Proboscis monkey: facts, habitat and what its large nose does

The enlarged nose is most developed in adult males, which points to a role in sexual and social signalling rather than a general breathing requirement. A larger nose makes the male's face more conspicuous and affects the acoustic properties of calls.

Research and field observations link male calls with group coordination, warnings and interactions with rivals. The nose should not be described as a simple loudspeaker with one proven purpose; it is part of a broader display involving body size, sound and social position.

Proboscis monkeys live in coastal and riverside habitats across Borneo. Groups often sleep near water and move through the canopy during the day.

They are capable swimmers and can cross rivers when necessary. Partly webbed digits and strong swimming behaviour are unusual among primates, but water also exposes them to crocodiles and makes intact riverside forest essential.

Their diet is dominated by leaves, seeds and unripe fruit, with seasonal changes and occasional invertebrates. A specialised multi-chambered stomach supports fermentation of fibrous plant material.

The rounded belly is therefore connected to digestion rather than obesity. Captive or wild diets rich in ripe sugary fruit can be unsuitable for a digestive system adapted to leaves and less sugary foods.