page_title: Slow loris
section_title: Anatomy and physiology
hierarchical_section_title: Slow loris / Anatomy and physiology
caption_attribution_description: English: Sunda slow loris (Nycticebus coucang) skull photographs from four sides.
context_page_description: Slow lorises are a group of several species of nocturnal strepsirrhine primates that make up the genus Nycticebus. Found in Southeast Asia and bordering 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. Although many previous classifications recognized as few as a single all-inclusive species, there are now at least eight that are considered valid: the Sunda slow loris, Bengal slow loris, pygmy slow loris, Javan slow loris, Philippine slow loris, Bangka slow loris, Bornean slow loris, and Kayan River slow loris. The group's closest relatives are the slender lorises of southern India and Sri Lanka. Their next closest relatives are the African lorisids, the pottos, false pottos, and angwantibos. They are less closely related to the remaining lorisoids, and more distantly to the lemurs of Madagascar. Their evolutionary history is uncertain since their fossil record is patchy and molecular clock studies have given inconsistent results.
context_section_description: Slow lorises have a round head because their skull is shorter than in other living strepsirrhine. Like other lorisids, their snout does not taper towards the front of the face as it does in lemurs, making the face appear less long and pointed. Compared with the slender lorises, the snout of the slow loris is even less pointed. As with other members of Lorisidae, its interorbital distance is shorter than in lemurs. The skull has prominent crests (ridges of bone). A distinguishing feature of the slow loris skull is that the occipital bone is flattened and faces backward. The foramen magnum (hole through which the spinal cord enters) faces directly backward. The brains of slow lorises have more folds (convolutions) than the brains of galagos.
  The ears are small, sparsely covered in hair, and hidden in the fur.  Similar to the slender lorises, the fur around and directly above the eyes is dark.  Unlike the slender lorises, however, the white stripe that separates the eye rings broadens both on the tip of the nose and on the forehead while also fading out on the forehead.  Like other strepsirrhine primates, the nose and lip are covered by a moist skin called the rhinarium ("wet nose"), which is a sense organ.
  The eyes of slow lorises are forward-facing, which gives stereo vision. Their eyes are large and possess a reflective layer, called the tapetum lucidum, that improves low-light vision. It is possible that this layer blurs the images they see, as the reflected light may interfere with the incoming light. Slow lorises have monochromatic vision, meaning they see in shades of only one color. They lack the opsin gene that would allow them to detect short wavelength light, which includes the colors blue and green.
  The dental formula of slow lorises is  × 2 = 36, meaning that on each side of the mouth there are two upper (maxillary) and lower (mandibular) incisors, one upper and lower canine tooth, three upper and lower premolars, and three upper and lower molars, giving a total of 36 permanent teeth. As in all other crown strepsirrhines, their lower incisors and canine are procumbent (lie down and face outwards), forming a toothcomb, which is used for personal and social grooming and feeding. The toothcomb is kept clean by the sublingua or "under-tongue", a specialized structure that acts like a toothbrush to remove hair and other debris.  The sublingua extends below the tip of the tongue and is tipped with keratinized, serrated points that rake between the front teeth.
  Slow lorises have relatively large maxillary canine teeth, their inner (mesial) maxillary incisors are larger than the outer (distal) maxillary incisors, and they have a diastema (gap) between the canine and the first premolar. The first mandibular premolar is elongated, and the last molar has three cusps on the crown, the shortest of which is near the back. The bony palate (roof of the mouth) only goes as far back as the second molar.
  Slow lorises range in weight from the Bornean slow loris at 265 grams (9.3 oz) to as much as 2,100 grams (74 oz) for the Bengal slow loris. Slow lorises have stout bodies, and their tails are only stubs and hidden beneath the dense fur. Their combined head and body lengths vary by species, but range from 18 to 38 cm (7.1 to 15.0 in) between all species. The trunk is longer than in other living strepsirrhines because they have 15–16 thoracic vertebrae, compared to 12–14 in other living strepsirrhines. This gives them greater mobility when twisting and extending towards nearby branches. Their other vertebrae include seven cervical vertebrae, six or seven lumbar vertebrae, six or seven sacral vertebrae, and seven to eleven caudal vertebrae.
  Unlike galagos, which have longer legs than arms, slow lorises have arms and legs of nearly equal length. Their intermembral index (ratio of arm to leg length) averages 89, indicating that their forelimbs are slightly shorter than their hind limbs. As with the slender lorises, their arms are slightly longer than their body, but the extremities of slow lorises are more stout.
  Slow lorises have a p
caption: The skull of the Sunda slow loris
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