Proboscis Monkey

The proboscis monkey (Nasalis larvatus) is a large leaf eating monkey that lives only on Borneo, the third largest island in the world. Its English name comes from the Latin word proboscis, which means a long nose or snout, and it is the only living member of the genus Nasalis. Many people simply call it the long nose monkey or the big nose monkey, and both names point to the huge, drooping nose of the adult males. In Indonesia it is known as bekantan. In both Indonesian and Malay it also carries the old nickname monyet belanda or orang belanda, the "Dutch monkey", a joke from colonial times about the big noses and round bellies of the European settlers. The species belongs to the colobines, the group of Old World monkeys that feed mainly on leaves, together with the langurs of Asia and the colobus monkeys of Africa. It was described in 1787 by the naturalist Friedrich von Wurmb, and the second part of its scientific name, larvatus, means masked. The IUCN now treats it as a single species, without the two subspecies that were once recognized.

Only adult males grow the famous nose, which can reach more than 10 cm and hangs down over the mouth. Females have a much smaller, pointed and slightly upturned nose, and young monkeys have short, snub noses. For a long time the big nose was explained as a simple sound box, an idea that came more from popular stories than from research. A study published in 2018 by a team led by researchers from Kyoto University gave a clearer answer. Measuring wild males in Sabah and captive ones, they found that males with bigger noses also had bigger bodies and larger testes, and that they led harems with more females. Their sound recordings showed that a larger nose changes the honking call of the male and makes it deeper, so other monkeys can judge a male both by looking at him and by listening to him. In other words, the nose works as an honest advertisement that tells females and rival males how strong and fit its owner is. Scientists believe it became so large over many generations because females chose big nosed males and rivals avoided fighting them. A later study confirmed the link between nose and body size, although, to the surprise of the researchers, those bigger males had smaller canine teeth.

Proboscis monkeys are among the largest monkeys in Asia, and the difference between the sexes is striking. Adult males measure about 66 to 76 cm from head to rump and usually weigh 16 to 22.5 kg, while females measure about 53 to 62 cm and weigh only 7 to 12 kg, roughly half as much. The tail is almost as long as the body, between 52 and 75 cm, but it cannot grip branches and is used only for balance. The famous pot belly appears in both sexes. It holds a large stomach divided into several chambers, and the stomach with its contents can account for about a quarter of the weight of the animal. The coat is orange to reddish brown on the head and back, with a pale cream collar around the neck, gray arms and legs and a whitish gray tail. The face is bare and pinkish orange. Babies are born with a striking dark blue face, which slowly fades over the first months until it reaches the color of the adults. The hands and feet are long, and the toes are partly joined by a web of skin, a rare trait among primates that helps them swim.

Borneo is the only place on Earth where proboscis monkeys live in the wild, and the IUCN lists them in the three countries that share the island: Brunei, the Malaysian states of Sabah and Sarawak, and Kalimantan, the Indonesian part of Borneo. Within the island they stay in the lowlands, from sea level to about 350 meters, and almost always close to water. Their main habitats are coastal mangroves, peat swamp forests, freshwater swamp forests and the forests that line the banks of rivers. During the day they may move some distance inland to feed, but every evening they return to tall trees on the river edge to sleep, which makes them easy to watch from a boat at dusk. Some of the best known populations live along the Kinabatangan River and on the Klias Peninsula in Sabah, in Bako National Park in Sarawak, along Brunei Bay and in Kalimantan parks such as Tanjung Puting and Kutai. Their range is severely fragmented, because the coastal and riverside forests they need are also the land that people clear first for farms, ponds and towns. A survey of the coastal rivers of Sabah counted at least 5,900 proboscis monkeys, but only about 15 percent of them lived inside protected forest reserves.

Proboscis monkeys eat almost only plants. Long field studies along the Kinabatangan River found that young leaves make up about two thirds of what they eat, unripe fruits and their seeds about a quarter, and flowers most of the rest, with the mix changing through the year as different trees sprout and fruit. They largely avoid ripe, sugary fruits. The reason is their stomach, which works like a fermentation tank: bacteria inside its chambers break down tough plant fibers, and too much sugar would ferment too fast and could cause a dangerous buildup of gas. Thanks to these bacteria they can live on leaves that most other primates could hardly digest. In 2011 researchers working in the Kinabatangan reported something never seen before in a primate: at least 23 wild proboscis monkeys were filmed bringing food back up into the mouth, chewing it again and swallowing it, much like a cow chewing its cud. The behavior is not as regular as in true ruminants, and it is still not clear whether all populations do it. Because leaves give little energy, the monkeys spend a large part of the day resting quietly between meals while their stomachs do the work.

Proboscis monkeys can swim, and very well. Proboscis monkeys are considered the best swimmers among primates, and swimming is part of everyday life for an animal that lives among rivers, channels and flooded forests. Their toes are partly joined by a web of skin, which helps them paddle with a sort of dog paddle stroke. A common sight on the rivers of Borneo is a group crossing from one bank to the other: the monkeys climb high into a tree, then leap one after another and land in the water with a loud splash before swimming to the far side. Researchers have seen that they often choose narrow stretches of river to cross, which shortens the time they spend in the water. That matters because saltwater crocodiles are one of their main predators, and a monkey in the water is far more exposed than one in the treetops. Jumping into the river is also a way to escape danger on land or in the canopy. The habit of swimming also lets the monkeys move between patches of forest that are separated by water, something very useful in a landscape cut up by rivers and mangrove channels.

The basic unit of proboscis monkey life is the harem, a group formed by one adult male, several adult females and their young. A field study in Sarawak found an average group size of about nine, and larger groups of up to around 30 have been reported. Males that do not have a harem live in bachelor groups. These units are not closed: every evening several of them gather in the same stretch of riverside trees to sleep, forming larger bands, and females often move from one harem to another, which is unusual among monkeys. Young males leave the group where they were born and join bachelor groups until they are strong enough to win females of their own. The adult male defends his harem with loud honking calls and threat displays rather than with long fights. Females give birth to a single baby after a pregnancy of about 166 days, and the newborn, with its dark blue face, clings to its mother for the first months. Most of the day follows a calm routine of feeding in the canopy and long rests, with more activity in the early morning and late afternoon.

The IUCN lists the proboscis monkey as Endangered, in an assessment made in November 2015 and published in its current form in 2021. It estimates that the population has fallen by at least half over three generations, a period of about 36 years, and it describes the population as still decreasing and severely fragmented. The IUCN does not give a total number, and local counts vary widely, but the species is believed to number in the tens of thousands at most. The main threat is the loss of the narrow strip of forest it depends on. Mangroves are cleared for shrimp and fish ponds and for coastal towns, peat swamps are drained and burned for oil palm plantations, and riverside forest is logged. Fires in dry years destroy large areas of peat forest in Kalimantan. Hunting adds more pressure: the monkeys are killed for meat in some areas and for bezoar stones, hard masses from the stomach that are sold for traditional medicine. The species is protected by law in all three countries and listed in Appendix I of CITES, which bans its international trade. There are some good signs: in Bako National Park the count rose from about 111 monkeys in 2005 to around 306 a decade later, and protected sites in South Kalimantan report new births every year.