Bornean Slow Loris, Nycticebus borneanus
BORNEAN SLOW LORIS
Nycticebus borneanus
Geographic Distribution and Habitat
The Bornean slow loris, known scientifically as Nycticebus borneanus, is a loris found in central south Borneo (West, South, and Central Kalimantan, excluding the extreme southwest), south of the Kapuas River, and extending as far east as the Barito River. The Bornean slow loris finds home in heavily degraded to pristine rainforests, plantations, and lowland and montane forests. Oil palm plantations are common in Borneo and have been slowly taking over many of the natural rainforests.
Borneo is known for its humid tropical equatorial climate with a wet season from November to April and a drier season from May to October.
The Bornean slow loris used to be classified under Nycticebus menagensis. In 2013, after an extensive investigation into their physical characteristics, scientists found differences in their “mask” that defined differences between the slow loris species. In fact, four species had been lumped together, which were eventually parsed out into the Philippine slow loris (N. menagensis), the Bornean slow loris (N. borneanus), the Bangka slow loris (N. bancanus), and the Kayan River slow loris (Nycticebus kayan).
Not only did these species look very similar (minus the masks, of course), they also live in very close proximity to one another. Most notably, the Bornean slow loris lives in environments alongside the Bangka slow loris (N. bancanus), creating a world of confusion for scientists identifying different species.
A study in 2024 using molecular studies confirmed the 4 species mentioned above, but also included other distinctions among the slow lorises, reflecting an ever-changing perspective on the classification of these primates.
Recent taxonomic revisions have made studying these animals more challenging. Scientists are still working to distinguish between closely related species and to clarify their specific ranges and habitats. As additional species are recognized, the need for further research becomes even more important to fully understand the diversity within slow lorises.
Size, Weight, and Lifespan
Museum specimens of the Bornean slow loris average about 10.24 inches (26 cm) in head-to-body length. To date, individuals have not been measured or weighed in the wild.
In other slow loris species, individuals living in professionally managed human care have reached up to 25 years of age and have weighed between 9.3 and 56.6 ounces (265–1,605 g). However, specific lifespan and weight data for the Bornean slow loris remain limited.
Sexual dimorphism—where males are larger than females—has been documented in some slow loris species, but it has not yet been confirmed in this species.
Appearance
A key feature of the Bornean slow loris, perhaps better described as the species-defining feature, is the dark and contrasting facemask. This facemask, which is characterized by a dark coloration around the individual’s eyes, led to the clarification of four different slow loris species. The dark mask never extends very far from their eyes and does not go past their cheekbones. A light stripe of fur goes between their eyes and further accentuates the mask around their eyes with white fur reaching the cheeks. Their mask is what some scientists call an aposematic signal, which is a physical appearance that communicates toxicity to other organisms. Take the poison dart frog, for instance. The bright coloration of the poison dart frog indicates its toxic qualities, using color as a warning. The same goes for the Bornean slow loris. The harsh black mask on their faces is a warning sign to predators about their innate toxicity. This toxicity hides in the crook of their elbow. An oblong-shaped gland, the brachial gland secretes a fluid that, when mixed with saliva, can create a venomous liquid.
Like other slow lorises, Bornean slow lorises have flat faces with forward-facing, large eyes. The eyes are light brown in color and help them see very well at night when they are most active. They are round, fluffy creatures with a brown and sometimes reddish-brown pelage. Their fur also has frosted tips. The hairs of their fur are a brown color towards the base and eventually reach a white color at the tips, giving the individual a frosted, dusty look.
Bornean slow lorises have opposable thumbs and opposable big toes, allowing them to hang securely on trees.
Bornean slow lorises, like all other lorises, are classified as strepsirrhine, or wet-nosed. This wet nose enhances their sense of smell.
All Bornean lorises, like the Bornean slow loris, lack an upper incisor, a sharp tooth meant to cut and shear food when biting. This feature helps identify them from other slow loris species. While they may not have an upper incisor, Bornean slow lorises, like all lorises, have a specialized set of lower teeth called a “toothcomb” that can aid in grooming.
Slow lorises also have a grooming claw on the second toe of the hind foot. The grooming claw aids in scratching hard-to-reach spots and helps groom fellow groupmates.
Diet
The diet of the Bornean slow loris has not been directly recorded. Other slow loris species have been recorded to be omnivores and will eat small birds, insects, reptiles, fruit, gum, sap and nectar. Highly dependent on the resources of their natural habitat, slow lorises will prioritize what is abundant in their ecosystem.
