No one is quite sure exactly why male proboscis monkeys have such enormous noses, but scientists think it helps make their calls louder and more impressive. When a male wants to warn rivals or impress females, he makes a loud honking sound through that wobbly nose - it can carry a long way through the forest.
Proboscis monkeys are excellent swimmers, which is unusual for primates. They leap from tree branches straight into rivers and swim with a doggy-paddle style, using their slightly webbed feet. They can even swim underwater for short stretches to avoid crocodiles - they make a huge splash as they plunge in from great heights.
Their favourite food is unripe leaves, fruit, and seeds found in mangrove and riverine forests. The leaves are hard for many animals to digest, but proboscis monkeys have special multi-chambered stomachs - a bit like cows - that can break them down. Groups of monkeys sleep in trees right on the riverbank, so they are easy to spot from a slow boat at dusk.