Geographic Distribution and Habitat
Proboscis monkeys, also known as bekantan in Indonesian and orang belanda in Malay, are endemic to Borneo. Here, they inhabit the island’s tropical forests in each of the three nations that politically divide it: Malaysia, Indonesia, and Brunei. While these monkeys are found mostly on the coast, in mangrove forests, or living along rivers in dipterocarp forests (tropical lowland rainforests with the tallest trees and greatest biodiversity), these one-of-a-kind monkeys are somewhat adaptable so long as they live near a slow-moving water source with trees growing along its banks.
Proboscis monkeys are the sole representatives of their genus, Nasalis, a name that comes from the Latin word for “nose.” At one point in history, some researchers speculated there may be two subspecies of proboscis monkeys. Following more research, only one type could be identified.
Size, Weight, and Lifespan
Proboscis monkeys are some of the largest monkeys in Asia. Males can weigh up to 50 pounds (22 kg) and measure between 29 and 30 inches (73–76 cm), head to belly. Their long tails add an extra 23.6–26.3 inches (60–67 cm).
Females are quite a bit smaller than their male counterparts, weighing in at 22 pounds (10 kg) on average. Head to belly, they measure between 24 and 25 inches (61–64 cm), and their tails are between 21.6 and 24.4 inches (55–62 cm).
Males have noticeably larger skulls and canine teeth than females, and also sport the largest noses of any primate, sometimes exceeding 4 inches (10.1 cm) in length!
Proboscis monkeys are sexually dimorphic in appearance, meaning that the males and females look noticeably different from each other. Apart from their difference in size, there are also key morphological differences between them, especially in their faces.
Proboscis monkeys are well-known (and frankly beloved) for their bulbous noses that droop down over their mouths. This feature is unique to males. However, females’ noses are still distinctive in their own right, sticking out quite far and turning slightly upward into a snout similar to a pig’s, with two prominent and gaping nostrils.
Another key difference that makes male and female proboscis monkeys distinct is the fur around their necks and upper torsos. Males sport impressive white collars. Depending on the angle, this can either frame a male’s face (making it look quite a bit larger) or give his upper body a formidably muscular air. On the lower half of their bodies, males have a bright pink penis, appearing almost neon in tone against the backdrop of their gray fur and black scrotum.
Both sexes share similar coloring and markings. Their faces are pink and leathery. The fur running under their chins from ear-to-ear is white. On the top of their heads, just above the brow, a patch of russet orange fur looks almost like a bad toupee. This orange fur runs down the length of their backs while the white fur on the napes of their necks turns slightly orange on their upper arms and bellies. The fur on their forearms and waists dulls to gray before exploding back into white on their legs and feet. The naked skin on their palms and the bottoms of their feet is black.
Look past the proboscis monkey’s clownish nose, and you’ll find a fascinating creature with quite a number of fascinating and surprising traits. Their large pot bellies encase a four-chambered stomach that helps them digest a cellulose-rich diet, and a close-up on their hands and feet reveals webbed digits, an adaptation rarely found in primates!
Webbed fingers and toes help proboscis monkeys to navigate their river and coastal environments. Acting similar to snowshoes, they distribute the monkeys’ weight just enough to keep them from sinking into the muddy forest floor. Perhaps more mind-blowing: their webbed extremities help them to swim, allowing them to make river crossings when necessary. The only other primates that are master swimmers are some macaque species and, of course, humans.
Proboscis monkeys eat mostly leaves and fruit, together making up about 90% of their diet. The remaining 10% is a hodgepodge of foods like flowers, bark, insects, and insect larvae. Individuals have also been observed eating crabs and mushrooms, but this behavior seems rather rare.
While their diets change according to seasonal availability, proboscis monkeys prefer young leaves and unripe, non-fleshy fruits. When chowing down on ripe fruits, they usually pick out the seeds and discard the rest as the sugars they contain can cause them severe indigestion and sometimes even death.
In spite of their preferences, proboscis monkeys keep a surprisingly rich and diverse diet. One study observed a group eating a total of 188 different plant species in a single evening!
Some researchers speculate that proboscis monkeys originally evolved to live in mangrove forests along Borneo’s coast as they are one of the only species of primates capable of digesting the tannin-rich plants found in these ecosystems. This is made possible by their chambered stomachs, which break down these toxic tannins and the extreme amounts of fiber they ingest. This amazing adaptation made it possible for proboscis monkeys to spread to other habitats further inland. The proximity of a slow-moving river of swamp seems to be their only requirement, as these provide certain minerals and salts necessary for their health and well-being.