Behavior and Lifestyle
The Bornean slow loris is an arboreal primate, meaning they spend their lives up in the trees. Their bodies are well adapted for their lifestyle, and evolution equips them with dextrous hands and gripping fingers. The opposable thumb and big toe allow the Bornean slow loris to have a secure grip on a tree branch or trunk at all times. When traversing the canopies, they will start with one grasping hand to securely latch on to a branch and follow with their foot. This method of moving closely resembles crawling and is very slow and steady. While they are very skilled at moving through the trees, they are also very good at simply staying very still. Resembling leaves or branches, they can stay still for long swaths of time. Their calm movement allows them to stay hidden from predators and keeps them undetected by potential unsuspecting prey as well.
Their peaceful and calm exterior also reflects their slow metabolic rate, meaning they burn fewer calories in their daily life compared to other primates of the same size. At night, when they are active, they spend 93.3% of their time alone. Eating food takes up about 20.5% of the time they are awake, and rest takes up around 5.4%. By being alone for the majority of their lives, they decrease the possibility of being spotted by predators.
Slow lorises are the only venomous primates. When the Bornean slow loris feels threatened, they raise an arm to get the brachial gland fluid into their mouth. Once mixed with saliva, the toxin can be applied to their heads to defend themselves against predators. Not recorded in Bornean slow lorises but present in other species is the tendency for mothers to spread the venomous fluid on the heads of their offspring to protect them. The dangerous poison can also be kept in the mouth for a powerful and sometimes deadly bite. Bites with this venom are known to fester and, with enough venom, can even cause anaphylaxis.
The sleep pattern of the Bornean slow loris has not yet been recorded, and there is great variability among other species of slow loris. Some species find their resting spots in tree holes or dense brush, while others curl up into a ball to sleep in between big leaves, twigs, or branches. Most importantly, across all slow lorises is the need to stay hidden when they are at their most vulnerable.
Because of their highly sensitive, moist nose, Borean slow lorises are very attuned to scent. Their sophisticated sense of smell has allowed them to communicate with scent. Using the oils naturally secreted from their body and their urine, Bornean slow lorises leave their traces everywhere. This way, they can communicate without having to congregate, which would make them more noticeable and vulnerable to predators.
Slow lorises are the only venomous primates.
Mothers care for their young and protect them from predators by applying a little bit of their venom onto the heads of their offspring.
Bornean slow lorises have a dark mask around their eyes to signal to other animals that they are venomous.
The face masks of different slow loris species are their key identifier
Not much is known about the Bornean slow loris directly due to their cryptic nature and hard-to-find populations, but understanding other slow loris species can help us get a better picture of this fascinating species
Bornean slow lorises are a quiet bunch. With a very covert way of traveling through trees and their hyper-vigilant eyes and ears, they spend the night finding food alone. While they are often alone, Bornean slow lorises are not lonely. Using scent marking and with overlapping home ranges, they are in touch with their friends and family.
Home range areas are not directly recorded in the Bornean slow loris but can vary widely across and within other slow loris species. For example, the Sunda slow loris (Nycticebus coucang) overlaps in home ranges with other Sunda slow lorises, forming what is called a “spatial group”. These spatial groups consist of a single adult female, a single adult male, and a varying number of younger individuals. This composition indicates a monogamous mating system, meaning that both males and females will have just one mate. This mating system is not the case across all slow lorises, however, with some species exhibiting polyandrous systems where a female finds multiple males. In the Sunda slow loris, both females and males both leave their natal, or home, group. Interactions between spatial group members are friendly, and they allogroom, follow one another, vocalize with each other, and sleep in contact with each other. When interactions are not so nice, their special venom is turned on members of their own species. Another species of slow loris, the Javan slow loris (Nycticebus javanicus), has been observed to use the venom to defend territories and mates. This does not happen frequently, but when they don’t see each other, they leave messages in scent. While these behaviors are not recorded in the Bornean slow loris, understanding the behavior of a similar species can give a better insight into their behavior.
The Bornean slow loris communicates in mysterious ways. Since they prioritize stealth to stay undercover from predators, they are extremely selective when they use their vocalizations. Other slow loris species, when they do make sounds, mainly use calls to signify extreme distress or communicate with other members of the groups about potential threats. The Bornean slow loris has not been recorded to perform the usual repertoire of whistles, short keckers, snarls, grunts, screams, and long keckers to communicate affiliative and aggressive interactions, but it is probable that they participate!
