Proboscis Monkey Facts | Borneo Wildlife Guide

Habitat

Proboscis monkeys (Nasalis larvatus) are confined to the island of Borneo, inhabiting mangrove forests along rivers and estuaries, swamp land and lowland rain forests.

Physical Characteristics

A male uses his sizable pendulous nose to attract a mate, as the organ creates a resounding call meant to impress females and intimidate vying males. Females have a smaller nose and are about half the size of males, which weigh up to 50 pounds. Adults have coats of pinkish-brown fur, which turns red by the head and shoulders and gray around the legs, arms and tail. Male proboscis monkeys possess a scarlet penis and a black scrotum.

Behavior and Communication

Proboscis monkeys are most active from late afternoon to sunset. This highly arboreal species is never far from the riverbanks where they congregate to forage and sleep. Proboscis monkeys are the most skilled swimmers of all primates, aided by their webbed hands and feet, as they leap from trees limbs into rivers with a great splash.

Proboscis monkeys form groups of between 3 and 32 members, which either consist entirely of males or are harems of a dominant male with several females and their offspring. These groups come together to form bands during the evening, selecting different riverbanks as their resting sites from one night to the next.

Proboscis monkeys communicate using several vocalizations. When warning a group of potential predators, males will honk. When distressed or excited, females and their young shriek and males growl in order to calm a group. During aggressive encounters, proboscis monkeys scream.

Feeding Habits

Proboscis monkeys favor unripe fruit, young leaves and tender shoots. Less frequently, they consume seeds, flowers and insects such as caterpillars. Between January and May, these monkeys are mainly frugivorous and for the rest of the year are primarily a folivore.

Reproduction

The gestation period of a proboscis monkey lasts 166 days, after which the female gives birth to a single infant in the branches of a tree at night. The mother then ingests the placenta and licks the newborn clean. The infant has a blue face that turns gray at two and a half months old and becomes a fleshy pink at eight and a half months. Young are weaned at seven months old and stay by their mother’s side for one year. Though the male plays no direct role in caring for his young, he provides potential protection by fending off rival males. If a new male replaces the old in a harem, he may commit infanticide.

Threats

The proboscis monkey is an endangered species, primarily threatened by habitat loss. Borneo’s landscape is evolving, as vast tracts of rainforest are cleared to make way for infrastructure, oil palm plantations, and logging industries. This typically arboreal species is forced to migrate between fragmented habitats in search of food on the ground, leaving it vulnerable to predators such as clouded leopards, crocodiles, monitor lizards, pythons, eagles and poachers. Proboscis monkeys are hunted by local people who consider their meat a delicacy. They are also killed for their bezoar stones, which fetch a price on the black market for use in traditional Chinese medicine. Numbers have fallen by more than 50 percent over the past 40 years, as the widespread depletion of forests continues. The proboscis monkey has gained protected status in Borneo.

Header Credit: Gavin Lautenbach

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