Found in Southeast Asia and nearby areas, they range from Bangladesh and Northeast India in the west to the Sulu Archipelago in the Philippines in the east, and from Yunnan province in China in the north to the island of Java in the south.

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.

Unknown [24] Other than the pygmy slow loris in sister genus Xanthonycticebus,[6] the group's closest relatives are the slender lorises of southern India and Sri Lanka.

The earliest known mention of a slow loris in scientific literature is from 1770, when Dutchman Arnout Vosmaer (1720–1799) described a specimen of what we know today as N. bengalensis that he had received two years earlier.

The French naturalist Georges-Louis Leclerc, Comte de Buffon, later questioned Vosmaer's decision to affiliate the animal with sloths, arguing that it was more closely aligned with the lorises of Ceylon (now Sri Lanka) and Bengal.

The name derives from the Ancient Greek: νυκτός, romanized: (nyktos), genitive form of νύξ (nyx, "night"), and κῆβος (kêbos, "monkey").

[55] However, in 1939 Reginald Innes Pocock consolidated all slow lorises into a single species, N. coucang,[56] and in his influential 1953 book Primates: Comparative Anatomy and Taxonomy, primatologist William Charles Osman Hill also followed this course.

[60] In 2006, the Bornean slow loris was elevated to the species level (as Nycticebus menagensis) based on molecular analysis of DNA sequences of the D-loop and the cytochrome b gene.

[66] Previous molecular analyses using karyotypes,[67] restriction enzymes,[68] and DNA sequences[69] were focused on understanding the relationships between a few species, not the phylogeny of the entire genus.

[62] Nekaris and Nijman (2022) combined morphological, behavioural, karyotypical and genetic data and suggested that the pygmy slow loris is best placed in its own genus, Xanthonycticebus.

[82][94] They can tightly grasp branches with little effort because of a special muscular arrangement in their hands and feet, where the thumb diverges at nearly 180° from the rest of the fingers, while the hallux (big toe) ranges between being perpendicular and pointing slightly backwards.

[27][94][95] The toes have a large flexor muscle that originates on the lower end of the thigh bone, which helps to impart a strong grasping ability to the hind limbs.

[27] The strong grip can be held for hours without losing sensation due to the presence of a rete mirabile (network of capillaries), a trait shared among all lorises.

Since they consume a relatively high-calorie diet that is available year-round, it has been proposed that this slow metabolism is due primarily to the need to eliminate toxic compounds from their food.

[98] They are found in India (Northeastern states),[98][99][100] China (Yunnan province), Laos, Vietnam, Cambodia, Bangladesh, Burma, Thailand, Malaysia, the Philippines, Indonesia,[98] Brunei,[19] and Singapore.

The Bornean slow loris (N. menagensis), found on Borneo and nearby islands, including the Sulu Archipelago,[19] and in 2012 was split into four distinct species (adding N. bancanus, N. borneanus, and N. kayan).

[103] Due largely to their nocturnal behavior and the subsequent difficulties in accurately quantifying abundance, data about the population size or distribution patterns of slow lorises is limited.

[106][107] 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 nocturnal primate.

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.

According to Nekaris, this adaptation—along with vocalizations, movement, and coloration patterns similar to those of true cobras—may have evolved through Müllerian mimicry to protect slow lorises when they need to move across the ground due to breaks in the canopy.

[94] Slow lorises are omnivores, eating insects and other arthropods, small birds and reptiles, eggs, fruits, gums, nectar and miscellaneous vegetation.

[127][129] A more detailed study of another Sunda slow loris population in 2002 and 2003 showed different dietary proportions, consisting of 43.3% gum, 31.7% nectar, 22.5% fruit, and just 2.5% arthropods and other animal prey.

[130] The pygmy slow loris often returns to the same gum feeding sites and leaves conspicuous gouges on tree trunks when inducing the flow of exudates.

[139]In the Mondulkiri Province of Cambodia, hunters believe that lorises can heal their own broken bones immediately after falling from a branch so that they can climb back up the tree.

The gall bladder of the Bengal slow loris has historically been used to make ink for tattoos by the village elders in Pursat and Koh Kong Provinces of Cambodia.

[143] When they were all considered a single species, imprecise population data together with their regular occurrence in Southeast Asian animal markets combined to incorrectly suggest that slow lorises were common.

A survey by primatologist Anna Nekaris and colleagues (2010) showed that these belief systems were so strong that the majority of respondents expressed reluctance to consider alternatives to loris-based medicines.

[154] According to National Geographic, slow lorises are protected by both local laws in southern Asia and by the Convention on International Trade in Endangered Species (CITES).

[157][121][156] According to data compiled from monthly surveys and interviews with local traders, nearly a thousand locally-sourced slow lorises exchanged hands in the Medan bird market in North Sumatra during the late first decade of the 21st century.

[158] As part of the trade, infants are pulled prematurely from their parents, leaving them unable to remove their own urine, feces, and oily skin secretions from their fur.