Few animals look as improbable as the proboscis monkey (Nasalis larvatus), a leaf-eating primate found only on the island of Borneo. With a pendulous nose, a round belly and a talent for swimming, it breaks almost every rule we expect a monkey to follow. Here are ten facts that explain why.
10 fascinating proboscis monkey facts
1. The nose is a built-in loudspeaker
A dominant male's nose can hang lower than his mouth and reach around 10 cm long. Research published in Science Advances shows it works as a resonating chamber, deepening his honking calls and advertising his size across dense forest.
2. Females choose the biggest noses
Nose size correlates with body mass and the number of females a male attracts, evidence that sexual selection drove the trait through female choice and male rivalry alike.
3. Males and females look like different species
Females and infants have small, neat, upturned noses, while males grow the famous droop. Males are also roughly twice the weight of females.
4. They are among the best swimmers of all primates
Proboscis monkeys belly-flop from branches into rivers and can swim underwater for up to about 20 metres to dodge crocodiles or reach feeding trees.
5. They have partly webbed hands and feet
Partial webbing between the fingers and toes helps them swim and spreads their weight on soft mangrove mud.
6. Their belly is a fermentation tank
A multi-chambered stomach full of bacteria ferments tough leaves, similar to a cow's digestion, giving them their pot-bellied silhouette.
7. They live nowhere else on Earth
The species is endemic to Borneo, shared by Indonesia, Malaysia and Brunei, and clings to coastal mangroves, peat swamps and riverbanks.
8. Society runs on harems and bachelor bands
Groups are typically one male with several females and young, alongside separate all-male bachelor bands that gather and split along the same river.
9. They are gentle and shy
Despite their size, they are plant-eaters that avoid people and spend most of their time in trees near water.
10. They are Endangered
The IUCN estimates fewer than 7,000 mature individuals remain after a decline of more than 50% in about 40 years, driven by habitat loss. Read the full proboscis monkey species guide for the complete picture.
Protecting them means protecting Borneo's riverside forests. WARN channels donor support to local rescue and ranger teams in Indonesia and Malaysia who keep those mangroves standing.
Read the full Proboscis monkey wildlife guide for the facts, IUCN status and conservation context — or donate to WARN to help fund the local partners protecting this species.
