Tarsiers

Tarsiers are prosimian primates of the genus Tarsius, a monotypic 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.

Fossil record
Fossils of tarsiers and 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. They also have the longest continuous fossil record of any primate genus, and the fossil record indicates that their dentition has not changed much, except in size, in the past 45 million years.

Classification
The phylogenetic position of extant tarsiers within the order Primates has been debated for much of the past century, and tarsiers have alternately been classified with strepsirrhine primates in the suborder Prosimii, or as the sister group to the simians (=Anthropoidea) in the infraorder Haplorrhini. Analysis of SINE insertions, a type of macromutation to the DNA, is argued to offer very persuasive evidence for the monophyly of Haplorrhini, where other lines of evidence, such as DNA sequence data, had remained ambiguous. Thus, some systematists argue that the debate is conclusively settled in favor of a monophyletic Haplorrhini.

At a lower level, it has been indicated that the tarsiers, currently all placed in the genus Tarsius, actually should be placed in two (a Sulawesi and a Philippine-Western group) or three separate genera (a Sulawesi, Philippine and Western group). Species level taxonomy is complex, with morphology often being of limited use compared to vocalizations. Several "vocal morphs" may represent undescribed taxa (such as North Sulawesi "T. tarsier", and a tarsier from the Togian Islands), as may also be the case for a number of poorly known isolated populations (such as the Basilan, Leyte and Dinagat populations of the T. syrichta group, and tarsiers on Siau Island that tentatively have been assigned to T. sangirensis). Further confusion exists over the validity of certain names. Among others, the widely used T. dianae has been shown to be a junior synonym of T. dentatus, and comparably T. spectrum is now considered a junior synonym of T. tarsier. On the contrary T. tarsier has been considered a junior synonym of T. syrichta, but features of the holotype indicate this is incorrect.


 * Infraorder Tarsiiformes
 * Family Tarsiidae: tarsiers
 * Genus Tarsius
 * T. syrichta group
 * Philippine Tarsier, Tarsius syrichta
 * Western or Horsfield's Tarsier, Tarsius bancanus
 * T. tarsier group
 * Spectral Tarsier, Tarsius tarsier
 * Dian's Tarsier, Tarsius dentatus
 * Lariang Tarsier, Tarsius lariang
 * Peleng Tarsier, Tarsius pelengensis
 * Sangihe Tarsier, Tarsius sangirensis
 * Siau Island Tarsier, Tarsius tumpara
 * Pygmy Tarsier, Tarsius pumilus

Anatomy and physiology
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 have no toothcomb, and their dental formula is also unique:

Vision
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 animals, however, tarsiers lack a light-reflecting area (tapetum lucidum) of the eye. They also have a fovea, which is atypical for nocturnal animals.

The tarsier 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".

Behaviour
They are primarily insectivorous, and catch insects by jumping at them. They are also known to prey on small vertebrates, such as birds, snakes, lizards, and bats. As they jump from tree to tree, tarsiers can catch even 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 after one year. Adults live in pairs, with a home range of around one hectare.

Conservation
One tarsier species, Dian's Tarsier (T. dentatus; listed by the junior synonym T. dianae by the IUCN), is listed on the IUCN Red List as being Vulnerable. Two other species/subspecies, Horsfield's Tarsier (T. bancanus) and its nominate subspecies, are listed as Least Concern. The Spectral Tarsier (T. tarsier; listed by the junior synonym T. spectrum) is categorized as Near Threatened. All other tarsier that have been rated by the IUCN are listed as Data Deficient.

Tarsiers have never formed successful breeding colonies in captivity, and when caged, tarsiers have been known to injure and even kill themselves because of the stress.

One site having some success at restoring tarsier populations is in the Philippine Island of Bohol. The Philippine Tarsier Foundation has developed a large semi-wild enclosure that uses lights to attract the nocturnal insects that make up the tarsier's diet.

The 2008 described Siau Island Tarsier is regarded as critically endangered and was listed among the 25 most threatened primates by Conservation International and the IUCN/SCC Primate Specialist Group in 2008.