Nycticebus javanicus

Geographic Distribution and Habitat

Javan slow lorises (Nycticebus javanicus) are endemic to the Indonesian island of Java, where they inhabit the western and southern regions of the island. They make their homes in primary and secondary forests, bamboo and mangrove forests, and even chocolate plantations. Unfortunately, their habitat is shrinking, with an estimated 20% of their historic habitat remaining.


Javan slow lorises were originally thought to be a subspecies of the closely related Sunda slow loris (Nycticebus coucang), but were separated out as their own species in 2008. While lorises share an ancestry with other “primitive” primates including the bushbabies of Africa and lemurs of Madagascar, their branch is at least 20 million years old.

Javan slow loris range, circled in red. IUCN 2020.

Size, Weight, and Lifespan

​Javan slow lorises are the largest of the Indonesian slow lorises, with an average weight between 1.3 and 1.5 lbs (570–690 g), and a body length of 11 inches (29 cm) on average. 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?

Ex situ conservation:
Conservation actions that occur outside of the species’ natural habitat, such as captive breeding programs in zoos.

Visit the Glossary for more definitions


Javan slow lorises have a very unique look compared to the rest of the primate world. Their most striking feature is their large brown doe eyes that contribute to their overall charismatic charm. These globe-like eyes are forward-facing and specially adapted to their dark forest nighttime environment. Their face is a light tan or white, and their eyes are framed by thick brown stripes that run vertically down their face, resembling a raccoon’s mask. It is theorized that these stripes are an adaptation to make their eyes appear bigger, which can make predators who catch a glimpse of them believe them to be a much larger animal. Their body is a buffy beige and they have a brown dorsal stripe running down their spine. Long arms and legs, with long splayed-out fingers and toes, help them to grip branches. Their vestigial tail is only about an inch in length. Unlike most other arboreal (tree-dwelling) primates, slow lorises are not particularly agile, so they don’t require a long tail to help them balance in the trees. The only form of sexual dimorphism they display is the males’ slightly larger size. It is not uncommon for individuals to be seen with bite wounds, as underneath their cuddly appearance, Javan slow lorises are known to be very territorial and defensive—and their bite packs a venomous punch!

Photo credit: Dr K A I Nekaris/Creative Commons

​Javan slow lorises are reliant on tree exudates such as sap and gum, which are an important year-round food source for them. They supplement this diet with fruit, insects, lizards, and eggs.

Behavior and Lifestyle

Like other lorises, Javan slow lorises are nocturnal and arboreal. As their name implies, they are slow-moving. They move about in a very distinctive, deliberate way by crawling on branches. They have specialized blood vessels in their wrists and hands that provide extra oxygen to those muscles, meaning that they are able to grip onto branches without moving for hours on end without getting exhausted. They also have a very flexible back, which is useful to them as they often hang upside down by their feet from branches. While it may seem that this unhurried lifestyle makes them extremely susceptible to predators, it can actually work to their advantage: they can move about virtually silently. One disadvantage, however, is that they require a great deal of connectivity in their forest habitat, such as vines and lianas that connect trees, as they can’t jump significant distances. They can, if pressed, cross short distances on the ground, though this leaves them very vulnerable to predators. In their natural, undisturbed habitat, this wouldn’t have been a problem, although human alteration of their habitat has made it so. Additionally, while slow lorises are usually slow, they don’t have to be. They can pick up the pace significantly if confronted by a predator. They also have very thick skin that can make the bite of a predator less harmful.

Javan slow lorises have a noxious oil-producing gland in the crook of their elbows. When they lick this gland, the oil combines with their saliva to produce a deadly venom. The venom fills in grooves in their canines, which wick the venom up so it can be used later to give a truly fierce bite that is capable of piercing bones, not to mention infecting their victim with their toxin. This venom is effective against other lorises and parasites, and can even cause anaphylactic shock which can result in death in humans, although that is very rare. However, the primary use of their venom is against others of their species. The venom is very painful and it causes long-term damage by decaying flesh. One field study found that 57% of all males and about a third of all females examined had signs of at least one venomous bite wound.

