The species was originally thought to be a part of the Bornean slow loris (N. menagensis) population until 2013, when a study of museum specimens and photographs identified distinct facial markings, which helped to differentiate it.

As with other slow lorises, this arboreal and nocturnal species primarily eats insects, tree gum, nectar, and fruit and has a toxic bite, a unique feature among primates.

[4] Primatologist William Charles Osman Hill continued this course in his influential 1953 book, Primates: Comparative Anatomy and Taxonomy.

[5] In 1971, that view was refined by Colin Groves, who recognized the pygmy slow loris (N. pygmaeus) as a separate species.

[9] A 2013 review of all museum specimens and photographs attributed to N. menagensis showed that they in fact comprised a species complex.

[12] The holotype, AMNH 106012, was originally collected on 8 October 1935 by Baron V. von Plessen near Peleben in the province of East Kalimantan in Borneo and is housed in the American Museum of Natural History in New York.

[16] On the ventral side of its elbow, it has a small swelling called the brachial gland, which secretes a pungent, clear oily toxin that the animal uses defensively by wiping it on its toothcomb.

First, the top of the dark ring around its eyes is either rounded or pointed (not diffuse at the edges) and the bottom stretches below the zygomatic arch, and sometimes extends as far down as the jaw.

[18] Like other slow lorises, N. kayan is arboreal, nocturnal,[13] and omnivorous, eating primarily insects, tree gum, nectar, and fruit.

When threatened, slow lorises may also lick their brachial glands and bite the aggressors, delivering the toxin into the wound.

The illegal wildlife trade is also a major factor,[10] with loris parts commonly sold for traditional medicine.