Anatomy and Physiology
The pygmy slow loris has a head and body length (measured from the top of the head to the base of the tail) of 195–230 mm (7.7–9.1 in); there is no significant difference in size between the sexes. The length of the skull is less than 55 mm (2.2 in). The tail is short, averaging 1.8 cm (0.71 in) in length. The bodyweight ranges between 360 and 580 grams (13 and 20 oz), with an average mass of 420 grams (15 oz) for males and 428 grams (15.1 oz) for females. There are, however, large seasonal variations in bodyweight, and individuals up to 700 grams (25 oz) have been recorded. The animal tends to have significantly higher bodyweights during the winter months, about 50 percent higher than the lowest values in the summer. The weight gains, achieved largely by increasing food intake, are triggered by changes in the length of the day and night. This seasonal change in bodyweight occurs in both sexes, in both pregnant and non-pregnant females—an adaptation thought to help ensure survival during winter when food resources become scarce. The species has distinctive teeth morphology: its third molar is triangular in outline and only slightly smaller than the first molar; its second molar is the largest. The incisors and canines on its lower jaw are procumbent (tilt forward) and together form a toothcomb that is used in grooming and feeding.
Like other prosimians, the pygmy slow loris has tapeta lucida in its eyes to assist with night vision. In adults, the rings circling the eyes are seal brown; they are darker in young individuals. There is a white stripe extending from the nose to the forehead, and the sides of the head and upper lip are silvery gray, while the rest of the face and top of the head is rufous. It has small black ears, typically about 23 mm (0.91 in) long, which do not have fur on the tips. On the dorsal side of the animal, a rufous to brownish-black stripe runs from the nape to the middle of the lower back. The upper parts, including the shoulders and upper back, are russet to reddish-buff to brownish, and are sometimes "frosted" with silvery gray white hairs. The presence or absence of a dorsal stripe and silvery hair tips appear to be a seasonal variation and have led some to postulate the existence of an additional species, N. intermedius, although DNA analysis has since confirmed this to be an adult version of the pygmy slow loris. The pygmy slow loris has buffy flanks, paler than the back. The upper sides of the arms are ochraceous, and have silvery hairs mingled with the darker ones. The buff legs are also tipped with silvery white hairs. The underparts are plumbeous (lead-colored) at the base, with ochraceous apical portions. The hands and feet are silvery white, with yellowish-white nails. Foot length is relatively consistent, averaging about 45 mm (1.8 in).
The pygmy slow loris has a diploid chromosome number of 2n=50. Although the banding patterns on the chromosomes of all slow lorises are similar, this species may be distinguished from the Bengal slow loris (N. bengalensis) by distinct differences in the number and location of nucleolus organizer regions.
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