Nycticebus kayan

Geographic Distribution and Habitat

The Kayan River slow loris (Nycticebus kayan), also called the Kayan slow loris, lives on the island of Borneo and is named for the Kayan River that runs through its range. 

Distinguished as a unique species in 2013 and only assessed by the International Union for Conservation of Nature in 2015, the species’ full geographic distribution remains somewhat unclear. They are endemic to central and northern Borneo in Brunei, Sarawak, Sabah, and East Kalimantan. Research approximates that Kayan River slow lorises are distributed across the central and northern portions of the island, from the Mahakam River in the south to the southern base of Mount Kinabulu in the north. Though its range probably spans the entire width of the island, the species does not live near the coast.

A large portion of the species’ range likely falls within some of the last intact ancient rainforests still remaining in Borneo. Known today as the Heart of Borneo, this area is a lush mosiac of habitats. Kayan River slow lorises probably call many of its peat swamps, swamp forests, ironwood, heath, and montane forests home.


The Kayan River slow loris was considered a subspecies of the Philippine slow loris (Nycticebus menagensis) until 2013 when researchers determined its unique facial markings warranted its own identification.

Kayan Slow Loris geographic range. Map: IUCN, 2020

Size, Weight, and Lifespan

The average Kayan River slow loris is 10 inches (27 cm) in length and weighs approximately 410 grams (just under a single pound).

The lifespan of a Kayan River slow loris is currently unknown. In general, slow lorises have been known to live up to 25 years in captivity, although their average lifespan in the wild is likely closer to 20 years.

What Does It Mean?

In ecology, the ability of an animal to avoid observation or detection by other animals through methods such as camouflage, nocturnality, subterranean lifestyle and mimicry.

Visit the Glossary for more definitions


Species of slow lorises share the same basic appearance. Though their sizes do vary, all are quite small. They have furry bodies, round heads, short ears, and broad flat faces. The big toes on their hind feet are opposable, giving them the exceptional gripping power essential to moving through the trees. Their most prominent features are their large, forward-facing eyes. These capture every scrap of light available to them during their late-night forages.

Bornean species, like the Kayan River slow loris, are only subtly distinct from others. Though only apparent under close examination, one distinguishing feature is their lack of a second upper incisor.

Slow loris species often have signature facial markings or face masks. The Kayan River slow loris is a prime example of this phenomenon, its own face mask being crucial to its 2013 recognition as a species distinct from the Philippine slow loris (Nycticebus menagensis).

A Kayan River slow loris’s face mask is only subtly distinguishable from that of the Bornean slow loris (Nycticebus borneanus), which was also formerly considered a subspecies of the Philippine slow loris. The rings around her eyes are dark, rounded, and clear-cut. The white stripe that runs between her eyes is not rectangular (like a Bornean’s) but bulb-shaped. With these two varieties of slow loris being sympatric, it is important that individuals be able to easily recognize other members of their own species.

Photo credit: © mike_hoit/iNaturalist/Creative Commons

Slow lorises are omnivorous. They primarily forage for a variety of leaves, fruits, seeds, berries, gums, saps, and bird eggs but also hunt arthropods and sometimes even small vertebrates like lizards.

Due to living in bio-diverse habitats, a slow loris’s dietary needs are quite particular to the ecosystems where he or she lives.

Saps are an especially crucial part of a slow loris’s diet. Deprived of these and her other natural staples, a slow loris in captivity quickly grows malnourished and dies.

The diet of the Kayan River slow loris has yet to be assessed.

Behavior and Lifestyle

The nocturnal slow loris is arboreal (tree-dwelling) with limbs uniquely adapted for this lifestyle. Her hands and feet all have opposable digits that enable her to keep a firm grip at all times. Her style of locomotion closely resembles crawling—bringing one hand forward, her opposing foot follows. Her pace is slow and her movements methodical. While other arboreal primates typically swing and leap to cover gaps in the foliage, her own method is far less dramatic—though still exceptionally acrobatic! With only her feet latched to their perch, she reaches toward the next with her entire body, holding herself aloft with incredible balance and poise. When her two hands are wrapped firmly around the target branch, she hoists herself onto it, continuing on her way without skipping a beat.

