Slow lorises have a round head, narrow snout, large eyes, and a variety of distinctive coloration patterns that are species-dependent. Their arms and legs are nearly equal in length, and their trunk is long, allowing them to twist and extend to nearby branches. The hands and feet of slow lorises have several adaptations that give them a pincer-like grip and enable them to grasp branches for long periods of time. Slow lorises have a toxic bite, a rare trait among mammals. The toxin is produced by licking a gland on their arm, and the secretion mixes with its saliva to activate it. Their toxic bite is a deterrent to predators, and the toxin is also applied to the fur during grooming as a form of protection for their infants.
They move slowly and deliberately, making little or no noise, and when threatened, they freeze and become docile. Their only documented predators—apart from humans—include snakes, hawk-eagles, and orangutans, although cats, civets and sun bears are suspected. Little is known about their social structure, but they are known to communicate by scent marking. Males are highly territorial. Slow lorises reproduce slowly, and the infants are initially parked on branches or carried by either parent. They are omnivores, eating small animals, fruit, tree gum, and other vegetation.
All five species are listed as either “Vulnerable” or “Endangered” on the IUCN Red List and are threatened by the wildlife trade and habitat loss. Although their habitat is rapidly disappearing and becoming fragmented, making it nearly impossible for slow lorises to disperse between forest fragments, unsustainable demand from the exotic pet trade and traditional medicine has been the greatest cause for their decline. Deep-rooted beliefs about the supernatural powers of slow lorises, such as their purported ability to ward off evil spirits or cure wounds, have popularized their use in traditional medicine. Despite local laws prohibiting trade in slow lorises and slow loris products, as well as protection from international commercial trade under Appendix I, slow lorises are openly sold in animal markets in Southeast Asia and smuggled to other countries, such as Japan. They have also been popularized as pets in viral videos on YouTube. Slow lorises have their teeth cut or pulled out for the pet trade, and often die from infection, blood loss, poor handling, or poor nutrition.
Little is known about the social structure of slow lorises, but they generally spend most of the night foraging alone. Individuals sleep during the day, usually alone but occasionally with other slow lorises. Home ranges of adults may significantly overlap, and those of males are generally larger than those of females. In the absence of direct studies of the genus, primatologist Simon Bearder speculated that slow loris social behavior is similar to that of the potto, another slow-moving nocturnal primate. Such a social system is distinguished by a lack of matriarchy and by factors that allow the slow loris to remain inconspicuous and minimize energy expenditure. Vocal exchanges and alarm calls are limited; scent marking with urine is the dominant form of communication. Adult males are highly territorial and are aggressive towards other males. Vocalizations include an affiliative (friendly) call krik, and a louder call resembling a crow’s caw. When disturbed, slow lorises can also produce a low buzzing hiss or growl. To make contact with other individuals, they emit a single high-pitched rising tone, and females use a high whistle when in estrus.
Slow lorises are slow and deliberate climbers, and often hold onto branches with three of their four limbs. To move between trees, they carefully grip the terminal branches of the neighboring tree and pull themselves across the small gap. They will also grip branches with only their hind feet, lift themselves upright, and quickly launch forward with their hands to catch prey. Due to their slow movement, all lorises, including the slow lorises, have a specially adapted mechanism for defense against predation. Their slow, deliberate movement hardly disturbs the vegetation and is almost completely silent. Once disturbed, they immediately freeze and remain docile. In Indonesia, slow lorises are called malu malu or “shy one” because they freeze and cover their face when spotted. If cornered, they may adopt a defensive posture by curling up and lunging at the predato The Acehnese name, buah angin (“wind monkey”), refers to their ability to “fleetingly but silently escape”.Little is known about the predation of slow lorises. Documented predators include snakes, the crested hawk-eagle (Nisaetus cirrhatus), and Sumatran orangutans (Pongo abelii). Other potential predators include cats, sun bears (Ursus malayanus), binturongs (Arctictis binturong), and civets.
Slow lorises produce a secretion on their brachial gland (a gland on their arm), which, when mixed with their saliva, creates a volatile, noxious toxin that is stored in the mouth. In tests, three predators—binturongs, clouded leopards (Neofelis nebulosa), and sun bears—retreated or showed other signs of displeasure when presented with cotton swabs anointed with a mixture of the toxic secretion and the saliva, whereas the toxic secretion alone generated mild interest. Before stashing their offspring in a secure location, female slow lorises will lick their brachial glands, and then groom their young with their toothcomb, depositing the toxin on their fur. When threatened, slow lorises may also lick their brachial glands and bite their aggressors, delivering the toxin into the wounds. Slow lorises can be reluctant to release their bite, which is likely to maximize the transfer of toxins. Animal dealers in Southeast Asia keep tanks of water nearby so that in case of a bite, they can submerge both their arm and the slow loris to make the animal let go. The toxin is similar to the allergen in cat dander. Loris bites cause a painful swelling, but the toxin is mild and not fatal. The single case of human death reported in the scientific literature was believed to have resulted from anaphylactic shock. Although the trait is rare among mammals, slow lorises may not be the only primates with this type of defense—the closely-related potto and slender lorises may share the trait.
Slow lorises have a special network of capillaries in their hands and feet that allow them to cling to branches for hours without losing sensation.
Studies suggest that slow lorises are polygynandrous. Infants are either parked on branches while their parents find food or else are carried by one of the parents. Due to their long gestations (about six months), small litter sizes, low birth weights, long weaning times (three to six months), and long gaps between births, slow loris populations have one of the slowest growth rates among mammals of similar size. Pygmy slow lorises are likely to give birth to twins—from 50% to 100% of births, depending on the study; in contrast, this phenomenon is rare (3% occurrence) in Bengal slow lorises. A seven-year study of captive-bred pygmy slow lorises showed a skewed sex distribution, with 1.68 males born for every 1 female
Breeding may be continuous throughout the year. Copulation often occurs while suspended with the hands and feet clinging to horizontal branches for support.In captive Sunda slow lorises, mating primarily occurs between June and mid-September, with the estrus cycle lasting 29 to 45 days and estrus lasting one to five days. Likewise, gestation lasts 185 to 197 days, and the young weigh between 30 and 60 grams (1.1 and 2.1 oz) at birth. Females reach sexual maturity at 18 to 24 months, while males are capable of reproducing at 17 months. However, the fathers become hostile towards their male offspring after 12 to 14 months and will chase them out. In captivity, they can live 20 or more years.