When not using voices, alarm calls and regular chatter are communicated through scent. With their wet noses, Bornean slow lorises are experts at identifying who went where when just by picking up a unique smell.
The secretions from the brachial gland also help as well. While scientists are unsure whether specifically Bornean slow lorises exhibit this, or just other species of slow loris, the brachial gland’s oils can be used to demarcate territory and communicate with friends and foes.
Javan slow lorises (Nycticebus javanicus) have also been found to communicate ultrasonically. The doublet click is at a frequency that humans cannot register and is used primarily in affiliative contexts. This is the first instance of the use of ultrasonic vocalization in a strepsirrhine (wet-nosed) primate, and much more research is necessary to understand the scope of this kind of communication within Javan slow lorises and potentially the Bornean slow loris as well.
Species of the slow lorises are known to also communicate using facial expressions. Although not directly recorded in the Bornean slow loris, slow lorises are known to use their teeth, eyes, and body language to communicate aggression and friendship.
The exact details of Bornean slow loris reproduction and family are not well known, and there is considerable variation across slow loris species. In general, slow lorises are sexually mature around 17-24 months of age. Once sexually mature, they are able to sire and have offspring and typically leave their home groups. Studies have indicated that slow lorises leave their homes around 16-27 months, with both sexes having been observed dispersing in the wild.
Female estrous, or the cycle of the fertile period, can last from 29-45 days and is typically exhibited by reddish and swollen genitalia. While the change in appearance is present in some species, this is not found in others, leaving the Bornean slow loris somewhat of a mystery. Based on sparse observations of various slow loris species, copulation tends to occur on one day within this period of 29-45 days. Mating has been observed to be initiated by the female, with some species’ females hanging from trees and vocalizing for their mates.
Once pregnant, the gestation (pregnancy) averages around 190 days until an offspring is born. Lorises in general have 1 to 2 offspring per year. These young are precocial, meaning they are born with eyes open and fur on their bodies. They quickly gain the strength to cling to their mothers’ underbelly when she is foraging or can be “parked” in tree holes while the mother is off on important business. After a couple of months, the slow loris can sustain itself, and weaning begins.
Bornean slow lorises are an important part of their ecosystem. As omnivories they work to keep the species of vegetation and animals they eat in check. Their digestive systems pass the foods they eat to further nourish the ecosystems with their fertilizer. They also participate in the balance of the ecosystem by being prey of orangutans, eagles, and pythons.
The Bornean slow loris is classified as Vulnerable by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Habitat loss due to deforestation is a critical problem for all organisms living in Borneo. Between 1973 and 2015, 50% of the forest was lost, with a third of that disappearing in just the last three decades. Largely due to oil palm and other large industries, the habitats for the Bornean slow loris are diminishing rapidly. While the Bornean slow loris has been somewhat resistant to the devastating effects of the removal of forests, it is only a matter of time before the loss of habitat becomes catastrophic.
Aside from habitat destruction, the wildlife trade is causing incredible harm to the population. Captured for the pet trade, for use in traditional medicine, and for bushmeat, Bornean slow lorises are not well protected by the laws and regulations in place to protect them.
Bornean slow lorises are also the main characters of many local folklores. For example, people from Borneo believed that slow lorises were the gatekeepers for the heavens and that each person had a personal slow loris waiting for them in the afterlife. While the exact species of slow loris is not named, the Bornean slow loris is a top contender to be the object of many folklores originating in Borneo. More common is the belief in the medicinal qualities of the slow lorises, which is reflected in the use of Bornean slow lorises in traditional medicine.
As for the pet trade. Even with many new regulations in place, smuggling Bornean slow lorises is not uncommon. Due to their small size, docile personalities, and overall “cute” appearance, viral videos and pet advertisements become real purchases. Bornean slow lorises are captured at a young age, unable to learn many of the skills needed for development, exposed to tremendous amounts of stress, and oftentimes will have severe disfigurements to ensure a docile pet. As a primate that deals with stress very poorly, these animals suffer at the hands of inexperienced owners, not only mentally but also from malnutrition, stress-related injuries, and infections.
Another worry for the preservation of this species is hybridization. Since these lorises are in close proximity to other slow loris species with ever-closer habitat circumstances caused by forest destruction, hybridization (or the combining of species) could erase the Bornean slow loris forever.
The Bornean slow loris is listed in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), an international agreement between governments designed to ensure that international trade in wild animals and plants does not threaten their survival.
