Like other slow lorises, it has a wet nose (rhinarium), a round head, small ears hidden in thick fur, a flat face, large eyes and a vestigial tail.

The Sunda slow loris is nocturnal and arboreal, typically occurring in evergreen forests.

It prefers rainforests with continuous dense canopies and has an extremely low metabolic rate compared to other mammals of its size.

It sleeps during the day, rolled up in a ball in hidden parts of trees above the ground, often on branches, twigs, palm fronds, or lianas.

The species is polyoestrous, usually giving birth to a single offspring after a gestation period of 192 days.

[8][10] It is sometimes called kuskus, because local people do not distinguish between the slow loris and cuscus, a group of Australasian possums.

[10] The Sunda slow loris was first described (in part) in 1785 by the Dutch physician and naturalist Pieter Boddaert under the name Tardigradus coucang.

[14] In 1971 Colin Groves recognized the pygmy slow loris (N. pygmaeus) as a separate species,[15] and divided N. coucang into four subspecies.

[18] Species differentiation was based largely on differences in morphology, such as size, fur color, and head markings.

)[20]: 46 When Étienne Geoffroy Saint-Hilaire defined the genus Nycticebus in 1812, he made the Sunda slow loris the type species.

[26] To help clarify species and subspecies boundaries, and to establish whether morphology-based classifications were consistent with evolutionary relationships, the phylogenetic relationships within the genus Nycticebus have been investigated using DNA sequences derived from the mitochondrial markers D-loop and cytochrome b.

[27] This hypothesis was corroborated by a 2007 study that compared the variations in mitochondrial DNA sequences between N. bengalensis and N. coucang, and suggested that there has indeed been gene flow between the two species.

The structure is generally used for grooming in other strepsirhine primates, but lorises also use it to scrape off gum when foraging.

The largest amount of time is spent eating phloem sap (34.9%), floral nectar and nectar-producing plant parts (31.7%), and fruits (22.5%).

[45] They are also known to feed on molluscs, including the giant land snail Achatina fulica,[46] and birds' eggs.

[44][50] The Asiatic reticulated python, the changeable hawk-eagle and the Sumatran orangutan have been recorded as predators of the Sunda slow loris.

[39][50][51] The Sunda slow loris may fit into the monogamous single male/single female social system,[44][52] though are mainly known to be solitary.

[40] The interactions between these individuals are largely friendly; they include allogrooming, following, pant-growling, and click-calling, although social behaviors only make up around 3% of the activity budget.

Males have shown antagonistic behaviors such as attacks, pursuits, threats, assertion, fighting, and subordination.

[54][55] When exploring new environments and during handling, it makes ultrasonic vocalisations out of the human hearing range.

The gland is licked to spread scent and is thought to have evolved for communication, but it is toxic to humans.

When stressed, infants may grin, while adults bear their teeth to show aggression or fear, but also during play.

[56] Reproduction is one of the few times the Sunda slow loris aggregates with conspecifics, as it is largely solitary.

[40] In the wild the mating system of the Sunda slow loris is thought to vary between populations.

[8][30][31][35][62] Despite being presumed extinct in Pulau Tioman, records indicate that slow lorises may still inhabit the island.

The facial markings and morphology of the Tioman slow loris are substantially different from mainland individuals, which hints at the potential distinctiveness of the population.

[28] According to the 2020 International Union for Conservation of Nature (IUCN) Red List assessment, the Sunda slow loris was evaluated as endangered.

[8][64] With a greater purchasing power, the increasing human populations in the species' range could have an even more serious impact.