Fun Facts

Slow lorises are the only venomous primate—and more than that, they join an exclusive list of only five other mammals that are known to produce venom: platypuses, two species of shrews, vampire bats, and solendons, a shrew-like mammal.

Daily Life and Group Dynamics

Javan slow lorises often live solitarily, although they may also form small family groups. They sleep alone or in their small groups. Their sleeping sites are 7–100 feet (2–30 m) high in the trees, usually in dense bamboo or branch tangles. Their home range size varies between 7 and 74 acres (3–30 ha), depending on the type of habitat they live in. They are fiercely territorial, and scientists have speculated this may be the reason they have evolved venom: to protect their territory, offspring, mate, and food.


​Slow lorises are usually quiet, although they use scent marking and occasional vocalizations to communicate with each other. They mark their territory with urine and the oil secreted from their underarm glands. They vocalize to each other to communicate danger, during mating, or for communication between a mother and her baby. Slow lorises use hissing and buzzing noises to ward off predators and communicate the danger to others in the area. They use low-pitched calls to signal others in non-dangerous situations. Females call to their mates with high-pitched or rising notes, and babies can call out to their mothers with calls too high-pitched for humans to hear.

Reproduction and Family

Females are known to be very picky in choosing their mates. They are no damsels in distress, either—females regularly fight with males by slapping or biting them, or even throwing them out of trees. Once a male has sufficiently impressed a female to be chosen as her mate, they form very strong bonds with each other. Following conception, a female slow loris is pregnant for about 6 months, after which she gives birth to one or, rarely, two babies. On average, she will have another baby about 16 months later.

Newborn slow lorises are born already strong enough to hold onto branches by themselves. A baby clings to its mother’s stomach almost constantly for about seven weeks. When she goes out foraging, a mother loris gives her baby an extremely thorough cleaning before leaving it in a safe place. Not only is the baby cleaned, but her venom is spread over her baby as a protective coating while she’s gone. At about two weeks of age, an infant loris begins to crawl through the trees independently. The young will nurse from their mother for about six months, although they will begin experimenting with solid food at about four weeks old. At six weeks old, young slow lorises begin to produce their own venom. The offspring stay in their family unit for about three years, and sometimes help to care for their younger siblings. Their generation length is about eight years.

​Ecological Role

​Javan slow lorises are known to be preyed on by a variety of other species, including hawks, pythons, and felines. They can protect themselves from predators by falling out of the tree they’re in, using their venomous bite, or rolling into a ball with their arms up, which can release venom from their underarm glands. Javan slow lorises are predators themselves to the trees from which they consume sap and gum, as well as the other plants and animals with which they supplement their diet. Slow lorises are also known to contribute to pollination.

Conservation Status and Threats

The International Union for Conservation of Nature lists the Javan slow loris as Critically Endangered (IUCN 2015), appearing on the IUCN’s Red List of Threatened Species. The major threats facing them are habitat destruction and fragmentation and collection for the pet trade. An estimated 20% of their historic habitat is remaining, and studies show that the remaining populations of Javan slow lorises are extremely fragmented. Only about 17% of the potential population exists within protected areas. Javan slow lorises are believed to have undergone a population loss of at least 80% over the last 24 years, or three generations.

In addition to the amazing diversity of animal and plant life it is home to, the island of Java is the most populous in the world—home to 130 million people on an island the size of New York State. The huge population of humans has placed a significant strain on the natural ecosystem—in fact, a mere 7% of Java’s forests remain. Javan slow lorises are significantly more heavily impacted by human activity than other Indonesian slow lorises. It’s not just a loss of habitat that is impacting the lorises, it’s the lack of habitat connectivity. The forest habitat of Java is so fragmented that every patch of forest with slow lorises is separated from other patches by at least several miles/kilometers of intensely modified land. As Anna Nekaris, a professor of primate conservation and biological anthropology at Oxford Brookes University and founder of the Little Fireface Project, puts it, “It’s a miracle there are any [lorises] at all.”

