Where Do Proboscis Monkeys Live? Habitat Explained

Proboscis monkeys live only on the island of Borneo, making them one of the most geographically restricted primates on Earth. They occupy a narrow band of coastal and riverside forests across the Malaysian states of Sabah and Sarawak, the Indonesian province of Kalimantan, and the tiny nation of Brunei. But saying they live “on Borneo” undersells how specific their needs are. These monkeys are tied to water in ways that shape nearly every aspect of their daily lives, from what they eat to where they sleep to how they avoid being eaten themselves.

A Borneo-Only Primate

Proboscis monkeys (Nasalis larvatus) are endemic to Borneo, the world’s third-largest island. They have no wild population anywhere else. Their closest living relative, the pig-tailed langur, is separated from them by hundreds of kilometers of open sea on the Mentawai Islands off Sumatra’s western coast. Genetic and fossil evidence suggests that the ancestors of the broader “odd-nosed monkey” group likely originated in the region around southwestern China and the Hengduan Mountains along the borders of present-day Myanmar, India, and China, dispersing southward over millions of years as climates shifted.¹ Proboscis monkeys ended up isolated on Borneo, where they adapted to a life few other primates could manage: one built almost entirely around wetlands.

Within Borneo, their distribution is not uniform. Populations cluster along coastlines and major river systems. In Sabah (the northeastern state of Malaysian Borneo), surveys have found them concentrated along the eastern coastal zone, with smaller and sometimes isolated pockets elsewhere.² Researchers conducting boat-based surveys at wetland forest sites across northwest Sabah have confirmed how tightly the species sticks to waterlogged habitats.³ You do not find proboscis monkeys on Borneo’s mountainous interior or in its upland rainforests. They are lowland, water-dependent animals through and through.

The Forest Types They Depend On

If you think of proboscis monkey habitat, think of forests with wet feet. Surveys in Sabah have cataloged their suitable habitats as including mangrove forest, nipah palm forest, riverine forest (or mixed lowland forest along rivers), freshwater swamp, peat swamp, and a unique habitat type found on the Klias Peninsula made up mostly of herbaceous scrubs and swampy grasses.⁴ That is a range of forest types, but the common thread is standing or flowing water nearby.

Mangrove forests are perhaps the most iconic proboscis monkey habitat. These are the tidal forests along Borneo’s coastlines, dominated by salt-tolerant trees like Rhizophora and Avicennia species. The monkeys eat the leaves and fruits of several mangrove tree species, and the tangled root systems provide routes for climbing and escaping danger. Nipah palm swamps, which grow in brackish water along estuaries, serve a similar role. Both habitats sit right at the interface of land and sea.

Riverine forests line the banks of Borneo’s major rivers and their tributaries. These are often mixed lowland forests where large dipterocarp trees grow alongside the waterway, providing tall sleeping trees and a diverse food supply. Peat swamp forests, which develop over thick layers of partially decomposed plant material, are waterlogged year-round and represent another important habitat. The vegetation in these different forest types varies considerably, and so does the monkeys’ behavior within them. Research in East Kalimantan, for instance, found that daily movement distances differed across habitat types, and the monkeys adjusted which levels of the canopy they used depending on tree height, diameter, and density.⁵

Why Rivers Matter So Much

Proboscis monkeys do not just live near water; they organize their entire daily routine around it. A study along the Menanggul River, a tributary of the Kinabatangan in Sabah, tracked one group’s ranging behavior in detail. The group never ventured more than about 800 meters from the riverbank, using a total home range of roughly 138 hectares. Their average daily travel distance was around 800 meters, and on days when they moved away from the river, they rarely exceeded 1,000 meters because they had to return to the riverside before sunset.⁶ The river functions as both a highway and a home base.

This riverbank fidelity is closely linked to fruit availability. In that same study, the monkeys spent more time in areas closer to the river and in patches with more fruit. When fruit was scarce, they traveled farther in a day, but still kept the river as their anchor point.⁶ Rivers provide a reliable corridor of productive vegetation, which is part of why the monkeys are so reluctant to stray far from them.