Populations living in mangrove forests along the coast have different staple foods than those living more inland in dipterocarp forests. A study on proboscis monkeys’ gut microbiomes (the collection of bacteria and other microbes living in an organism’s digestive tract) found that the former populations had less diverse biomes than the latter. Maintaining a healthy gut microbiome is important since microbes (organisms, like bacteria, too small to be seen without a microscope) aid in the digestive process, influencing both their physical and mental health. While both types of populations were found to be healthy, the findings suggest that the menu of dipterocarp forests is a step-up from the fare offered by mangrove forests.
Behavior and Lifestyle
Proboscis monkeys are elusive and shy primates known to bolt at the sight or sound of threats, including humans. Historically, this made them quite hard to study in the wild; this and their chosen habitats, where water and mud tend to make navigation difficult. Just the noise of a motorboat in the distance sends these attentive monkeys packing.
Proboscis monkeys spend most of their time in the trees where they have all the food they need and where they are generally safe to rest during their long digestion process. When digesting, they rarely lie on their side. Instead, they are often found sitting upright, a posture that aids in the digestion of their fiber-rich diets. A recent study discovered why.
While chambered stomachs are found in all colobines (the taxonomic subfamily of leaf-eating monkeys that includes proboscis monkeys), proboscis monkeys are the only ones (and the only primate for that matter) who regurgitate and remasticate their food. To put this more simply, they actively bring food they have already eaten back up from their stomachs in order to rechew it and swallow it again. This is the same eating strategy a cow uses. In fact, because of this behavior, the particle size of proboscis monkey poop is more like that of a cow’s (tiny) than of their fellow primates.
When on the go, proboscis monkeys are usually looking for their next meal. To navigate the treetops, they clamber and climb along branches and make daring leaps across gaps in the canopy. In general, these arboreal monkeys rarely descend to the forest floor. When they do leave the safety of the trees, it’s usually to escape a predator or to cross a river.
Proboscis monkeys are one of few nonhuman primates known to engage regularly in swimming, and their webbed fingers and toes make them quite adept at it! They ply through the water with ease, doggy-paddling to the opposite bank. Since most of the swamps and rivers they dip into are infested with crocodiles and other potential predators, speed is of the essence. If necessary, or spooked, a proboscis monkey can even dive and swim underwater.
When crossing bodies of water, proboscis monkeys have a few different strategies. In order to limit the time they are vulnerable to water-dwelling predators, groups often try to cross at areas where a river is narrowest, and the monkeys may choose to enter the water silently so they are less likely to attract predators. Very often, however, they leap!
Leaping is more efficient. Though it ensures they make a loud splash (one that the water will carry up- and downstream for quite a distance), proboscis monkeys employ other strategies to make this practice safer. For instance, they know to leap as far out as possible in order to minimize the distance they will have to swim. In order to ensure optimal distance, they leap from high up in the canopy, often staggering heights. Falling, they splay their arms and legs out wide before tucking them back in just before hitting the water.
In certain situations, water can also provide safety for proboscis monkeys. When at rest, groups tend to choose branches directly above the water. Here, the slender boughs are more sensitive to movement, acting as a natural alarm system the moment a predator, like a hungry clouded leopard, comes too close. If the monkeys suddenly have to flee, they can drop down into the water and swim to safety.
Proboscis monkeys share their habitat with long-tailed macaques (Macaca fascicularis). But neither are particularly territorial. The two species generally get along and, probably because they know there is safety in numbers, even occasionally share sleeping and feeding trees. Long-tailed macaques also happen to be one of the other nonhuman primate species known to swim. Is this a coincidence? Or could the macaques have learned the behavior by watching and mimicking their long-nosed cousins?
The Malay name for proboscis monkeys, orang belanda, means “Dutchman.” Originally, the joke was that, with their big noses and portly bellies, the monkeys resembled the Malay peoples’ European colonizers.
Early observers of proboscis monkeys assumed males had to move their noses out of the way in order to put food in their mouths. (They don’t.)
A recent study uncovered some fascinating insights into proboscis monkeys’ social lives. These monkeys live in intricate communities where group membership can change easily. They peacefully share territories, they rarely engage in conflicts, and multiple small groups often gather to eat, sleep, and socialize. What’s truly exceptional is that both males and females, but especially females, tend to switch groups many times during their lives—a behavior not seen in your average primate. This complexity in their social structure is said to be even more intricate than that observed in great apes and, in some respects, more closely mirrors the dynamics of human communities.
