Slow lorises of the genus Nycticebus are an anomaly in the primate world. They are small, wide-eyed, and appear gentle—yet they possess a biological weapon found in no other primate: venom. Yes, the slow loris is the world’s only venomous primate, and its toxin can be lethal.
This nocturnal animal is distributed across South and Southeast Asia, from Bangladesh and Northeast India to the Philippines, and from Yunnan in China to the island of Java.
There are nine species within this genus: the Philippine slow loris, Bengal slow loris, greater slow loris, Kayan slow loris, Bangka slow loris, Bornean slow loris, Sumatran slow loris, Javan slow loris, and pygmy slow loris.
All of them share one defining trait that sets them apart from other primates: an active venom delivery system.
How Slow Loris Venom Works
The source of a slow loris’s venom is not its mouth, but its brachial glands—small bald patches located on the inside of the upper arms. These glands secrete a pungent, oily substance.
When threatened, a slow loris raises its arms and licks this area. The oil mixes with saliva, forming an active toxin. This mixture then flows to the canine teeth through tiny grooves in the tooth structure.
When the animal bites, the venom is injected directly into the wound. A slow loris bite is strong enough to penetrate bone.
The venom can kill arthropods and small mammals. More than 200 chemical compounds have been identified from the brachial glands of the pygmy slow loris alone. Biologically, this represents an exceptionally sophisticated defensive and offensive system for such a small primate.
What Happens If a Slow Loris Bites You?
It is a serious mistake to assume that slow loris venom is dangerous only to arthropods or small mammals and therefore harmless to humans. This assumption is incorrect. The same toxin can trigger severe reactions when introduced into the human body.
Slow loris bites can cause intense pain, swelling, numbness, and wounds that heal poorly and are prone to infection. In some cases, nerve damage and long-term skin complications occur.
The most dangerous reaction is anaphylaxis—a severe, life-threatening allergic response. Medical reports document a man in the interior of Kalimantan who experienced nausea, shortness of breath, numbness of the lips, and excessive sweating after being bitten by a slow loris, despite having no prior history of allergies. He arrived at the hospital in anaphylactic shock.
Another case in Japan involved a 37-year-old woman who was bitten by her pet slow loris and suffered severe anaphylactic shock.
The effects of the venom depend heavily on the victim’s sensitivity. In mild cases, the bite area becomes numb. In severe cases, cardiac disturbances, anaphylaxis, and even death may occur. For this reason, the perception of slow lorises as safe pets is profoundly misguided.
They Don’t Just Bite Predators, They Bite Each Other
Another surprising fact is that slow lorises do not use their venom solely against predators or to subdue prey. They also deploy it against other slow lorises.
In a study published in Current Biology examining 82 wild Javan slow lorises, around 20 percent were found to have fresh bite wounds inflicted by other individuals. Approximately one-third of females and 57 percent of males showed evidence of bite injuries.
This behavior is closely linked to territoriality. Males defend mates, females protect offspring and food resources, and both sexes actively guard their territories.
The use of venom in such internal conflicts is extremely rare among mammals. Slow lorises belong to a very small group of venomous mammals that use venom against members of their own species, alongside vampire bats, certain shrew species, the platypus, and the solenodon.
Venom is also used passively by these primates. Before leaving her infant alone in a tree to forage, a mother slow loris will lick her brachial glands and groom her offspring. The venom deposited on the infant’s fur is believed to help protect it from predators such as clouded leopards and sun bears.
For decades, scientists debated the evolutionary purpose of venom in primates. Initially, it was thought to function primarily as a defense against predators or parasites. However, thousands of hours of field observation of Javan slow lorises indicate that venom plays a major role in competition between individuals.
Slow Lorises Are Not Sloths
Because of their slow movements, slow lorises are often mistaken for sloths. In reality, the two are fundamentally different. Sloths live in Central and South America, while slow lorises are found only in South and Southeast Asia.
In terms of speed, slow lorises are actually faster. Three-toed sloths move at roughly 0.011 miles per hour. Slow lorises can reach speeds of up to 1.18 miles per hour and move even faster when ambushing prey. Sloths rely on camouflage to survive; slow lorises rely on venom.
Tragically, all slow loris species are now experiencing population declines, with conservation statuses ranging from Vulnerable to Critically Endangered. Some species, such as the Bangka slow loris and the Javan slow loris, are classified as Critically Endangered.
All slow loris species are legally protected across South Asia, including Indonesia, and are listed under CITES. Despite this, their exact numbers in the wild remain unknown.
Ironically, the slow loris’s cute appearance has fueled illegal wildlife trade and a trend of keeping them as exotic pets. In reality, a slow loris bite is far from harmless.
