Geographic Distribution and Habitat
Proboscis monkeys, called bekantan in Indonesia, and sometimes otherwise referred to as long-nosed monkeys, are endemic to Borneo, including all three nations that divide the island: Brunei, Indonesia, and Malaysia. During Dutch colonization, proboscis monkeys were accorded an additional local moniker, monyet belanda (Dutch Monkey), reflecting the perceived resemblance to the Dutchmen’s portly bellies and bulbous noses.
Proboscis monkeys inhabit mangrove forests along rivers and estuaries, swamp-land, and lowland rainforest and rarely range more than .6 mi (1 km) from water. They can also be found in wetlands that are not associated with the coast, such as swamp forests, limestone hill forests, rubber forests, and tropical heath forests. Populations that are found inland usually gather along rivers. Since forested areas along waterways are those that are first habituated by people, proboscis monkeys live in some of the most threatened habitats in Borneo.
Size, Weight, and Lifespan
Proboscis monkeys are one of the largest colobines (leaf-eating monkeys), and are the among the largest monkeys in Asia. They are sexually dimorphic in size; that is, males are distinctively larger and heavier than females. Males measure 28.7-29.9 inches (73-76 cm) in length and females measure 24.0-25.2 inches (61-64 cm) in length. Adult males can reach around 44-53 lb (20-24 kg). Females, however, are considerably smaller, reaching around half the weight of males, weighing approximately 22-24 lb (10-12 kg). Their long tails can reach at least the same length as their body, 26-29 inches (66-75 cm) in males and 20-24 inches (52-62 cm) in females.
Proboscis monkeys can live up to 20 years in the wild.
Sexually dimorphic in appearance as well as size, the primary differentiating feature between males and females is the nose—their most striking attribute. Noses of adult males hang down over their mouths, and may even extend past their chins, exceeding 4 inches (10 cm) in length and reaching up to 7 inches (17.5 cm). These large noses are used in sexual displays and as amplifiers, producing loud honking calls. Although female noses are substantial compared to other primates, they are considerably smaller in relation to the noses of the adult males, and are slightly turned upward. At birth, the noses of both males and females are more typically monkey-like in size, and they develop into their signature larger noses as they mature.
The coat of the proboscis monkey is light brown, sometimes with a hint of red on the head and shoulders. Their throat and neck pelage is cream colored. Their limbs and tail are gray. Males have a bright red visible penis and black scrotum. Infants are born with black fur and a blue face.
Another conspicuous feature of proboscis monkeys is their large bellies, which are a result of their diet and digestive system. Proboscis monkeys are highly selective feeders, and have a specialized chambered stomach (sacculated stomachs) that allows them to consume foods other primates cannot.
Their distinct characteristics do not end there, however. As proboscis monkeys live in waterways throughout the coastal areas of Borneo, they are adapted to swim. They have been observed swimming fully submerged for 65 ft (20 m). With webbing between their 2nd and 3rd toes, proboscis monkeys are not only accomplished swimmers, but can maneuver easily across soft mangrove substrate. Due to this adaptation, more forest patches are accessible for proboscis monkey group life, as their movements are not restricted by water.
What Does It Mean?
A layer of social structure in the multi-level societies of some monkeys that consists of multiple one-male units and generally remains relatively cohesive over space and time, analogous to the group or troop of other monkeys.
Having a series of different sections (e.g., langurs have sacculated stomachs, or stomachs with three sections)
Visit the Glossary for more definitions
Although proboscis monkeys are both seasonal folivores and frugivores, the main source of their diet consists of leaves. Their large and swollen bellies are made up of sacculated chambers that contain a special cellulose-digesting bacteria that breaks down complex leaf matter. These chambered stomachs digest leaves, an easily available but difficult-to-digest food source, through fermentation.
