Tarsiers are small primates found only in the islands of Southeast Asia. They all belong to the genus Tarsus, but the exact number of species is open to debate. Most recently, tarsiers have been divided into three species groups, or genera: Western, Eastern, and Philippine, containing 18 species or subspecies.

Across species, all tarsiers share certain characteristics. They are all nocturnal and tend to be small, weighing between 80 and 150 g. They have soft, velvety fur, which is generally gray, buff, beige, or ochre. All tarsiers have long hind legs and long tails that are either sparsely covered in fur or have a tuft at the end. Although their bodies are slender, tarsiers look round due to their habit of crouching while clinging to branches. Their most distinctive features may be their round heads, remarkably large eyes, and their mobile, bat-like ears.

Read on to learn more about these adorable and unusual primates.

1. They were once found all around the world. Tarsiers were once more widespread — fossils have been found in Asia, Europe, and North America. All the species living today are restricted to the islands of Southeast Asia, including the Philippines, Sulawesi, Borneo, and Sumatra.

2. They haveenormous eyes. Tarsiers have the largest eyes of any mammal in relation to their body size. Each eyeball is about 16 mm in diameter and weighs nearly as much as the animal's brain.

3. They can turn their heads like they're in The Exorcist. Tarsiers, thanks to specially adapted vertebrae, are able to turn their heads 180 degrees in each direction.

4. Tarsiers are built for leaping. The hind limbs of tarsiers are about twice as long as their bodies, due mostly to the extremely elongated tarsus bones of the feet, from which the animals get their name. Their hind limbs are longer in proportion to body length than in any other mammal. In addition to their long tarsals, tarsiers also have well-developed leg muscles. These specializations enable tarsiers to leap over 5 meters, more than 40 times their own body length. Tarsiers spend most of their time clinging to vertical tree branches and move through their environment primarily through leaping, although they can also climb, walk, and hop.

5. Their fingers are elongated and sticky. Tarsiers have long, thin fingers, with the longest third finger about the same length as the upper arm. Their fingers are tipped with sticky pads that help them grip and cling to surfaces. The second and third fingers bear long, curved claws which are used for grooming (these are sometimes called "toilet claws").

6. Tarsier brains differ from those of other primates. They have a unique connection between their eyes and the lateral geniculate nucleus, an area of the brain that receives visual information. This distinguishes tarsiers from lemurs, lorises, and monkeys, suggesting that they arose in an early, independent line of primate evolution.

7. Some tarsiers live in families, while others are loners. Social behavior varies between the tarsier species. Eastern tarsiers are the most sociable, living in small family groups, while Western tarsiers appear to be more solitary.

8. Tarsiers are the only entirely carnivorous primate. Tarsiers eat mostly insects, but also prey on birds, snakes, and lizards. They have strong jaws and teeth and a wide mouth for their small size, which enables them to consume larger prey.

9. Tarsier babies are the largest relative to the size of the mother of any mammal. Female tarsiers give birth to a single baby that weighs 25-30% of the mother's body weight. Young tarsiers are born with fur and their eyes open and can climb trees within a day of birth. Mothers carry their young in their mouths or on their backs.

10. Tarsiers are very vocal. The number and types of vocalizations vary between species, with spectral tarsiers producing 15 different groups of vocalizations (including alarm calls, food calls, infant calls, and play whistles) and Western tarsiers producing just four kinds of calls. In some tarsier species, mated pairs perform vocal duets, often at dusk and dawn.In addition to vocalizing, all tarsiers use scent marking as a form of communication. They have scent glands in their face, stomach, and genital regions and rub their glands on trees, bushes, and each other to defend territories and confirm group membership. 

References and Other Resources:

Collins, C. E., Hendrickson, A., and Kaas, J. H. (2005). Overview of the visual system of tarsius. The Anatomical Record Part A 287(1): 1013–1025. doi:10.1002/ar.a.20263.Dagosto, M., Gebo, D. L., and Dolino, C. (2001).

Positional behavior and social organization of the Philippine tarsier (Tarsius syrichta). Primates 42: 233–243. doi:10.1007/bf02629639.Gron, K. J. (2010).

Primate Factsheets: Tarsier (Tarsius) Taxonomy, Morphology, & Ecology. Accessed 2015 August 21 at http://pin.primate.wisc.edu/factsheets/entry/tarsier. Groves, C. and Shekelle, M. (2010).

The Genera and Species of Tarsiidae. International Journal of Primatology 31(6): 1071–1082. doi:10.1007/s10764-010-9443-1.Myers, P.  Tarsiidae. Animal Diversity Web. Accessed August 21, 2015 at http://animaldiversity.org/accounts/Tarsiidae/.Ramsier, M. A., Cunningham, A. J., Moritz, G. L., Finneran, J. J., Williams, C. V., Ong, P. S., Gursky-Doyen, S. L., and Dominy, N. J. (2012).

Primate communication in the pure ultrasound. Biology Letters 8(4): 508–11. doi:10.1098/rsbl.2011.1149.Rosa, M. G., Pettigrew, J. D., and Cooper, H. M. (1996). Unusual pattern of retinogeniculate projections in the controversial primate Tarsius. Brain Behavior and Evolution 48(3): 121–129. doi:10.1159/000113191.