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The Proboscis monkey has one of the most unusual appearances of any of the leaf-eating monkeys of the family Cercopithecidae. Both the Latin and common names of this species refer to the mature males' large pendulous nose that hangs down over their mouth. Local people referred to these large monkeys with their potbellies and red noses as 'Dutch monkeys' as they were considered such a caricature of the Dutch sailors and plantation owners of the area. Apart from their large noses, male Proboscis monkeys are also distinctive by being much larger and heavier than females, and having a bright red, visible penis and black scrotum. The coat is a light brown with red on both the crown of the head and the shoulders; the limbs and tail are grey in colour and there are cream patches on the throat. Infants are born with black fur and a vivid blue face. The cause of the males' large nose is still a matter of contention but may be a form of sexual selection, with females preferring males with large noses possibly as these enhance their vocalizations.

Proboscis monkeys are found in either coastal mangrove forests or in lowland rainforest close to freshwater rivers. Groups consist of single mature males and around 6 (wives) females and their young; adolescent males form bachelor groups until they can take over their own harem. Groups join together in larger more fluid troops to rest at dusk; these encounters may be noisy with rival males displaying to each other and often crashing through the branches. Unusually, females may switch harems several times in their lives, and they compete between each other to mate with the male of their group. When a female is ready to mate she will perform a head shaking and presenting display. A single offspring is born after a gestation period of nearly 6 months, remaining with their mother for the first few years; males will then leave to join bachelor groups.  Young leaves make up the majority of the proboscis monkey diet between June and December, and fruit from January to May, although seeds and flowers are also consumed. These monkeys are excellent swimmers and have partially webbed feet; they can be seen readily leaping into the water with dramatic belly flop in order to cross rainforest rivers.

Numbers of Proboscis monkeys in Borneo have fallen dramatically in the last 40 years primarily as a result of habitat loss. Vast areas of the native rainforest have been cleared for timber and for the construction of oil-palm plantations, which now constitute one of Malaysia's top exports. Proboscis monkeys do not adapt to degraded habitat and recent technical advances have meant that even mangrove swamps may now be logged. Hunting is also a threat to the survival of this species; their propensity to gather in large groups on the river's edge makes these monkeys easy targets.