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Facts About Proboscis Monkeys You Didn't Know

Smita Pandit
Native to the island of Borneo in Southeast Asia, proboscis monkeys are placed in the category of the Old World monkeys. This post provides some interesting facts about the proboscis monkey.

Did You Know?

The proboscis monkey has the 'Endangered' status in the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, and is listed in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).
Proboscis monkeys are native to Borneo, an island situated to the southeast of the Malay Peninsula. These mammals can be found in Kalimantan (Indonesia), Sabah (Northeastern Borneo), as well as Brunei and Sarawak in Northwestern Borneo. Their name is derived from the term 'proboscis', which refers to the long, flexible nose of a mammal.
The older males have a very long, pendulous nose, that almost hangs over their mouths. While you might find these long-nosed, pot-bellied monkeys rather odd to look at, their long nose is believed to be a type of sexual selection, with females showing a preference for males with a long nose.
These monkeys exhibit sexual dimorphism, which means that there are differences in the appearance of the males and females. During the Dutch colonial rule in Indonesia, the locals used to call them 'Dutch monkeys', while comparing these monkeys to the long-nosed, pot-bellied Dutch sailors and plantation owners.

Information About Proboscis Monkeys


Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Family: Cercopithecidae
Subfamily: Colobinae
Genus: Nasalis
Scientific name: Nasalis larvatus
Physical Characteristics

Length: 73 - 76 cm (males), 54 - 64 cm (females)
Average Weight: 44 pounds (males), 22 pounds (females)
Feet: Partially-webbed toes
Tail Length: 66 - 75 cm (males), 52 - 62 cm (females)
Breeding and Lifespan

Gestation Period: 166 - 200 days
Age of Sexual Maturity: 5 years (Females)
Breeding Season: February to November
Lifespan: 15 - 20 years


The proboscis monkey is endemic to the island of Borneo in Southeast Asia. As far as their habitat is concerned, these monkeys seem to have a preference for coastal mangrove forests, but they can also be found in mixed diterocarp-kerangas forests or lowland rainforests that lie close to freshwater rivers.
They rarely move farther than a kilometer from water. The availability of food is a determining factor, when it comes to the choice of habitat. They avoid deforested areas. The deforestation of the rainforests for timber has been the reason behind their declining numbers or their endangered status.


These monkeys are folivores (leaf eaters) and frugavores (fruit eaters). It is believed that they are more likely to consume fruits from January through May, and leaves from June through December.
Their diet mainly comprises tender leaves, shoots of mangrove, fruits, seeds, flowers, and unripened fruits. However, they might occasionally consume caterpillars or larvae.

Interesting Facts About Proboscis Monkeys

✦ Only the males have a very long nose. Though the nose of the females is larger in comparison to monkeys belonging to other genera, it is considerably shorter than that of the males. The juveniles or young monkeys have small, upturned noses.
✦ When excited or angry, the nose of the male monkeys turns red and swells up. The nose acts as a resonating chamber during vocalizations. During vocalizations, the nose extends and amplifies the sounds. It is believed that females get attracted to monkeys with a longer nose, as the long nose helps them make louder calls.
✦ Loud honking calls are made by adult males of the group to warn others about the presence of a predator or to threaten other members of the group. Adult males might also growl for calming or reassuring the members of a group.
Males protect the young ones from predators, which might include false gavial, pythons, crested serpent eagles, estuarine crocodile, and clouded leopards.
✦ These monkeys are social by nature. They live in groups, with a group comprising a male with a harem of around six females and their offspring. The dominant male protects the group. While the group feeds, the adult might climb a tall tree to look out for predators.
✦ The females compete with other females for the attention of the dominant male. Usually, females pout or shake their heads to express their desire to mate.
✦ Several harems or groups might even join together to form troops, which can have as many as 50 males and females. Young females might move from one harem to another.
✦ Several groups might come together at night to sleep. They prefer to sleep near water.
✦ Births mostly occur during the night. The mother cleans the offspring by licking, and is also known to consume the placenta.
✦ The offspring stays with the mother for the first few years. After 18 months, juvenile males leave the group for joining all-male groups.
✦ These monkeys are born with blackish blue fur on their face. Within 3-4 months, the color of the fur changes.
✦ Adults have light brown fur, with red-colored fur on the crown and the shoulders. The fur on the underparts and the tail is gray in color and there are cream patches on the throat. The fur on the back is reddish brown in color. They have cream-colored fur around the neck, which looks like a collar.
✦ Their diet comprises unripe fruits and leaves, which are rich in cellulose. The colonies of bacteria present in their digestive tract helps in the digestion of cellulose.
✦ Both males and females have a large belly, which accounts for about one-fourth of their body weight. Their stomach is divided into chambers like cows. Symbiotic bacteria in the first two chambers help in the digestion of cellulose, whereas the enzymes present in the other two chambers digest these bacteria.
✦ The partially-webbed feet of these monkeys helps them swim. Usually, these monkeys resort to swimming for crossing rivers. They can walk upright in the water. They have been known to leap from high branches into rivers.
✦ These monkeys vocalize in different ways. The males may honk to communicate territorial information. They might honk for reassuring infants. In case of an impending threat, the males give an alarm call. Threat calls are given by both males and females. When agitated, the females and offspring are known to shriek.
Over the years, the population of these unusual-looking Proboscis monkeys has nosedived. In Sarawak, their population in the wild was estimated to be 6,400 in 1977. It has now declined to 1,000. Currently, their population in the province of Sabah and Kalimantan is estimated to be 2,000 and 4,000, respectively.
Since 2000, they have been placed in the category of 'endangered species'. This is mainly attributed to the degradation or loss of their habitat due to deforestation and hunting. These monkeys are protected by law, and poaching attracts stiff penalties.