Adult males develop a fleshy nose that can hang past the mouth and must be pushed aside to eat. Acoustic analysis has shown that larger noses shift call formants downward, which listeners can use as a proxy for the size of the animal making the sound. Measurements across males have found nose size associated with body mass, testis size and the number of females in a male's group.
The rest of the anatomy is built around leaves. Proboscis monkeys have a chambered, foregut-fermenting stomach with bacteria that break down cellulose, which is why they look permanently swollen. The same bacteria make sugary ripe fruit risky, since rapid fermentation can produce dangerous bloating, so the diet leans on unripe fruit, seeds and young leaves.
They are also strong swimmers with partly webbed toes, crossing rivers where crocodiles are present, a trade-off between predation risk and reaching feeding trees.