Proboscis Monkey

The oversized nose is a product of sexual selection, meaning females appear to prefer, or at least respond strongly to, males with larger noses, and the trait has been driven to an exaggerated size over generations as a result. The nose is not just for show — it acts as a resonating chamber that amplifies and deepens the male's loud honking calls, which he uses both to warn off rival males and to keep his group of females together and moving as they travel through the forest. A bigger nose produces a louder, lower call, which likely signals size, strength, and dominance to both rivals and potential mates without the male needing to fight to prove it.

Yes, unusually so for a primate. Most monkeys avoid deep water, but proboscis monkeys live in coastal and riverine forest where crossing water is often unavoidable, and they have partially webbed fingers and toes that improve their swimming efficiency. They readily enter rivers, swim across using a dog-paddle-like stroke, and can swim underwater for short distances, sometimes diving in dramatically from a high branch to escape a predator on land, such as a clouded leopard, or in the water itself, most notably estuarine crocodiles, which are a real threat during river crossings.

Their digestive system is built around fermenting tough, fibrous, young leaves in a large, multi-chambered stomach that works much like a cow's, using symbiotic bacteria to break down cellulose that the monkey's own body cannot digest. That fermentation process does not handle a sudden load of sugar well — eating a lot of sugary ripe fruit can cause excessive gas production and dangerous bloating in this kind of stomach, so proboscis monkeys mostly avoid ripe fruit and favour young leaves, shoots, and unripe, less sugary fruit and seeds instead, in contrast to many other primates that prefer the sweetest fruit they can find.

It is found only on the island of Borneo and almost always close to water, in coastal mangroves, swamp forest, and lowland forest along rivers, which happens to be exactly the kind of accessible, low-lying land that has been most heavily logged and then converted to oil palm plantations over the past few decades. As that riverine and coastal forest has been cleared, proboscis monkey populations have become increasingly fragmented and restricted to a shrinking network of protected areas and remaining forest strips along rivers, and hunting for meat and habitat disturbance add further pressure. Its narrow habitat requirements and total dependence on Borneo's lowland forest make it one of the primates most directly threatened by the palm oil industry's expansion.