Within the intricate social framework of proboscis monkeys, smaller groups typically range in size from three to 32 members, following a few common patterns. Many groups are composed of a single adult male, several adult females, and their offspring. This arrangement, known as polygyny, involves all the females mating with the dominant male, who in return offers them protection. Meanwhile, bachelor groups, consisting of multiple adult males, offer safety and companionship as they await the opportunity to form their own single-male groups. Interestingly, bachelor groups occasionally include one or two exclusive female members.
Proboscis monkeys are masters at maintaining harmony within their groups. Unlike many other species, they rarely engage in violent fights or show excessive dominance. Instead, they actively steer clear of conflicts or address them swiftly. These monkeys form strong bonds with one another, which helps prevent disagreements and makes them easier to resolve over time. Additionally, they retain a playful nature throughout their lives, which keeps the atmosphere light and easygoing among adult members.
In the world of proboscis monkeys, the high-ranking males are more likely to be nurturing leaders than cruel despots. In many primate species, big canine teeth are used as weapons to establish dominance, but proboscis monkeys have a different approach. While males generally have larger canines than females, a recent study revealed that the males with the biggest noses, who are more popular with females as group leaders, actually have smaller canine teeth compared to their less popular, smaller-nosed competitors. This suggests that in proboscis monkeys, having a large nose is more important than having large canines when it comes to attracting mates, and males tend to avoid challenging their larger-nosed counterparts. The reasons behind this complex balance of natural and social selection processes are not fully understood, but they seem to favor male leaders who prioritize cooperation and kindness over violence and intimidation.
All primates are naturally social creatures, and as a result, they’ve developed various ways to communicate, such as unique facial expressions, body language, physical gestures, and vocalizations. Proboscis monkeys are no exception. They too have complex social dynamics that involve equally intricate methods of communication. However, our current knowledge on this subject is limited, leaving much more to discover. Nonetheless, ongoing research has already uncovered some fascinating details.
Vocalizations are crucial for proboscis monkeys, and they use a range of sounds like honks, brays, groans, squeals, and loud roars to communicate information about their surroundings and emotions. Honks serve as warnings about predators or threats. Females and young monkeys emit shrieks when excited or upset. Males use growls to calm agitated group members and maintain peace. When conflicts arise, brays are used by monkeys outside the disagreement to encourage resolution. Researchers frequently witness “chorus” events when many individuals, or even whole groups, create a cacophony of sounds in the forest. Finally, loud roars are exclusive to males and useful to intimidate.
Proboscis monkey males have an advantage in vocalizing due to their large noses, which amplify their calls and make them louder than those of females. These louder calls are valuable for alerting the group to dangers. As a result, females prefer to join groups led by males with the largest noses, as it signifies their ability to protect the group. In this way, a male’s nose size communicates his attractiveness as a potential mate, and those with the largest noses earn respect and prestige as leaders that smaller-nosed males are unlikely to challenge.
In the rare event of a male challenger, he uses a physical display to assert himself. This includes opening his mouth to reveal his canine teeth, standing upright to appear larger, and emitting a powerful and intimidating call to establish dominance. Males also like to flaunt their bright pink penises.
Proboscis monkeys use physical actions to communicate and strengthen their bonds and relationships. They engage in affiliative behaviors such as grooming, playing (even as adults), and sitting close to each other. While these actions may appear unremarkable, they convey closeness and foster affection among individuals, reducing the need for conflicts and disputes. Typically, females invest more time in forming these close connections with each other compared to males.
Proboscis monkeys engage in year-round mating, and both males and females can have multiple mates over their lifetimes. It’s the female who initiates copulation. She communicates her interest by making a pouty face and pursing her lips at her potential mate before presenting her hindquarters. If the male is also interested, he mounts her for copulation.
Once pregnant, a female proboscis monkey carries her baby for an average of 5.5 months. After birth, she has a single infant who clings to her fur. The baby’s dark blue face and nearly black fur signal its young age to other adults, who can assist the mother in caring for it. Although the mother primarily takes care of her offspring, female proboscis monkeys often cooperate in raising each other’s babies, and they can even nurse infants that aren’t their own, displaying a remarkable level of care and kindness within their social groups.
