Tarsiers are nocturnal primates known for their large, round eyes and the extended tarsus bone in their feet. Once widespread across the globe, they are now found only on the islands of Maritime Southeast Asia. They belong to the family Tarsiidae, the last surviving family of the infraorder Tarsiiformes within the suborder Haplorhini.
These mammals are expert climbers, gripping tree vines tightly and leaping impressive distances when on the ground. Notably, they are the only primates that are strictly carnivorous, maintaining a diet mostly consisting of insects.
The total body length, including the head, ranges between 3.9 and 5.9 in (10 and 15 cm), but the hindlimbs, including the feet, are almost twice that length. In addition, their tail is almost 7.8 to 9.8 in (20 to 25 cm) long.
One of the largest tarsiers, the Sangihe tarsier (Tarsius sangirensis), measures 5.9 in (15 cm) from the head to the base of the tail and weighs only 5 to 5.3 oz (143 to 150 g).
Though short and round to look at because of the way they cling to trees, these primates actually have long and slender bodies covered by soft and velvety fur. This fur ranges from buff and beige to ochre, and in some species, it is grayish-brown on the underside. Their tail is scaly towards the bottom and, in most species, culminates in a tuft of hair.
Their most distinctive features include the elongated tarsus bone of the feet (hence their name) and their conspicuously large, goggling eyes, with each eyeball measuring 16 mm (0.63 in) in diameter and weighing almost as much as their brain. Their elongated tarsi, along with tibiofibulae (fused tibia and fibula), help them cling and leap from tree to tree efficiently.
Their muzzle is short, and ears are large, hairless, membranous, and easily movable.
Tarsiers’ skulls have large, forward-facing eye sockets with expanded rims. A thin interorbital septum separates the orbits.
Their upper medial incisors are pointed and large, followed by small canines and tritubercular (three-cusped) upper molars. They have a dental formula of 2.1.3.3/1.1.3.3.
The neural connections between the two eyes and the lateral geniculate nucleus (a structure in the thalamus, a key component in the mammalian visual pathway) distinguish tarsiers from other primates, like monkeys, lemurs, and lorises. The difference lies in the sequence of cellular layers that receive signals from the ipsilateral (same side of the head) and contralateral (opposite side of the head) eyes of the lateral geniculate nucleus.
Their eyes possess a central pit of cone cells called the fovea and, unlike many nocturnal vertebrates, lack the light-reflecting layer tapetum lucidum in the retina.
For much of the 20th century, the phylogenetic position of tarsiers has been under debate. They were alternately classified as strepsirrhine (infraorder Strepsirrhini) primates under the suborder Prosimii but currently belong to the infraorder Haplorhini. Very recently, all tarsiers have been placed under a single genus, Tarsius. However, in 2010, scientists Colin Groves and Myron Shekelle suggested splitting Tarsius into 3 genera: Carlito (Philippine tarsiers), Cephalopachus (western tarsiers), and Tarsius (eastern tarsiers). Such grouping was based on differences in eye size, limb length, dentition, tail tufts, tail sitting pads, chromosome count, number of mammary glands, and several other factors.
The Philippine tarsier is further subdivided into 3 subspecies, whereas the Horsfield’s or western tarsier has 4 subspecies.
Tarsiers are among the oldest primates, dating back at least 55 million years. While all living tarsiers today are confined to Southeast Asian islands, they were once widespread across Asia, Europe, and North America. Fossil evidence indicates that their dental structure has remained largely unchanged over the last 45 million years, except for size.
So far, four fossil species have been identified, all from the Eocene and Miocene Epochs. Tarsius eocaenus from the Middle Eocene has been found in China, while Tarsius sirindhornae of the Middle Miocene has been unearthed in Thailand. Additionally, Hesperotarsius thailandicus and Hesperotarsius sindhensis, both dating back to the Miocene, have been discovered in northwestern Thailand and Pakistan, respectively.
Though once widespread globally, these mammals are currently restricted to Maritime Southeast Asia, particularly in the islands of Brunei, Indonesia, Malaysia, and the Philippines.
They primarily inhabit deciduous and mangrove forests, thorn scrub, shrublands, grasslands, swamps, and even urban gardens. Secondary habitats include plantations, areas with small-scale farming, and forests cleared for bamboo and rattan extraction.
These animals typically prefer forests with liana or a long-stemmed woody vine. These vines provide the best vertical support for climbing trees. They also occupy the lower vegetation layers, just above the ground, to avoid attacks from aerial and terrestrial predators.
Tarsiers are the only primates that are exclusively carnivorous. Although they mostly feed on insects, like beetles, arachnids, cockroaches, grasshoppers, cicadas, and walking sticks, they opportunistically consume flying frogs, lizards, and crabs that crawl up the land.
These animals sometimes eat baby birds, small snakes, and even baby bats.
These are nocturnal creatures (sometimes crepuscular, too), which are most active around the night and typically rest throughout the day.
These animals generally live in monogamous pairs comprising a single male and a female. However, sometimes, a few groups may consist of a dominant male, multiple females, and their young. In contrast, some species, like the western tarsier, primarily live solitarily.
Tarsiers usually forage individually and rest in groups. When these groups gather, they often groom each other.
When a predator is around, tarsiers produce warning calls to gather their group members. As a response to the call, around 2 to 10 tarsiers assemble around the predator, mobbing and harassing it to reduce chances of being attacked.
Tarsiers characteristically move by leaping. It is due to their unique heel morphology that these primates can leap quite far, with some species, like the western tarsier, able to leap over 16 ft.
They also climb and walk on all four legs (quadrupedalism), besides hopping and clambering on trees. When not actively moving from one place to another, they cling to the trees.
Although there is insufficient data on the lifespan of tarsiers in the wild, they are estimated to live for as long as 16 years.
Most tarsiers reproduce seasonally, with most births usually taking place at the end of the rainy season (between February and July). Some species, like Spectral tarsiers, breed twice a year and usually copulate either in May or November.
After a gestation period of about six months, they give birth to a single offspring. These furred newborns weigh 25 to 30% of their mother’s body weight (the largest mammalian young relative to maternal body mass) and have open eyes. They are able to climb within a day of their birth and attain sexual maturity by two years.
They are targeted by terrestrial predators, like snakes, lizards, cats, and slow lorises, as well as aerial predators, such as owls and other birds of prey.
Tarsiers are adversely impacted by habitat loss and degradation, as well as the use of harmful insecticides. The IUCN Red List classifies the 14 species into the following categories.
| Species | IUCN Category |
|---|---|
| Siau Island tarsier | Critically Endangered (CR) |
| Niemitz’s tarsier, Sangihe tarsier, Peleng tarsier, Pygmy tarsier | Endangered (EN) |
| Spectral tarsier, Gursky’s spectral tarsier, Jatna’s tarsier, Dian’s tarsier, Makassar tarsier, Horsfield’s tarsier, Wallace’s tarsier | Vulnerable (VU) |
| Philippine tarsier | Near Threatened (NT) |
| Lariang tarsier | Data Deficient (DD) |
As generally shy animals, tarsiers are stressed by certain activities associated with captivity, such as being touched or kept in an enclosure and sudden camera flashes. Under such stressful situations, they hit their heads against hard objects and kill themselves.
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