Habitat destruction alone is enough to put a significant strain on the survival of Javan slow lorises as a species. Unfortunately, they face another major threat: the pet trade. Some people have been so taken by the Javan slow loris’s wide-eyed charm that they have taken them in as pets. Online videos showcasing the pet lorises have unfortunately only made the problem worse. This has become a major detriment to the species as a whole and, sadly, a death sentence for many of the animals subjected to this fate.

They are easy targets for capture due to their lack of jumping skills and their accessible daytime sleeping sites. Most of the animals captured for the pet trade stay in Indonesia, although some are smuggled to the Middle East and Japan. The lorises face many perils in their journeys as pets: they are subject to unsafe transportation conditions such as being stuffed into boxes or bags, and because of their fierce territoriality and venomous bite, they can severely injure each other in transit. As they are venomous, their captors often cut or extract their canine teeth, which can kill them. It doesn’t even completely remove the danger of their venom to their human handlers, because they can still give a nasty, venomous bite with their incisors. 

They are often not properly cared for when they reach their new home. They may be forced into a diurnal lifestyle, which can cause permanent damage to their sensitive eyes that have adapted over millions of years to dark nighttime forests. Their diet, composed largely of tree gum and sap, is very difficult to replicate in captivity, and they can become bloated due to a poor diet. They often are provided with inappropriate housing. Further, it can also be difficult for their captors to correctly interpret their behavior. For example, when they are tickled, they shoot their arms up. A human may interpret this as joy, but it is actually a sign of distress—the action gives them quick access to the venom-producing glands under their arms. Their cries are often at too high a frequency to be heard by humans.

Once captured for the pet trade, it is very difficult to successfully reintroduce them to the wild. Because they rely heavily on their teeth for defense and to gouge bark for tree gum, individuals who have had their teeth removed can never be released, and with those who are, their survival rate is very low. In one case, 11 slow lorises were released back to the wild from the pet trade, and only two survived long-term.

Conservation Efforts

Javan slow lorises are protected under Indonesian law and are listed on the Convention on International Trade in Endangered Species (CITES) Appendix I. The protected national parks on Java are at too high an altitude to be of much benefit to Javan slow lorises. While there are three zoos in the world successfully housing them captive, there is no viable ex situ breeding program. 

Education is a major component of the Javan slow loris conservation effort. There are training programs to help law enforcement officers, CITES officials, and zoo and rescue center staff improve their identification skills for slow lorises, which are notoriously difficult to differentiate. There are education programs that teach the Javan community about slow lorises, and specifically to educate farmers on the benefits of the species for pest control and pollination, and to share growing methods that benefit both the farmer and the lorises. One recent effort to aid the species is to install irrigation pipes. These were already being used by farmers and the lorises had been observed using them to cross between forest patches. One researcher tried installing more to connect patches of forest and, sure enough, the lorises have been documented using them to expand their home range. As a plus, this also aids local farmers, who invest in and benefit from the infrastructure. The new pipes allow the animals to reach new feeding trees and they can spend more time feeding and sleeping and less time traveling. Because the pipes are installed high off the ground, the lorises are safe from terrestrial predators while traveling.

Another major component of Javan slow loris conservation is tackling the pet trade. Efforts have been made to remove “cute” videos of pet slow lorises on YouTube and Instagram, in an effort to reduce the demand for them as pets. Searching for slow loris content on Instagram results in a wildlife protection alert, warning the user that the search term may result in content that is harmful to wildlife, and may be a useful tool to help curb the illegal slow loris trade. There are a number of rescue programs across the Javan slow loris range that take in confiscated animals and care for them. Sometimes attempts are made to re-release them. More data and long-term monitoring is necessary to know how successful these releases have been to the conservation of the species.

Learn about the effort of the Little Fireface Project here

  • Nekaris, K. A. I., Campera, M., Nijman, V., Birot, H. Rode-Margono, E. J., Fry, B. G, … Imron, M. A. 2020. Slow lorises use venom as a weapon in intraspecific competition. Current Biology, 30(20), 1252-1253. doi: 10.1016/j.cub.2020.08.084

Written by K. Clare Quinlan, October 2021