Tarsiers are very small nocturnal and crepuscular primates. They possess a short-snouted, round head, ranging between 1.4 and 1.7 in (3.5-4.4 cm) in length, and middle-sized to very large, skinny, mobile ears. Their eyes are huge, with one eye weighing nearly as much as the whole brain. The eyes do not fit into the cranial orbit, but protrude from their sockets like an egg in an egg cup. The owl-like appearance of tarsiers is a consequence of many common features in the biology of both these nocturnal predators. More than 30 such convergences of tarsiers with owls have been described, such as the same pelage color in both sexes, similarities in the anatomy of the eyes, the semicircular canals in the inner ear, and the sensory biology for prey location. The dental formula for tarsiers is (I2/2 C1/1 P3/3 M3/3) X 2 = 36.
The fur of all species shows the colors of dead leaves, i.e., the tarsiers are sand-colored to ochre or grayish buff, with a considerable variation towards reddish or brownish. The Philippine species (Tarsius syrichta) tends to be lighter than the western tarsier (T. bancanus) and the Sulawesi species (T. spectrum). The fur is velvet-like but sometimes somewhat curly. A curly pelage seems to be more frequent in tarsiers from higher altitudes. All sparsely haired or naked parts of
the skin are pigmented ranging from a more sandy color, e.g. in the Philippine species, to a rich dark brown, e.g., in Dian's tarsier (T. dianae). Orange skin color at the testicles or dark brown patches in the ears, however, are caused by secretions from skin glands.
The slender body, reaching up to about 4 in (10 cm) in length, often appears round in the clinging or sitting animals. The hands are equipped with long or even extraordinarily long, very prehensile fingers for clinging and climbing, but especially for catching prey. Their tips have round discs of finger pads for an enhanced grip when clinging to vertical stems. The thumb is opposable to the palm and the fifth digit through movements in its basal joint. As an extreme adaptation to leaping between vertical supports, the hind limbs are longer in relation to body size than in any other mammal, reaching 2.3 times the length of the precaudal spine. As a portion of the hind extremity, the foot is proportionately longer than the thigh and the lower leg, which is mainly due to the strongly elongated calcaneal and navicular bones. There is a strong, opposable great toe. Except for toilet claws on the second and third toes, all toes and fingers have nails.
The tail of all species is long and rodlike. Except for the pygmy tarsier (T. pumilus), where it is certainly shorter, the tail measures between 7.8 and 9.8 in (20-25 cm), which is about 2.5 times the trunk length. In the Sulawesi tarsiers, the tail retains scaly skin structures, a most ancestral and, therefore, most spectacular feature, which is not found in any other primate. The tails of all species appear to be naked, at least partially. This is most obvious in the Philippine tarsier, which possesses only a thin, roughly 2.4 in (6 cm) long, sparsely haired tuft, the length of the single hair being only about 0.1 in (3 mm) long. In the western tarsier, the tuft hair is about 0.3 in (7 mm) long, whereas in the Sulawesi species the sin gle hairs may measure 0.2-0.5 in (5-12 mm). Although far from being bushy, more than half of the distal part of the tail of the Sulawesi tarsiers is hairy.
There is a sensitive skin area on the ventral side of the tail of all tarsier species. Being endowed with papillary skin ridges, this is friction skin that is used as a support area. Tarsiers spare much of their energy budget by sitting on their tails when resting on a vertical support, much like woodpeckers do.
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