Because their digestive system is so specialized, they are somewhat restricted in what they can eat. For example, foods high in sugar content, such as ripe pulpy fruits, can cause rapid fermentation and bloat, which can be fatal. An improper diet can also cause complications such as: the fore-stomach becoming too acidic for the microflora within it to function properly; liver and blood disorders; or obstructions in the intestines or stomach, which can also result in death. This specialized diet is a major cause of mortality of proboscis monkeys in zoos, and a crucial reason why they are difficult to maintain and should not be kept in captivity.
Behavior and Lifestyle
Proboscis monkeys are the primate world’s most accomplished swimmers, frequently leaping from tree branches into the river and out again. Their webbed feet help them to successfully out-swim aquatic predators, such as crocodiles, and to swim across deep waters to find food or escape potential danger on land.
Arboreal (tree-dwelling), they generally otherwise avoid descending to the ground. The majority of their day is spent feeding, resting, and traveling.
Social grooming among troop members is primarily performed by females and is mainly directed at infants. Adult males rarely participate in grooming. Most of the allogrooming between individuals is done while seated.
Proboscis monkeys live sympatrically with a number of other primate species including surilis, gibbons, orangutans, langurs, macaques, slow lorises, and tarsiers. This means that they occupy the same geographical ranges with these other species. If one of these species is encountered, proboscis monkeys will usually ignore them and go about their business. There may be some competition between proboscis monkeys and the sympatric species due to overlap of food resources; however, aggressive behaviors between species is rare.
Proboscis monkeys leap from trees and sometimes hit the water with a comical belly flop.
Proboscis monkeys’ stomachs can contain up to a quarter of an individuals’ total body weight.
Proboscis monkeys are not only good swimmers, but can also walk upright in the water.
Daily Life and Group Dynamics
Proboscis monkeys live in stable social groups, the sizes of which can vary from 3 to 26 individuals and consist of one adult male, several females, and their infants and juveniles. Groups are found near one another and may join together. These larger groups serve to help reduce predation or avoid displacement. Groups also travel together in “bands,” which are formed by the fission and fusion of social groups; that is, the size and composition of a group changes as time passes. Proboscis monkeys merge into one group to sleep or travel (fusion) and split into smaller groups to forage (fission). Individuals do not play or groom with members of a different group.
On a typical day, following morning feeding, the groups disperse into the forest away from waterways, where they spend the rest of their day until they head back to waterways to feed again in the afternoon. In late afternoon, proboscis monkeys congregate along the waterways to feed, play, and communicate with other groups. This is an opportunity for all-male groups to check out their competition as well as potential mates before settling in for the night. These encounters can be noisy as rival males act out in front of one another, crashing through the branches. They usually sleep by waterways in large groups, high enough on branches to avoid predation.
Males leave their natal group before adulthood and join all-male groups. These adolescent males form bachelor groups until they are old enough to form their own harem (a group of females sharing one mate). Single-male group size may vary from 9 to 19 individuals, but larger groups, such as band-level social groupings, may consist of 60 individuals.
Occasionally, a female may switch through several harems in her life. Females may switch groups to avoid inbreeding and infanticide, to increase their dominance status, or to reduce feeding competition. Female proboscis monkeys may compete with one another to mate with the adult male in their group.
Proboscis monkeys use several forms of vocal communication: honk, alarm call, threat call, infant call, and female call. Occasionally males will make growling noises that are usually used to calm the group members. They honk as a threat, specifically to warn off predators or to warn the group of predators. Honking is also sounded to communicate territorial information. Males honk to infants for reassurance and to communicate information about group cohesion. Alarm calls differ between sexes, but males often emit these calls in situations of danger. Shrieks are used by females and juveniles to either show excitement or agitation. Screams occur during agonistic encounters.
Visual cues, such as displays of branch-shaking and open-mouthed bared teeth are used in low intensity agonistic encounters. Branch-shaking between males is accompanied by vocalizations and helps to maintain proper spacing between groups at sleeping locations during the night and in the morning.
