Tarsier

The tarsier (pronounced TAR-see-urr) is a small, nocturnal primate with large eyes found only on a few islands in Southeast Asia. Tarsiers are ancient animals that began appearing in the fossil record about 45 million years ago. They are considered to be a possible evolutionary link between lemur-like animals and humans. They are called tarsiers because of the very long tarsus bones found in their feet.

Many aspects about tarsier evolution and taxonomy (the system scientists use to define how organisms are related) and even the number of species remain unclear. Tarsiers share physical and evolutionary characteristics with both lemurs and more complex primates but have physiological differences that have made classifying them with either group problematic. Even differentiating among various species of tarsiers has proven difficult, and biologists continue to review how to categorize them within their own genus. The Wallace's tarsier (Tarsius wallacei) was only discovered in 2010, and scientists believe that more undiscovered species may exist.

Background

The taxonomy of tarsiers is complicated and still under review. Since 2010, tarsiers have been split into three genera: Carlito, the Philippine tarsier; Cephalopachus, the Horsfield's, or western, tarsier; and Tarsius, which includes the nine species of eastern tarsiers. Together, these animals belong to the taxonomic family Tarsiidae. Distinctions among the various tarsiers at the species level are primarily based on differences in vocalization, social structure, distribution, teeth structure, and appearance. However, scientists have discovered genetic differences among some individual populations of tarsiers that may warrant the further splitting of these populations into more distinct species.

According to the fossil record, modern species of tarsiers differ very little from their ancestors that lived 45 million years ago. They are often called "living fossils" as a result. Tarsiers have the least species diversity of any primate group due, in part, to their highly specialized physiology. Nonetheless, the unique traits that have made categorizing them so difficult also make them the most distinct group of primates within the larger primate taxonomic order.

Tarsiers are defined as both prosimians (alongside strepsirrhine, or "wet-nosed," primates such as lemurs and lorises) and haplorrhine ("dry-nosed" or "simple-nosed) primates (such as both Old World and New World monkeys and apes and humans). Prosimians are a collection of comparatively primitive primates that share similar behavioral characteristics, including living mainly in trees and eating mainly live prey. As a result, their jaws and teeth closely resemble those of strepsirrhine primates. However, scientists believe that tarsiers are more closely related to other haplorrhine primates on the evolutionary scale. Haplorrhines share certain physical characteristics not seen in the strepsirrhine primates. These include the inability to naturally produce vitamin C, the capacity to produce a wide variety of facial expressions, and a reliance upon sight as their primary sense. These adaptations have resulted in eyes and noses that are more similar to haplorrhine primates. In 2016, scientists were able to sequence the entire tarsier genome. Based on these results, they believe that tarsiers' are genetically more closely related to other haplorrhine primates.

Overview

Tarsiers have a number of unusual characteristics. Each of their eyes is larger than their brains. Their brains, which are small and smooth, more closely resemble the brains of carnivores than those of other primates. They are the only known primates to have a completely carnivorous (meat-eating) diet. They are able to turn their heads a full 180 degrees in either direction, an evolutionary adaptation that allows them to see both their prey and predators more easily. They are the only mammals capable of such spinal movement. They have very long legs and tails. In particular, their anklebones are highly elongated, and their tibia and fibula bones are fused halfway down. These specialized anatomical traits give them strong legs that allow them to jump from tree to tree with ease.

Tarsiers are among the smallest primates in the world, weighing between 3 and 5 ounces (80 and 150 grams) depending on the species. Despite their small size, tarsier mothers produce young that weigh up to 30 percent of their own weight—a trait that is very unusual among mammals. To produce such large infants, tarsiers typically gestate (carry an embryo within their womb) for six months, a period that is far longer than other, larger primates. Tarsiers wean their young only two months after giving birth; other primates of comparable size generally take more than a year to do this. Tarsiers give birth to only one infant yet have between four and six nipples.

The known species of tarsiers are found on islands located within Indonesia, Malaysia, and the Philippines. Due to environmental degradation and the continuing deforestation of their jungle habitats, most tarsier species are regarded as either threatened or endangered. In particular, the Siau Island tarsier, which is located only on the small island of Siau, is listed as critically endangered and was placed on the World's 25 Most Endangered Primates by the International Union for Conservation of Nature (IUCN) between 2006 and 2010. In addition to these threats to their native habitats, tarsiers have proven very difficult to keep in captivity. About half of all wild-caught tarsiers die quickly in captivity due to the stress of their capture. Attempts to breed tarsiers in captivity have had limited success. Sanctuaries in the Philippines and Indonesia have had some success in preserving, but not breeding, tarsiers.

Tarsiers spend their entire lives living in trees. They are powerful leapers who can jump as far as 6.5 feet (2 meters) in distance and 5 feet (1.5 meters) in height. They mostly eat insects, but they may consume birds, snakes, and lizards. They have been seen catching some forms of prey in mid-flight. Another specialized adaptation for life in trees is adhesive finger pads that enable them to stick to trees while asleep or when jumping between trees.

Perhaps their most identifiable trait is their enormous eyes. Tarsiers have the biggest eyes relative to weight of any mammal. Tarsiers are very vocal and can make many calls. These calls differ among species and even subspecies of tarsiers, and they are one of the ways scientists differentiate among species and colonies. Mated males and females often vocalize together, creating a series of song-like noises.

Bibliography

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