With no way to actively protect herself against attacking predators, her best defense is in keeping as low a profile as possible, so as to avoid them altogether. Moving silently and scrupulously is just one of the many cryptic strategies she uses to this end. In fact, this need to remain under the radar and out of harm’s way is so key to her survival that it distinctly shapes her way of life. She also eludes predators by foraging under the cover of darkness. Her two large and sensitive eyes give her exceptional night vision. With these, she can attend to her forage while keeping her sharp sense of smell and keen hearing attuned to her surroundings and wary of any misgivings. Of course, her cryptic maneuvers advantage her as a hunter as well. Any unfortunate cicadas who cross her path will likely never realize until she’s already gobbling them up.

Some researchers have theorized that a slow loris’s markings have evolved to mimic the form of a cobra. Though in no way a perfect imitation, they could make enough of an impression to keep a predator at bay lest it does turn out to be a deadly cobra in the end.

Oddly enough, like cobras, slow lorises are venomous. Specialized glands hidden in her underarms secrete a special oil. When this oil mixes with her saliva, a toxin is created. She may spread the toxin through her fur with her tongue. This not only renders her an unsavory snack for larger predators but also helps to manage and kill pesky infectious parasites. If actively under attack, she uses her extremely powerful bite to inject venom into her attacker. When given, a slow loris bite is powerful enough to send a grown human into anaphylactic shock!

Her specialized glands are also used in her ritual of scent marking. As strepsirrhine primates, slow lorises have wet noses. This feature makes them highly receptive to smell—a sense that has diminished in many of the other primate lineages. As she crawls through the rainforest, she dutifully transfers her oils to the various substrates she passes. She also leaves her trace by urinating wherever she goes, a behavior researchers call “rhythmic urination.” Her scent marks are not used to mark her territory but as a way of communicating with other slow lorises.

After all, like so many primates, she is a social creature. Once more, however, her need to keep a low profile greatly influences the means by which she socializes with others. Congregating in large groups would make her and her fellow slow lorises far too visible to predators. Instead, scent marking is a way of communicating and bonding with members of her spatial group. By leaving her scent in various ways as she traverses her range, she keeps members of her spatial group privy to her movements. In the same manner, she remains knowledgeable of theirs. Scent marks also allow her to attract potential mates and even help them to track her down in the dense jungle foliage. Once one has located her, the more physical means of their courting commence.

The inner workings of slow loris social behavior are complex and not yet profoundly understood. Each species likely has its own unique set of mannerisms, rituals, and social dynamics that help individuals to recognize each other and bond as groups. For now, the socially driven behaviors of the Kayan River slow loris remain uninvestigated.

Fun Facts

The distinct face mask of the Kayan River slow loris is the feature responsible for its 2013 recognition as its own species.

The Kayan River slow loris has a venomous bite, strong enough to send a grown human into anaphylactic shock.

Daily Life and Group Dynamics 

A Kayan River slow loris wakes at night. Having spent the daylight hours asleep, he emerges from his hiding spot resolved to forage.

He sets off, moving delicately but persistently through the trees. He hardly makes a noise. His large eyes search the branches for his next meal. Spotting a tasty leaf, he uses his two grasping feet to anchor himself to his perch before reaching out with both hands to grab the leaf and bring it to his mouth.

Solitary and self-reliant, he spends most of his night hunting and foraging alone. However, his inclination toward solitude by no means makes him an unsocial creature. His range overlaps with several other members of his particular species. These individuals comprise what researchers call his “spatial group.” The members of this group lead mostly solitary lives. However, as they make their nightly forages, each one leaves behind scent marks. When other members happen upon one of these scent marks, they learn a great deal of information about the group member who left it.

Generally, no more than two members of the same spatial group encounter each other at one time. When this happens, they may or may not completely ignore each other. If they choose to connect, they are likely to spend time grooming.

Again, the social workings of slow lorises are not well understood at this time and those of the Kayan River slow loris in particular have not yet been researched.


As communicators, slow lorises make fantastic use of their olfactory senses. As strepsirrhine primates, they have wet noses that give them better-smelling capabilities than their haplorrhine cousins, who have evolved to have dry (and, therefore, less sensitive) noses. By relying on scent to communicate, slow lorises are able to maintain their low profiles while still keeping in touch with members of their spatial groups.