Between 12 and 18 days after birth, the proboscis monkey infant is weaned and starts exploring the canopy on its own. During this time, they lose their blue face and darker coat, typically around three to four months of age. As a juvenile, which is between 1.5 to 3 years old, they still heavily rely on adults for food and comfort. They don’t begin venturing out on their own until they’re between three and five years old.
Male proboscis monkeys typically leave their natal group and often join a bachelor group before eventually forming their own group with multiple females. Females, on the other hand, tend to stay with their natal group, but it’s not a strict rule. Recent research has shown that female group membership is more flexible than previously thought, and they too can change groups when needed.
Proboscis monkeys act as seed dispersers. When they eat both ripe and unripe fruit, the seeds travel through their digestive tracts. They drop to the ground, still intact, in their feces miles away from where they were eaten. Through this process, the forest is constantly being regenerated, and the biodiversity of forests is preserved.
Many primates are known to disperse seeds in this way. However, a recent study revealed something rather special about proboscis monkeys’ roles as seed dispersers. Researchers found the germination rates (the likelihood that seeds sprout) of seeds consumed by proboscis monkeys are greater than those of seeds that never traveled through a monkey’s gut. Rates increased for seeds of both ripe and unripe fruit after being ingested. However, seeds of unripe fruit (proboscis monkeys’ preference) showed an even higher rate of germination. In short, proboscis monkeys are not just dispersing the seeds they eat, they’re also helping to ensure they grow!
The proboscis monkey is classified as Endangered by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species, and populations are currently in decline.
Proboscis monkeys face a host of threats in the wild. In particular, habitat loss is a major contributor to this species’ decline. All across Borneo, forests are being degraded and destroyed, cleared for timber or to make space for farms and human settlements. Palm oil and rubber plantations have replaced swaths of what were once some of the world’s most biologically diverse forests with carefully managed monocultures. The island’s mangrove forests are being cut down to make way for shrimp farms. Without these habitats, healthy and intact, proboscis monkeys cannot survive.
As arboreal primates, proboscis monkeys are vulnerable to any and all changes in the canopy. This means that forests don’t have to be completely cleared in order to negatively affect them. When patches of forest are left, they are often cut off from each other. This phenomenon, called forest fragmentation, limits an arboreal primate’s ability to roam freely and widely. Even a small gap in the canopy made by a road, can become an impassable obstacle for them. In this way, groups become isolated, limiting their access to food as well as mates and increasing the likelihood of inbreeding.
With inbreeding come a host of other threats. When gene flow slows or halts altogether, new generations become more vulnerable to diseases and parasites, and individuals may be born with debilitating birth defects. All of these changes severely decrease the chances that new generations live long enough to ever procreate. For proboscis monkeys, who never stray far from their natal rivers, the problems that forest fragmentation creates become even more detrimental. By greatly reducing their area of possible distribution, the clearing of riverbanks and mangrove forests has highly adverse impacts on proboscis monkey populations throughout Borneo.
As a biodiversity hotspot, Borneo represents some of the most unique and fragile ecosystems in the world. The extinction of one species initiates a domino effect, ensuring that others begin to die out in turn. As seed dispersers, the loss of proboscis monkeys brings their forest ecosystems to a screeching halt. A lack of new trees renders the few remaining forests unhealthy, limiting the diversity of species capable of thriving in them. The effects of global climate change are already, and will continue to, exacerbate these problems, not to mention create their own. Borneo is not just fragile, it’s irreplaceable. Once Borneo’s habitats become degraded, rejuvenation is unlikely; once species are destroyed, there is no way to bring them back. In this way, Borneo’s identity and fate are both tied to that of the proboscis monkey, not to mention all of its countless endemic species, both discovered and undiscovered.
Borneo’s habitats are also threatened by forest fires. Southern Borneo’s rainforests grow in an accumulation of partially decayed vegetation or organic matter known as peat. Peat builds up in wetlands and, because it holds water well, causes conditions in the area to gradually become wetter and wetter. As a result, the wetlands grow. During the last several decades, draining Borneo’s peatlands has become a common practice for farmers and loggers. After drainage, the peat dries out, becoming highly flammable. Farmers have been known to purposely set drained peatlands on fire in order to quickly clear the area. Once it breaks out, a fire is nearly impossible to extinguish as it will continue to burn invisibly underground. The 1997–98 forest fires inflicted extensive damage on the habitat of proboscis monkeys and are believed to have caused the most significant loss of habitat among all primate species in Kalimantan (Indonesia).