Reproduction and Family
Females reach sexual maturity at about 5 years of age and males at 7 years. Females exhibit sexual swelling, in which her genitals appear to be pink or red. It is not certain if there is reproductive seasonality in proboscis monkeys, but in western Borneo there are some indications that they mate most during mid-year and give birth from March-May near the end of the rainy season. Populations of proboscis monkeys at different locations may have different birth seasons and mating seasons. Copulation is initiated by females. Males may also initiate copulation, but much less frequently than females. Females initiate copulation by puckering her lips, shaking her head, or presenting her rump to the male.
After a gestation period of five and a half months, a single infant is born. Births usually occur at night or in the early morning. The female sits on a tree branch during birthing and afterward she consumes the placenta.
Infanticide by adult males sometimes occurs after a group take-over. Infants are cared for by their mothers as well as by other adult females in the same group. This is called allomothering. A child remains with his or her mother up until about 1 year of age. Young females stay with their natal group (into which they were born), but males leave to join a bachelor group.
As herbivores, proboscis monkeys have an effect on plant populations by dispersing seeds.
Conservation Status and Threats
The proboscis monkey is classified as Endangered on the International Union for Conservation of Nature Red List (IUCN, 2015). Numbers of proboscis monkeys in Borneo have fallen in the last 40 years. There are estimated to be fewer than 20,000 proboscis monkeys remaining in Borneo, comprised of many small, isolated populations. Due to their specific habitat requirements, proboscis monkeys are effectively restricted to forest strips along rivers and the interlinking waterways of swamp forests and coastal mangroves.
Although fires, hunting, and illegal wildlife trade all are threats to proboscis monkeys, the major threat is habitat destruction. Waterways are typically the first areas to be disturbed by people, as they can provide a source of fuel, shelter, food, and income. Deforestation in Borneo is occurring at 1.7% a year, although it can reach as much as 7.92% in mangrove forests.
The rampant clearing of rainforests (especially riverine forests and mangroves) in Borneo for timber, palm oil plantations, and settlements has depleted huge regions of the proboscis monkey’s habitat. Forest fires along riverbanks have also taken out large areas in which the proboscis monkey once roamed. Habitat destruction and habitat fragmentation has impacted the distribution of the species.
Proboscis monkeys are being forced to descend from trees more often and travel long distances for food. Land predators include jaguars and some humans who consider them a delicacy. Since proboscis monkeys are relatively slow-moving, they are easy to hunt. They are often hunted for food, most significantly in the Bornean interior, but it is now increasing to coastal regions. They are also hunted for bezoar stones found in intestinal secretion, which is used in traditional Chinese medicine.
Even living in a protected area does not guarantee survival for proboscis monkeys. Despite an estimated 5,000 proboscis monkeys living in protected areas, entire populations still lose their habitat and die. Twenty years after establishing the Pulau Kaget Nature Reserve, in Indonsian Borneo (Kalimantan), 90% of the island had been cleared for agriculture, only providing narrow strips of forests surrounding the clearings for proboscis monkeys. This led to the remaining trees dying from over-use by proboscis monkeys, as they were forced to survive in these narrow strips. The end result was that proboscis monkeys began to die, forcing the remainder of the population to be moved to unprotected areas or zoos, where many died en route or following arrival. Protected lowland forests in Kalimantan decreased by 56% from 1985 to 2001, and by 2001, 53% of the remaining protected lowland forests in Kalimantan were overlapping with concessions and plantations.
The proboscis monkey is listed on Appendix I of the Convention on International Trade in Endangered Species (CITES) and is protected by law throughout its range. However, the legal protection of the proboscis monkey suffers in some proportions of its range due to governmental and institutional deficiencies such as lack of conservation funding, lack of knowledge, and poor management. A vital area of wetland in Sabah, called Kinabatangan Wildlife Sanctuary, has been designated as a sanctuary for Asian elephants, orangutans, and proboscis monkeys. However, even this protected area is fragmented by plantations, which species cannot cross. Proboscis monkeys are rarely seen in captivity due to their special dietary needs, thus they are hard to keep alive. The lack of protected areas and ideal management of these areas will cause the proboscis monkey population to continue to decrease. The protection of contiguous habitat is vital for the survival of this species.
Written by Tara Covert, August 2018