Choosing Where to Sleep

One of the most striking aspects of proboscis monkey habitat use is their sleeping-site selection. Every evening, groups gather in tall trees along the riverbank to spend the night. A study along the Kinabatangan River identified 81 sleeping trees and found that the monkeys were highly selective. They chose trees that were taller than their neighbors, had larger trunks, and bore large, high first branches. Sleeping trees also tended to have more mature leaves and fruit, and fewer vines, than surrounding trees. The characteristics of neighboring trees mattered too, suggesting the monkeys evaluate not just the sleeping tree itself but the entire site.⁷

Sleeping in tall riverside trees is thought to serve multiple purposes. The height gives the monkeys a vantage point and makes it harder for predators to approach undetected. The location near open water means the tree is accessible from the river (useful for quick escapes) and benefits from whatever breeze comes off the water, which may help reduce insect harassment. Researchers have also noted that proboscis monkeys select sleeping sites and river-crossing locations partly based on the availability of arboreal escape routes that could protect them from predators like clouded leopards.⁸

How Flooding Changes the Rules

Borneo’s river systems flood seasonally, and this reshapes where proboscis monkeys sleep. Normally, groups cluster together at riverside sleeping sites, sometimes with multiple groups sharing adjacent trees. But during seasonal floods, when more than a meter of water covers the forest floor extending well over 700 meters inland, the monkeys abandon the riverbank and sleep deeper in the forest instead. They also stop aggregating with other groups and sleep alone.⁹

The likely explanation has to do with predators. When the forest floor is deeply flooded, ground-dwelling predators like clouded leopards cannot move through the area. With the terrestrial threat neutralized, the monkeys no longer need the safety-in-numbers strategy of riverside sleeping aggregations, so they spread out into the interior forest where food sources may be more accessible.⁹ This is a telling example of how the monkeys’ habitat use is not fixed but flexes in response to changing environmental conditions.

Predators and the Safety Calculus of Habitat

Understanding where proboscis monkeys live means understanding what is trying to eat them. The two main predator categories are clouded leopards on land and crocodilians in the water. Both the Sunda clouded leopard and two species of crocodilians, the false gharial and the saltwater crocodile, share the monkeys’ lowland river habitat. Researchers have suggested that both groups are significant potential predators of proboscis monkeys of any age or sex, and that predation pressure shapes anti-predator strategies including sleeping-site selection.¹⁰

The monkeys navigate this dual threat in their choice of where to cross rivers and where to sleep. River-crossing sites tend to be at narrower points, presumably minimizing the time spent in water where crocodilians are a danger. Sleeping trees offer arboreal escape routes, meaning branches that connect to neighboring trees, allowing monkeys to flee without descending to the ground where leopards hunt.⁸ The habitat itself, with its combination of water and tall forest canopy, is not just a place to live but an anti-predator landscape the monkeys have learned to read.

What They Eat and How It Ties to Habitat

Proboscis monkeys are primarily leaf-eaters, but their diet is broader than that label suggests. They consume young leaves, seeds, and unripe fruits, with a preference for specific plant species that varies by habitat type. A survey of plant diversity in proboscis monkey habitat along the Barito watershed in South Kalimantan identified 152 tree species, 41 of which served as food sources. In riverine forest, key food plants included species like Vitex pinnata, Dillenia excelsa, and Sonneratia caseolaris. In mangrove areas, Avicennia alba, Bruguiera gymnorrhiza, and Rhizophora apiculata were important food trees.¹¹

This diet helps explain why proboscis monkeys cannot simply relocate to any forest on Borneo. They need the specific assemblage of trees found in waterlogged, tidally influenced, or riverside habitats. Their specialized digestive system, which includes a multi-chambered stomach similar in function to a cow’s, allows them to ferment tough leaves. But it also makes them sensitive to dietary changes. They largely avoid ripe, sugary fruits that many other primates favor, because the rapid fermentation of sugar in their stomach can produce dangerous amounts of gas. The plants available in their wetland habitats suit this unusual physiology.

