Proboscis monkeys live in mangrove and riverine forest where channels cut the canopy into fragments. Rather than detour, troops often jump from riverside trees and swim, sometimes belly-flopping from several metres up. Partial webbing between the second, third and fourth toes of the hind foot increases the surface that pushes water.
The habit has an obvious cost. Bornean rivers hold saltwater crocodiles and false gharials, and crossings are made in groups, often at narrow points and in daylight. Swimming underwater for a stretch reduces the surface disturbance that a slow paddling crossing would create. Adults have been recorded moving twenty metres or more submerged.
The function of the male's enormous nose is less settled. It amplifies vocalisations and correlates with body size and testes size, and larger-nosed males hold more females, but whether females choose the nose directly or simply track the size that accompanies it has not been resolved.