Tarsiers are haplorrhine primates of the genus Tarsius, a genus in the family Tarsiidae, which is itself the lone extant family within the infraorder Tarsiiformes. Although the group was once more widespread, all the species living today are found in the islands of Southeast Asia.
Fossils of tarsiiform primates are found in Asia, Europe, and North America, and there are disputed fossils from Africa, but extant tarsiers are restricted to several Southeast Asian islands including the Philippines, Sulawesi, Borneo, and Sumatra. The fossil record indicates that their dentition has not changed much, except in size, in the past 45 million years.
Within the family Tarsiidae, there are two extinct genera, Xanthorhysis and Afrotarsius. However, the placement of Afrotarsius is not certain, and is sometimes listed in its own family, Afrotarsiidae, within the infraorder Tarsiiformes.
Two fossil species of with in the genus Tarsius are known from the fossil record. Tarsius eocaenus is known from the Middle Eocene in China while Tarsius thailandicus lived during the Early Miocene in northwestern Thailand. The genus Tarsius has a longer fossil record than any other primate genus, but the assignment of the Eocene and Miocene fossils to the genus is questionable.
Tarsiers are small animals with enormous eyes; each eyeball is approximately 16 mm in diameter and is as large as their entire brain. Tarsiers also have very long hind limbs. In fact, their feet have extremely elongated tarsus bones, from which the animals get their name. The head and body range from 10 to 15 cm in length, but the hind limbs are about twice this long (including the feet), and they also have a slender tail from 20 to 25 cm long. Their fingers are also elongated, with the third finger being about the same length as the upper arm. Most of the digits have nails, but the second and third toes of the hind feet bear claws instead, which are used for grooming. Tarsiers have very soft, velvety fur, which is generally buff, beige, or ochre in color.
Unlike other prosimians, tarsiers lack any toothcomb, and their dental formula is also unique:
All tarsier species are nocturnal in their habits, but like many nocturnal organisms some individuals may show more or less activity during the daytime. Unlike many nocturnal vertebrates, however, tarsiers lack a light-reflecting area (tapetum lucidum) of the eye and have a fovea.
The tarsier’s brain is different from other primates in terms of the arrangement of the connections between the two eyes and the lateral geniculate nucleus, which is the main region of the thalamus that receives visual information. The sequence of cellular layers receiving information from the ipsilateral (same side of the head) and contralateral (opposite side of the head) eyes in the lateral geniculate nucleus distinguishes tarsiers from lemurs, lorises, and monkeys, which are all similar in this respect. Some neuroscientists suggested that “this apparent difference distinguishes tarsiers from all other primates, reinforcing the view that they arose in an early, independent line of primate evolution.”
Tarsiers are the only extant entirely carnivorous primates: they are primarily insectivorous, and catch insects by jumping at them. They are also known to prey on birds, snakes, lizards, and bats. As they jump from tree to tree, tarsiers can even catch birds in motion.
Gestation takes about six months, and tarsiers give birth to single offspring. Young tarsiers are born furred, and with open eyes, and are able to climb within a day of birth. They reach sexual maturity by the end of their second year. Sociality and mating system varies, with tarsiers from Sulawesi living in small family groups, while Philippine and Western tarsiers are reported to sleep and forage alone.
Keeping with their tradition of using black and white drawings of interesting animals, the technical book publisher O’Reilly Media selected the tarsier for the cover of Unix in a Nutshell books as well as each of the 7 editions of the popular Learning the vi and vim Editors by Linda Lamb. When questioned about the animal choice, Publisher Tim O’Reilly described the tarsier as looking “like somebody who had been a text editor for too long.” This tarsier image has become a part of O’Reilly’s branding and is now included in their logo.