Proboscis Monkey
Common Name: Monyet Belanda
Genus: Nasalis

The proboscis monkey is found only on the island of Borneo in Southeast Asia. It prefers to live in the mixed diterocarp-kerangas forests, mangrove forests and lowland forests near fresh water and rivers. They live almost exclusively in mangrove forests, but can also be found in lowland rainforests. It depends on the mangroves near river edges for resting and sleeping. They avoid deforested areas and avoid human settlements.

This elusive monkey belongs to the Asian langurs. They are the only member of the Nasalis genus.

The proboscis monkey gets its name from its large, fleshy nose. Both males and females have the large noses. Female noses are not as large, although larger than most monkeys, and juveniles have small upturned noses. The male's are so large that they hang down over their mouths, reminding one of the old comic, Jimmy Durante. Sometimes they have to push it out of the way before putting something in their mouth. Their noses swell and turn red when they become excited or angry. They also make loud honking sounds as a warning when they sense danger, which make their noses stand out straight. The nose acts as a resonator when the monkey vocalizes.

Another obvious characteristic of proboscis monkeys are their large pot bellies. Proboscis monkeys are colobine monkeys and they have a unique digestive system that allows them to use leaves as their main food supply. Twice as large as any other colobine monkey, their stomachs are divided into compartments and filled with cellulose digesting bacteria. The bacteria help digest the leaves and neutralize toxins in the certain leaves. Their stomach contents make up one quarter of their body weight, and make them look permanently pregnant.

Their ability to eat mainly leaves gives them a niche where they are the only medium sized mammal living in the canopy of the forest.

They have reddish-brown fur on their back and shoulders, which ends at midsection . Their chests are creamy, with a creamy collar running around their neck and around their waist to their buttocks and tail. Their arms and legs have long, gray gloves and stockings. Orange fur covers their shoulders and a cap of darker red fur covers their head. Their faces are flesh colored with small, intelligent brown eyes. Their ears are small and lie flat against their heads.

The male proboscis monkey is much larger and heavier than the female. A male is 2 to 2.5 feet (66-72 cm) long, and weighs 35-51 lbs. (16-23 kgs), while a female is 1.7 to 2 feet (53-61 cm) long and weighs only (15-24 lbs.) 7-11 kgs. Their tails are as long as their body.

Troops of proboscis monkey average in size from 12 to 27 individuals but can be as big as 60 to 80 males and females. The groups don't appear to have a lot of structure. Both males and females will move out of their natal group Their social system has two levels. One is the all-male groups. These are made up of juveniles, adolescents and adult males. Juvenile males will leave the group they were born to at about 18 months, and join an all-male group. There is very little aggression between the males when they join together to form troops. The second level is made up of several groups which are led by separate males. Each male will have his own harem and several of these harems will come together to form a troop. Females may transfer from one harem to another when young, but the harems are mostly stable. Proboscis monkeys are not a territorial species and a group's range will overlap that of many other groups.

Adult females tend to coordinate troop movement and lead groups when feeding. One of the dominant males will keep a look out in a tall tree while the group feeds.

Proboscis monkeys are dependent on habitats that adjoin rivers, and usually don't move farther than 1,969 feet (600 m) from a river or stream. They sleep in trees, preferring thick branches growing over water, to protect themselves from predators. Proboscis monkeys are excellent swimmers, but only swim only when necessary, as when crossing rivers. They will cross at its narrowest point doing the dog paddle. Many will cross at the same time for protection from predators like crocodiles. They will quietly slip into the water and swim silently without splashing so as not to attract attention to themselves. In times of danger, the whole troop may jump into the water as a means of escape. Partially webbed feet help the proboscis monkey swim and distribute its weight when walking on the soft mangrove mud.

Proboscis monkeys swing through the trees and leap from branch to branch using all four limbs, moving slowly because of their weight. Usually feeding in the early morning, they will rest the whole day. Then will feed again just before nightfall. The greatest times of activity is from late afternoon until dark. Ninty five percent of their diet is leaves of the mangrove and pedada trees. They prefer immature leaves over older ones They will also eat fruits and seeds

Proboscis monkeys give birth to one baby at a time. Their gestation period is about 166 days. They will usually give birth at night The newborn has a deep blue face and sparse, almost black fur. The color changes to its adult colors at about 3-4 months. Females will help look after each others infants and sometimes eve suckle other babies. The young will stay close to their mother for about one year, or until she has another infant. Males will reach sexual maturity in about 4-5 years and females in 4 years. Their life span is about 20 years.

The proboscis monkey's main predators are man and the clouded leopard. The proboscis monkeys were once protected by its mangrove habitat, which was unhabitable for people. Now technology has made it possible to clear-cut the mangroves and fill them in. Logging has caused loss of the proboscis monkey's habitat and their population is going down. In 1977 it was estimated that 6,400 proboscis monkeys remained in the wild in the province of Sarawak. It is now estimated that only 1,000 remain. About 2,000 remain in the province of Sabah and 4,000 in Kalimantan. It is listed as endangered by the USDI and Appendix 1 of CITES. Appendix I lists species that are the most endangered among CITES-listed animals and plants. These are threatened with extinction, and commercial international trade of these species is generally prohibited. Attempts to keep proboscis monkeys in captivity have not been successful. The only place to see these monkeys are in their natural habitat, the rapidly disappearing mangrove forests of Borneo.


"Untitled Document", (3/11/03)

"Proboscis Monkey - Nasalis larvatus",  


World Biomes