The Proboscis Monkey: Why Its Nose Is So Large and Why the Species Is Endangered

In Borneo’s tropical forests, one mammal evolved a feature unlike any other in the world: an exceptionally large nose. The proboscis monkey, Nasalis larvatus, is now endangered, with fewer than 7,000 individuals remaining in the wild.

The species lives only on Borneo and is the largest of the colobine monkeys. Its population continues to decline as forests are cleared to expand agriculture and tourism. Hunting for use in traditional medicine has also reduced its numbers.

The evolutionary history of the proboscis monkey remains difficult to reconstruct. Researchers believe its ancestors separated from their African relatives more than 10 million years ago and reached present-day Borneo around 1.8 million to 3 million years ago. Few fossil or archaeological remains have been found, partly because the monkey favors mangroves, swamps and rivers, where recovering material is difficult.

Its scientific name also reflects its most striking feature. Nasalis refers to the nose, while larvatus comes from the Latin word for “mask” or “ghost,” often translated as “masked.” Males weigh between 16 and 22 kilograms and measure 66 to 77 centimeters. Females are smaller, weighing up to 11 kilograms, with tails nearly as long as their bodies.

The monkeys eat mainly mangrove leaves, along with seeds, fruit and young leaves. A large fermenting stomach, filled with a bacterial mixture, helps break down fibrous leaves and remove toxic or indigestible substances. Research published in Nature found that the animals choose plants according to nutritional value and digestibility rather than feeding opportunistically.

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The fact that the abundance of plant species was always negatively correlated with preference supports the observation that Nasalis larvatus does not feed opportunistically, but instead preferentially selects certain foods.