As she moves about, a slow loris leaves scent marks in her wake. She may rub the toxic oils from her brachial glands onto various substrates or use her urine. Some evidence suggests scent marking is particularly important to mating rituals. However, this behavior has only been studied in a select few species. While Kayan River slow lorises undoubtedly communicate using scent, it is possible that they use scent marking in ways unique to their species. We will not know until the issue has been properly researched.

Slow lorises do not make alarm calls as this would make their cryptic strategies for defense futile. However, for all their efforts to remain undetected, slow lorises do use vocalizations to communicate. Of the species of slow loris that have been studied so far, their repertoires consist of a variety of calls. Whistles and short keckers are probably used to show affiliation while snarls, grunts, screams, and long keckers are almost definitely signs of aggression. The Kayan River slow loris likely makes some, if not all of these sounds—and may even make others of which humans are not yet aware!

Research shows that Javan slow lorises make ultrasonic vocalizations that are beyond the range of human hearing. This gives them the ability to communicate at a frequency unheard by predators who might be nearby. However, this subject remains to be investigated in other slow loris species.

The ways in which slow lorises communicate are probably unique to their species—perhaps even their own spatial groups! Therefore, those in which Kayan River slow lorises communicate, while not altogether inconceivable, will remain ultimately unknown until research sheds light on them.

Reproduction and Family

Having only recently been distinguished among their fold, little is currently known about the mating rituals and familial relationships of Kayan River slow lorises. Furthermore, the nocturnal lifestyles (active at night) of slow lorises make studying them difficult for human researchers; and what little has been observed in one species cannot necessarily be assumed to be the case for others. Altogether, the more minute details of their habits remain difficult for us to apprehend.

Generally speaking, a female attracts a male using scent marks that likely signal to him her readiness to mate. Following her marks through the jungle, the male eventually locates her. In some species, a female initiates copulation by whistling while hanging upside down from a branch by her feet.

There is some debate as to whether or not slow lorises are monogamous (having one mate) or polyandrous (females having multiple mates). It likely varies depending on the species. Whatever the case, the mother seems to take complete responsibility for raising her offspring.

The birth of more than one slow loris at a time is a rare event. The mother leaves her newborn hidden in a nest while she forages. In case any predators should find him, she applies a coat of her toxic oils to his fur. Eventually, he develops the muscles and skills to leave the nest and join his mother on her nightly forages. From her, he learns the survival and social skills that will transform him from a helpless youth to the self-reliant creature into which a slow loris matures.

Photo Credit: © mike_hoit/iNaturalist/Creative Commons
​Ecological Role

The ecological role of the Kayan River slow loris is not understood at this time.

Conservation Status and Threats

The Kayan River slow loris has been assessed as Vulnerable by the International Union for Conservation of Nature (IUCN, 2015), appearing on IUCN’s Red List of Threatened Species. The species is suspected to be in decline due to loss of habitat from deforestation, forest fires, and the illegal pet trade.

Burning of habitat and forest conversion, especially to palm oil plantations, represent threats to the species. Because the Kayan slow loris is relatively adaptable to human settlements one might assume that the species could be less affected by forest loss than some other primate species. However, the severity of deforestation in general and recent forest fires specifically have had dramatic impact on the region and its inhabitants, including the Kayan slow loris. 

Kayan slow lorises are collected locally for use as pets. Subsequent uncontrolled release of pets in some areas is also a threat. Hybridization poses a real threat to this species both in the wild and among confiscated individuals in rescue centers. In addition, the species is at risk of exploitation in the illegal wildlife trade. Lack of law enforcement further threatens slow loris species across their range.

Until the late 2015 assessment of the species, which was only announced in the summer of 2020, the Kayan River slow loris was considered a subspecies of the Philippine slow loris whose 2008 IUCN evaluation deemed them Vulnerable. With the recognition of several new slow loris species native to Borneo since that time—all of which were once also considered subspecies of the Philippine slow loris—it is likely that each species’ population is in more critical condition than previously thought.

This taxonomic saga sheds light on the risks of unintentionally misrepresenting species based on insufficient data. When species are more biologically diverse than science previously assumed, it takes years for conservation efforts to catch up. In that time, the situations in which those previously unrecognized species find themselves are likely to grow more precarious.

