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, remain ambiguous. Thus, some systematists argue the debate is conclusively settled in favor of a monophyletic Haplorrhini. In common with simians, tarsiers have a mutation in the L-gulonolactone oxidase (GULO) gene, which confers the need for vitamin C in the diet. Since the strepsirrhines do not have this mutation and have retained the ability to make vitamin C, the genetic trait that confers the need for it in the diet would tend to place tarsiers with haplorrhines.
At a lower phylogenetic level, the tarsiers have, until recently, all been placed in the genus Tarsius, while it was debated whether the species should be placed in two (a Sulawesi and a Philippine-western group) or three separate genera (Sulawesi, Philippine and western groups). Species level taxonomyis complex, with morphology often being of limited use compared to vocalizations. Further confusion existed 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.
In 2010, Colin Groves and Myron Shekelle suggested splitting the genus Tarsius into three genera, the Philippine tarsiers (genus Carlito), the western tarsiers (genus Cephalopachus), and the eastern tarsiers (genus Tarsius). This was based on differences in dentition, eye size, limb and hand length, tail tufts, tail sitting pads, the number of mammae, chromosome count, socioecology, vocalizations, and distribution. The senior taxon of the species, T. tarsierwas restricted to the population of a Selayar island, which then required the resurrection of the defunct taxon T. fuscus.
In 2014, scientists from the University of the Philippines (Diliman Campus) - Institute of Biology in partnership with the University of Kansas has discovered a gentically distinct populations of Philiipines tarsier. The genetically distinct populations are found in the Dinagat Islands, Surigao del Norte, and probably Siargao Islands in Mindanao Island's northeast portion. Isolation is the key to the population's distinctiveness. Prior to the study, scientists generally accepted three subspecies of Philippine tarsier: the large island of Mindanao contained one subspecies, Tarsius syrichta carbonarius; while the islands of Samar and Leyte sported another, Tarsius syrichta syrichta; and Bohol held the third, Tarsius syrichta fraterculus. However the new genetic research found the relationships among the Philippine tarsier populations was even messier. Looking at mitochondrial and nuclear DNA, Brown's team uncovered three different evolutionary lineages: one lineage of tarsier makes their home on Bohol, Samar, and Leyte Islands (putting two presently accepted Philippine tarsier subspecies into a single subspecies); another has conquered the vast majority of Mindanao; while a long-cryptic branch has evolved in northeastern Mindanao and Dinagat Island (the new subspecies). For the purposes of the paper, the scientists refer to this as the Dinagat-Caraga tarsier. Rafe Brown of the University of Kansas' Biodiversity Institute, an author of the study, also said that through a more keen study, the only current Philippine tarsier species, Carlito syrichta, could be split into three distinct full species in the future.
Anatomy and Physiology Edit
Tarsiers are small animals with enormous eyes; each eyeball is approximately 16 mm in diameter and is as large as its entire brain. The unique cranial anatomy of the tarsier results from the need to balance their large eyes and heavy head so they are able to wait silently for nutritious prey. Tarsiers have an incredibly strong auditory sense because their auditory cortex is very distinct. Tarsiers also have very long hind limbs, due mostly to the extremely elongated tarsus bones of the feet, from which the animals get their name. The combination of their elongated tarsi and fused tibiofibulae makes them morphologically specialized for vertical clinging and leaping. 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:220.127.116.11.1.3.3. Unlike many nocturnal vertebrates, 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." Philippine tarsiers are capable of hearing frequencies as high as 91 kHz. They are also capable of vocalizations with a dominant frequency of 70 kHz.
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. Pygmy tarsiers differ from other species in terms of their morphology, communication, and behavior. The differences in morphology that distinguish pygmy tarsiers from other species are likely based on their high altitude environment.
All tarsier species are nocturnal in their habits, but like many nocturnal organisms, some individuals may show more or less activity during the daytime. Based on the anatomy of all tarsiers, they are all adapted for leaping even though they all vary based on their species. Ecological variation is responsible for differences in morphology and behavior in tarsiers because different species become adapted to local conditions based on the level of altitude. For example, the colder climate at higher elevations can influence cranial morphology.
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. Tarsiers tend to be extremely shy animals.