Proboscis monkey

Proboscis monkeys eat mostly leaves.

They like soft young leaves best. They also eat fruit and seeds.

Proboscis monkeys are mostly leaf-eaters and fruit-eaters. About half of their feeding is on leaves and about two-fifths on fruit. They prefer soft young leaves and fruits that are not yet ripe, along with seeds and mangrove shoots. Now and then they eat caterpillars or other small insect larvae.

What they eat changes through the year as different foods become available. Because their diet is so special, they are very hard to keep healthy in zoos, and they are almost never seen in zoos outside Asia.

The IUCN gives about 52 percent of feeding records as leaves and about 40 percent as fruit, with a preference for young leaves and unripe, non-fleshy fruits; the monthly diet tracks availability. The Animal Diversity Web adds seeds, mangrove shoots and occasional invertebrates such as caterpillars and larvae, and notes a more frugivorous diet from January to May.

This specialization has practical consequences: the species is almost never kept in captivity outside Asia, because its diet of particular leaves and other plant material makes it hard to keep alive.

Proboscis monkeys live in family groups.

A grown-up male leads most groups. At night, lots of groups sleep together in trees by the river.

Most proboscis monkey groups have one grown-up male, several females and their young. Other groups are made up only of males. Groups usually have about 3 to 32 members.

The male leads the group and decides where it goes. He growls to calm the group and honks to warn of danger. In the evening, several groups gather to sleep in trees close to the river, and they choose a different spot each night. Proboscis monkeys are most active from late afternoon until dark.

The Animal Diversity Web describes one-male groups with 2 to 7 adult females, and all-male groups, of 3 to 32 individuals; Britannica gives groups of about 20 with a single male and up to a dozen females. Adult males coordinate group movement. Several groups gather in the evening into sleeping bands near the river and do not sleep in the same place on consecutive nights.

Calls include male growls that calm the group, male honks used as threats or warnings of predators, shrieks from females and juveniles, and screams during conflicts. Grooming, mostly between females, lasts 1 to 5 minutes. Leopards and crocodiles are known predators.

Proboscis monkeys need their riverside forests.

Some of these forests are protected in parks. Laws say people must not hunt the monkeys.

The proboscis monkey is endangered. Its numbers have fallen by more than 70 percent in about 36 years. People clear riverbanks and mangroves for logging, farms, shrimp ponds and homes, and fires have burned large areas of forest. Because they are slow-moving and easy to hunt, hunting can wipe out a whole local population.

Proboscis monkeys are protected by law in every country where they live, and they are found in at least 16 protected areas, including the Lower Kinabatangan Wildlife Sanctuary in Malaysia.

The IUCN lists Nasalis larvatus as Endangered (assessed 2015, published 2021 and flagged as needing an update), estimating a decline of over 70 percent, probably under 80, across three generations (36 years). Habitat loss is the main driver because riverine and coastal forest is favored for logging, cultivation and settlement; the Mahakam Delta population was decimated by conversion to shrimp farms, and the 1997 to 1998 fires destroyed much habitat in Kalimantan.

The species is easily hunted, for food and for bezoar stones used in traditional medicine. It is protected by law throughout its range and on CITES Appendix I, and occurs in 16 protected areas, although enforcement is weak in places. In Sarawak fewer than 1,000 are thought to remain.