Indeed, misunderstanding the taxonomy of slow loris species has proven problematic for their conservation both generally and locally. While researchers wrestle with the raw data, conservationists may occasionally misidentify the individual slow lorises that wind up in their care. Since taxonomy determines what they should be fed, how they should be socialized, and where they should be released if they are eventually rehabilitated, proper care and rehabilitation are dependent on proper species identification.

This issue of taxonomy is doubly important given the sheer number of slow lorises that end up in rescue and rehabilitation centers. The biggest threat to all slow loris species is their value as commodities in the exotic pet trade and for traditional medicines. Slow lorises are illegally trafficked and sold all over the world. Buyers rarely have any concern about what variety of slow loris they are receiving. When rescued from these circumstances, a slow loris is inevitably already in rough shape. Suffering from infection, malnutrition, and blindness, and highly stressed by his ordeal, there is no room for error in getting him what he needs as fast as possible. Therefore, proper rapid identification is essential to his rescuers’ success.

Unfortunately, poached from the wild, a slow loris’s chances of survival are small—and rehabilitation leading to her release even smaller. Even if she is properly brought back to optimal health, a single consequence of her ordeal often remains that will permanently prevent her eventual release. When she was first taken, her captors prepared her for sale. By removing her teeth—an incredibly painful procedure typically done without any kind of anesthetic—they prevent her from biting their future owners, which can prove fatal. However, with this one action she, a competent and self-reliant creature, is rendered helpless and dependent for the rest of her life. Without her teeth, she cannot properly eat or defend herself in the wild. And so, ultimately, she has no hope of ever going back.

While life in the wild is certainly preferable to life in captivity, slow lorises who avoid becoming pets are still threatened by human encroachment into their natural habitats. When left to their own devices, slow lorises often prove to be quite adaptable and capable of surviving in suboptimal conditions. In many places where they live, however, forests have been degraded to the point where it significantly and adversely affects their rates of survival—and fragmentation of their ranges inevitably inhibits healthy gene flow between spatial groups. What’s more, such trends put them at even greater risk of being found, captured, and sold into the pet trade.

Kayan River slow lorises live in one of the last and largest stretches of continuous ancient forest left in Southeast Asia. Known as the Heart of Borneo, this vast stretch of forest is less than half of what the island once supported. In the last thirty years alone, illegal logging, the construction of dams and other infrastructure, mining, slash-and-burn agriculture, and many other human activities have drastically depleted Borneo’s wilderness, putting the animals that call it home—including many humans themselves—at great risk.

The Heart of Borneo, for now, remains intact. But these trends have not let up. Compounded with the overarching effects of climate change, the future for the Kayan River slow loris’s home appears bleak. Slash-and-burn agriculture has become especially problematic over the years. With its peat forests and swamps, Borneo is highly vulnerable to fire. When a fire ignites in one of these environments it can smolder invisibly underground for weeks. In the wake of such fires, the scorched soil more easily erodes, preventing the forests from ever recovering. September 2019 saw some of the worst fires on record in Borneo, many of which had been set intentionally in order to clear the land for palm oil plantations. So long as the Heart of Borneo is at risk, so too is the Kayan River slow loris.

Conservation Efforts

Despite the assessment of the Kayan slow loris as Vulnerable to extinction, there remain no recovery plans, systematic monitoring of the species, or conservation management plans in place on behalf of the species. There are few local and international trade management agreements with questionable enforcement.

Deprived of adequate recognition, direct efforts to conserve the Kayan River slow loris are not currently underway. Fortunately, a number of conservation efforts do serve to benefit the lives of Kayan River slow lorises, if only indirectly. For instance, the species likely resides within a number of protected areas, including the spacious Kayan Mentarang National Park in the Indonesian province of Kalimantan.

The Kayan River slow loris must also benefit from the Heart of Borneo Initiative. This is a trilateral agreement between the three countries of Borneo (Indonesia, Brunei, and Malaysia) to work in cooperation to sustainably manage the last remaining rainforests of Borneo, using the largest of them—the one known as the Heart of Borneo—as their focal point. Their efforts to protect, research, and educate others about the biodiversity of these ancient rainforests are paramount to the survival of all the creatures who call these rainforests home, including the Kayan River slow loris.

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Written by Zachary Lussier, April 2020. Conservation status updated July 2020.