The proboscis monkey (Nasalis larvatus) is one of the largest monkeys in Asia, and one of the most surprising. Males are heavyweights with enormous noses, females are far smaller, and both are unusually at home in the water. Here is how they measure up.
How big is a proboscis monkey?
Size depends heavily on sex. Males have a head-and-body length of roughly 66 to 76 cm and typically weigh about 16 to 22 kg, with exceptional individuals reaching around 30 kg. Females are much smaller, around 53 to 62 cm and roughly 7 to 12 kg. Both sexes add a long tail of similar length to the body. That makes a big male one of the heaviest of all the leaf-eating monkeys.
Why males are so much larger
This strong size difference between the sexes is tied to their breeding system. A dominant male defends a group of females, so larger body size and a bigger nose help in competition and in attracting mates, while females stay compact.
Can a proboscis monkey swim underwater?
Yes, and this is where they outshine almost every other primate. Proboscis monkeys regularly leap from branches into rivers, often with a comical belly-flop, and can swim submerged for up to about 20 metres. They do this to cross wide rivers, reach feeding trees, and escape predators such as crocodiles.
The body of a swimmer
They have partial webbing between their fingers and toes, which improves their stroke in water and helps spread their weight on soft mangrove mud. Living their whole lives beside coastal mangroves, swamps and rivers, water is central to how they move and survive.
How they compare at a glance
- Male weight: about 16 to 22 kg, up to roughly 30 kg.
- Female weight: about 7 to 12 kg.
- Underwater swim: up to about 20 metres.
- Lifespan: at least 23 years in captivity.
For the full breakdown of biology and behaviour, see the proboscis monkey species guide. These river specialists only survive where their forests do, and WARN helps fund the local teams in Borneo working to keep those river corridors intact.
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.