On top of these threats, proboscis monkeys are frequently hunted in Borneo. Because they spend hours at rest while they digest their leafy meals, they are easy targets for poachers. Once found, entire communities can be wiped out in little time and with little effort. While they are mostly hunted for meat, they are frequently killed in pursuit of bezoar stones. Bezoar stones are small masses that form in a creature’s intestine, which practitioners of ancient Chinese medicine claim (without scientific evidence) can cure ailments and neutralize poisons. The practice of hunting proboscis monkeys seems to have begun inland, but it is now spreading towards Borneo’s coasts.
While rehabilitation and release programs have worked to return rescued proboscis monkeys to the wild, their complex diets, impossible to replicate in captivity, make breeding programs unfeasible. Once they are gone from the wild, they are gone.
The proboscis monkey is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. It is also legally protected in Brunei, Malaysia, and Indonesia and can be found in several protected areas within all three of these countries. Unfortunately, due to a lack of institutional funds, inadequate knowledge and poor overall management, these protections are infrequently enforced.
Fortunately, there are a few organizations in Borneo dedicated to conserving proboscis monkeys. The Labuk Bay Proboscis Monkey Sanctuary in Sabah (Malaysia) has been in operation since before 2000, one positive outcome of the devastating droughts and forest fires of 1997–98. Its mission includes protecting the population of proboscis monkeys at Labuk Bay from extirpation, creating more sustainable habitat in the region (including rejuvenating its mangrove forests), educating locals about the importance of proboscis monkey conservation and leveraging their support. The organization also aids efforts to research the monkeys they protect, and multiple eye-opening and significant discoveries about proboscis monkeys have been made thanks to their project.
The Danau Girang Field Center, also located in Sabah and managed by the Sabah Wildlife Department and Cardiff University, conducts research aimed to reduce the loss of Asian biodiversity by understanding the effects of habitat fragmentation on tropical ecosystems. The center has been involved in a number of studies involving proboscis monkeys, investigating the effects fragmentation has on their kind and exploring new methods to help study them in the wild.
In 2013, Dr. Amalia Rezeki founded the Friends of Indonesian Bekantan (SBI) organization in Indonesia. Despite the proboscis monkey being declared the mascot of South Kalimantan, an Indonesian province, back in 1990, Dr. Rezeki noticed that not enough was being done to ensure the survival of this unique species. Her organization operates on multiple fronts to protect the proboscis monkey, the symbol of her province. Through a collaboration with Lambung Mangkurat University, SBI established the Bekantan Research Center on Curiak Island, a crucial conservation area for proboscis monkeys. Here, they conduct research and educate visitors through ecotourism programs. SBI also runs a rescue, rehabilitation, and release program, which has successfully aided 45 proboscis monkeys as of February 2020. Additionally, habitat restoration is a central part of SBI’s mission. They are actively involved in restoring riverbanks by planting thousands of rambai trees, the leaves of which are a staple of the proboscis monkey’s diet.
Over the years, proboscis monkeys have gained a devoted following. They’ve been featured on television programs put on by the likes of National Geographic and BBC Earth, and researchers have shown particular interest in studying this primate, perhaps even rivaling the attention typically reserved for great ape species. It’s worth noting that the New England Primate Conservancy’s page on proboscis monkeys consistently ranks among our most visited, underscoring the species’ unique allure.
What is it exactly that makes proboscis monkeys so captivating? Clearly, their distinctive physical traits, such as their prominent noses and webbed feet, are what draw most people in initially. However, it’s the surprising behaviors they exhibit that keep our attention. From their fluid group dynamics to their unique vocalizations and peaceful natures, proboscis monkeys challenge our mainstream understanding of nonhuman primates in ways that resonate with our own human experiences.
Whatever the reasons for it, this level of attention and fascination offers conservationists a unique opportunity to galvanize support for their efforts. The connections to be found, both in appearance and behavior, serve as poignant reminders of the interconnectedness of all life on our planet. They reinforce the urgency to protect these delightfully peculiar creatures, and indeed, all species facing extinction. By igniting our curiosity and affection, proboscis monkeys should leave us pondering what other primates we may have overlooked in our studies and what valuable insights we might uncover about them, ourselves, and our world by devoting more of our time and efforts to study them.
Written by Zach Lussier, September 2023