkeys.