With their large pendulous nose, giant bellies, and permanent erect penis, the proboscis monkey has one of the most unusual appearances of all primates. Their most distinctive trait is their large protruding nose. Although females already have a very large nose for a primate, the nose of males is so long it often hangs over their mouth, exceeding 10cm in length. Why males have such a large nose is still a matter of dispute but may be a form of sexual selection, with females preferring males with large noses because the nose amplifies vocalisations.
Another striking characteristic of Proboscis monkeys are their large pot bellies, making them look permanently pregnant. Their bellies are so enourmous because of a very special digestive system which consists of cellulose digesting bacteria, enabling them to eat leaves and unripe fruit. Their stomach is so large, its contents can make up to one quarter of their body weight.
Local people referred to these monkeys with their potbellies and red noses as ‘Dutch monkeys’ as they were considered such a caricature of the Dutch colonisers.
Proboscis monkeys can only be found in Borneo and inhabit mangrove, peat swamp, and riverine forests. Although they are mostly arboreal (tree-living), proboscis monkeys are good swimmers and have the most aquatic lifestyle among primates. They usually live in a harem which comprises one adult male and one to eight females and their offspring. The males are much larger, weigh more than twice as much as females, and have an erect penis almost permanently.
Due to ongoing habiat loss by logging and conversion to agriculture, numbers of proboscis monkeys have fallen dramatically in the last 40 years. Huge areas of native rainforest have been cleared for timber and oil-palm plantations. Therefore the proboscis monkey is evaluated as Endangered on the IUCN Red List of Threatened Species, with only some thousands are known to still exist.
I myself encountered proboscis monkeys in the wild on a number of rare occasions while working in Borneo’s tropical rainforest. Visiting Tanjung Puting national park , exploring its many rivers, I had the fortune of seeing a Proboscis monkey actually jumping over a river (see my photos). After shouting nervously for many minutes, the brave monkey eventually jumped and flew more than 15 meters through the air, landing safely in the canopy on the other side.