Everyone who has the opportunity to see a Proboscis monkey is struck by its giant nose. But there is a second feature which is very typical for Proboscis monkeys and their relatives, the so called Leaf-monkeys - it's their large belly. Indicated by the name of that group of apes, they almost exclusively feed on leaves - and leaves are not easy to digest. Moreover leaves are not very rich in energy-content. So Proboscis monkeys have to eat a lot of leaves and spend most of their time on feeding. In order to obtain enough energy from their meagre nourishment, Proboscis monkeys have got a complicated stomach divided into several parts. Their stomach is quite similar to that of ruminants, which have to face the same problems.
Adult Proboscis monkeys are large animals. Males reach a length of about 70 cm and a weight up to 24 kg. Females are significantly smaller, reaching lengths of 60 cm and weights of just 12 kg. The tails of Proboscis monkeys are about as long as their bodies. Their fur is brick-red, the front and the cheeks are of brighter colour and the arms and legs are more greyish. The most obvious feature of Proboscis monkeys is the male's extremely long nose, shaped like a cucumber. Noses of females and young are not small either but don't reach the sizes of the noses of males by far.
Proboscis monkeys inhabit Borneo. They are endemic on that South-Asian island. The prefered habitat of Proboscis monkeys are rainforests of the lowlands and most of all mangrove-forests at the river mouths. They live on trees and wander about in groups of sizes from 10 to 30 animals. Mostly several adult males can be found in one group, but groups with just one single adult male are known too. Proboscis monkeys move quite rapidly on the trees. They are able to swing from branch to branch using their hands but are also very skilled in running on all fours. If Proboscis monkeys are in a hurry, they jump from tree to tree. They prefer this way to move when they have to cross rivers or escape from predators like the Clouded leopard, a cat which is closely related to the Tiger. If there's no other opportunity to escape left, Proboscis monkeys drop themselves into the water even from high branches. They are very skilled swimmers and divers. This is not very common among apes, since most ape species are said to be more or less scared of water. But it's a necessity of life to be able to swim in a habitat that is dominated by rivers and river mouths. But accidents are very common; sometimes Proboscis monkeys are driven out to the open sea by river currents.
Adult male Proboscis monkeys are imposing figures. They are quite aggressive towards members of other groups. But before initiating a fight they try to clarify their positions by showing threatening behaviour. Doing a lot of fuss they break branches and give loud calls. But there are not much further details known about Proboscis monkey behaviour and their social life in the wild. Long-term studies of Proboscis monkey groups are extremely difficult, because they can be observed only by boat in the mangrove-forests. In spite of the unapproachability of its habitat the Proboscis monkey is regarded to be endangered, since habitat destruction doesn't stop even at such places.