Threats to Their Habitat

The lowland coastal and riverside forests that proboscis monkeys depend on are also some of the most economically valuable land on Borneo. This overlap has driven massive habitat loss. A detailed analysis of proboscis monkey habitat around Balikpapan Bay in East Kalimantan, covering 2000 to 2017, found that palm oil production caused habitat loss throughout the entire study period, evolving from plantation expansion to the construction of processing infrastructure. Unexpectedly, the study found that industry and infrastructure development had recently overtaken shrimp farming and wildfires as the most prominent drivers of habitat change.¹²

Mangrove forests have been particularly hard hit. Across Southeast Asia, mangroves have been cleared for shrimp aquaculture ponds, coastal development, and charcoal production. For proboscis monkeys, the loss of mangroves eliminates both food sources and the tidal-zone habitat they rely on. Peat swamp forests face a different set of pressures: drainage for agriculture makes them fire-prone, and once drained peatlands burn, the organic soil itself is destroyed, making recovery extremely difficult.

In Sabah, the Sulaman Lake Forest Reserve, the only Class V Mangrove Forest Reserve in the Kota Kinabalu forestry district, was found to harbor a small and isolated relict population of proboscis monkeys. But degradation of the mangrove forest’s integrity was identified as a major threat to their survival there.² Small, cut-off populations like this one face the additional risks that come with isolation: limited genetic diversity, vulnerability to local catastrophes, and the inability to recolonize if wiped out.

How Much Habitat Is Actually Protected

Conservation efforts for proboscis monkeys revolve around protecting the remaining patches of suitable habitat. But the picture is sobering. An 18-year monitoring study of the Klias Peninsula in Sabah, one of the species’ population strongholds, found that less than half of the available proboscis monkey habitat in the region is fully protected. Population strongholds remain in the central part of the peninsula, particularly around the Padas Damit Forest Reserve, and distribution patterns have stayed relatively stable over the study period. However, the data also revealed a decline in the number of individuals per breeding group and very small, vulnerable populations at fragmented sites like Bongawan.¹³

The species is classified as Endangered on the IUCN Red List. Population estimates are difficult to pin down because of the logistical challenges of surveying remote, swampy forests, but all recent assessments agree the trend is downward. Some populations, particularly those along major rivers where ecotourism brings economic incentive to preserve the forest, are doing better than those in areas where logging and agriculture dominate land-use decisions.

Habitat Restoration and What It Requires

Because proboscis monkeys are so habitat-specific, conservation strategies have increasingly focused on restoration of degraded wetland forests, not just protection of what remains. The challenge is figuring out what to plant. The survey that identified 152 tree species in proboscis monkey habitat, 41 of them food sources, was conducted specifically to create a reference list for restoration projects.¹¹ Getting the species mix right matters. Planting the wrong trees, even native species, in a restoration project could create forest that looks suitable but fails to provide the specific food resources proboscis monkeys need.

Restoration also has to account for hydrology. Simply planting trees on drained peatland does not recreate peat swamp forest. The water table has to be raised, which means blocking drainage canals, a process that can bring the restored land into conflict with neighboring agricultural operations. Mangrove restoration requires appropriate tidal influence, salinity levels, and substrate conditions. These are not quick fixes. Restoring functional proboscis monkey habitat can take decades, which is why protecting existing habitat is still the more effective strategy where possible.

Living Alongside People

Proboscis monkeys are not deep-wilderness animals in the way that, say, orangutans can be. They frequently live in forest patches adjacent to villages, oil palm plantations, and aquaculture ponds. Along the Kinabatangan River in Sabah, which is one of the most studied proboscis monkey populations, the forest has been reduced to a narrow ribbon along the riverbank, sometimes just a few hundred meters wide, hemmed in on both sides by oil palm. The monkeys persist in this thin corridor, though the habitat fragmentation limits their movement and genetic exchange between groups.

Ecotourism has become economically important in some of these areas. River cruises to see proboscis monkeys at their evening sleeping sites are a draw for tourists visiting Sabah, and this creates a financial incentive for local communities and governments to maintain the remaining riverside forest. The relationship is complicated: too many boats at dusk can disturb the monkeys’ sleeping-site selection and group aggregation behavior, but the tourism revenue also makes the argument for conservation tangible in a way that species reports alone often fail to.

Proboscis monkeys have also been documented occasionally feeding in oil palm plantations and other agricultural areas at forest edges, though they do not thrive there. They are not crop pests in the way macaques can be, and their forays outside the forest are a sign of habitat pressure rather than adaptability. Their specialized digestive system and reliance on specific plant species mean they cannot make a living in agricultural landscapes the way more generalist primates can. The forest is not optional for them. It is the whole story.