Tarsiers are small arboreal primates (genus Tarsius) that inhabit the forests of various islands of the Philippines, where they are active primarily during dawn and dusk. Their common and genus names reflect the fact that this small primate’s tarsal (ankle) bones are extremely long, making its “heel” a major component of hind limb elongation. Functionally, tarsal elongation acts as a lever in the tarsier’s leaping mode of locomotion. There are at least five recognizable species of Tarsius: T. syrichta, T. bancanus, T. spectrum, T. pumilus, and T. dianae. Solitariness among males and females is the most consistently documented social behavior.
Tarsius is unique in many ways compared to other extant primates. It is the only primate with reptilianlike scales, which are located around its nipples and on its tail (which bears a tuft of hair only at its tip). It is also the only primate whose tail is longer than its body and whose eyeball is larger than its brain. It has conical eyeballs that do not move in their sockets, however it can rotate its head 180 degrees in either direction. The tarsier grows sinus hairs externally on its nose and has interdigitating, sensory hairs around its upper and lower lips (forming an “oral organ”). It has a grooming claw on its third pedal digit. It is also the only primate that develops hemochorial placentation in the environment of a bicornuate uterus, and that begins amniogenesis via cavitation but then converts the process to folding. The tarsier is totally proteinivorous, eating prey larger than itself, including fish, birds, lizards, and poisonous, neurotoxic snakes. Whether as a result of becoming unique or remaining primitive compared to other primates, the tarsier bears a well-developed anterior cusp (paraconid) on all three of its lower molars that lies distinctly apart from the next cusp behind (metaconid). Its upper anterior teeth are caniniform and those behind increasingly premolariform; the lower anterior teeth resemble the lateral teeth of toothcombed primates, with the teeth behind becoming increasingly premolariform. Furthermore, its chromosome number is the greatest among primates (2N = 80).
Tarsiers have for the longest time been grouped with primates of Madagascar, lorises, and bushbabies in the suborder Prosimii. One compelling reason is that they all uniquely possess a grooming claw on the second pedal digit. The morphological counterpart to the lateral tooth of the uniquely prosimian toothcomb complements this hypothesis. Dental features link Tarsius with the loris group.
In 1918, Pocock grouped Tarsius and anthropoids in the suborder Haplorhini because they all supposedly have laterally continuous (not slit) nostrils. The remaining prosimians were relegated to Strepsirhini. Pocock was incorrect about Tarsius, but Hill kept Haplorhini alive by confusing “single nostril” with “single upper lip,” a feature that does not, however, correctly characterize all New World monkeys (making Tarsius and catarrhines the only primates with a smoothly continuous upper lip). Various questionable features have been invoked in support of Haplorhini, for example, hemochorial placentation (but cetaceans and various bats and rodents, not Tarsius, approach the anthropoid condition) and partial postorbital closure (which develops differently and involves different bones in Tarsius versus anthropoids). The absence of a retinal fovea and a rhinarium do, however, support this hypothesis.
References:
- Fleagle, J. G. (1999). Primate adaptation and evolution (2nd ed.). New York: Academic Press.
- Niemitz, C. (Ed.) (1984). Biology of tarsiers. Stuttgart: Gustav Fischer Verlag.
- Wright, P., Simons, E. L., & Gursky, S. (Eds.). (2003). Tarsiers: Past, present and future. Rutgers, NJ: University of Rutgers Press.