Indonesian law also protects this species, though not as much as one might hope. In recent years, more local initiatives are taking the place of what law and regulations should be, encouraging local communities to learn about the Bornean slow loris and to stop the capturing and selling of the animals in addition to halting the rapid destruction of their natural forests.
While rules and regulations may not be well enforced, Borneo does have its fair share of protected lands. The Heart of Borneo is a pristine rainforest the size of the whole of England and Scotland, it is a treasure trove of biodiversity and safe haven for the organisms that inhabit it. In 2007, the governments of Brunei, Indonesia and Malaysia committed to the Declaration on the Heart of Borneo Initiative to help conserve the biodiversity of the land through a local network of protected areas, sustainable management of forests and other sustainable land uses. The Bornean slow loris has been found in one of the many protected areas within the Heart of Borneo, the dipterocarp forest of Bukit Batikap Protected Forest, Central Kalimantan. With many more Bornean slow lorises most likely living in the Heart of Borneo, these protected lands provide a beacon of what natural biodiversity could look like.
- Ecosystems of the Heart of Borneo, WWF.org
- The Heart of Borneo, WWF.org
- Munds RA, Nekaris KA, Ford SM. Taxonomy of the Bornean slow loris, with new species Nycticebus kayan (Primates, Lorisidae). Am J Primatol. 2013 Jan;75(1):46-56. doi: 10.1002/ajp.22071. PMID: 23255350.
- Blair ME, Cao GTH, López-Nandam EH, Veronese-Paniagua DA, Birchette MG, Kenyon M, Md-Zain BM, Munds RA, Nekaris KA, Nijman V, Roos C, Thach HM, Sterling EJ, Le MD. Molecular Phylogenetic Relationships and Unveiling Novel Genetic Diversity among Slow and Pygmy Lorises, including Resurrection of Xanthonycticebus intermedius. Genes (Basel). 2023 Mar 3;14(3):643. doi: 10.3390/genes14030643. Erratum in: Genes (Basel). 2024 Apr 03;15(4):451. doi: 10.3390/genes15040451. PMID: 36980915; PMCID: PMC10048081.
- Duke Lemur Center Slow Loris
- Wisconsin National Primate Research Center Slow Loris
- Wiens, F. and Zitzmann, A. (2003), Social structure of the solitary slow loris Nycticebus coucang (Lorisidae). Journal of Zoology, 261: 35-46. https://doi.org/10.1017/S0952836903003947
- Makur, K.P., Utami-Atmoko, S.S., Setia, T.M. et al. Slow loris (Nycticebus borneanus) consumption by a wild Bornean orangutan (Pongo pygmaeus wurmbii). Primates 63, 25–31 (2022). https://doi.org/10.1007/s10329-021-00960-4
- K.A.I. Nekaris, Marco Campera, Vincent Nijman, Hélène Birot, Eva Johanna Rode-Margono, Bryan Grieg Fry, Ariana Weldon, Wirdateti Wirdateti, Muhammad Ali Imron, Slow lorises use venom as a weapon in intraspecific competition, Current Biology, Volume 30, Issue 20, 2020, Pages R1252-R1253, ISSN 0960-9822, https://doi.org/10.1016/j.cub.2020.08.084.
- BBC Science Focus: What is a slow loris? Everything you need to know about this cute but venomous primate
- Geerah DR, O’Hagan RP, Wirdateti W, Nekaris. The Use of Ultrasonic Communication to Maintain Social Cohesion in the Javan Slow Loris (Nycticebus javanicus). Folia Primatol (Basel). 2019;90(5):392-403. doi: 10.1159/000495077. Epub 2019 Aug 15. PMID: 31416069.
- Nekaris, K. Anne-Isola, Ariana Weldon, Muhammad Ali Imron, Keely Q. Maynard, Vincent Nijman, Stephanie A. Poindexter, and Thais Queiroz Morcatty. 2019. “Venom in Furs: Facial Masks as Aposematic Signals in a Venomous Mammal” Toxins 11, no. 2: 93. https://doi.org/10.3390/toxins11020093
- Ocampo-Peñuela Natalia , Garcia-Ulloa John , Kornecki Itto , Philipson Christopher D. , Ghazoul Jaboury. Impacts of Four Decades of Forest Loss on Vertebrate Functional Habitat on Borneo. Frontiers in Forests and Global Change. VOLUME=Volume 3 – 2020. doi =10.3389/ffgc.2020.00053
Written by Nami Kaneko, February 2026