Tarsier – Fossil Records, Anatomy, Behavior and Physiology

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Philippine tarsier (Carlito syrichta), one of the smallest primates.

Tarsiers are haplorrhine primates of the family Tarsiidae, which is itself the lone extant family within the infraorder Tarsiiformes. Although the group was once more widespread, all of its species living today are found in the islands of Southeast Asia, specifically the Philippines, Malaysia, Indonesia, and Brunei. They are found primarily in forested habitats, especially forests that have liana, since the vine gives tarsiers vertical support when climbing trees.

Fossil Record

Fossils of tarsiiform primates are found in Asia, Europe, and North America, with disputed fossils from Africa, but extant tarsiers are restricted to several Southeast Asian islands in Indonesia, Philippines, and Malaysia. The fossil record indicates that their dentition has not changed much, except in size, in the past 45 million years.

Within the family Tarsiidae, there are two extinct genera, Xanthorhysis and Afrotarsius. However, the placement of Afrotarsius is not certain, and it is sometimes listed in its own family, Afrotarsiidae, within the infraorder Tarsiiformes, or considered an anthropoid primate.

The phylogenetic position of extant tarsiers within the order Primates has been debated for much of the 20th 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.

Anatomy and physiology

Tarsiers are small animals with enormous eyes; each eyeball is approximately 16 millimetres (0.63 in) in diameter and is as large as, or in some cases larger than, 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 a strong auditory sense, and their auditory cortex is distinct. Tarsiers also have long hind limbs, owing mostly to the 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 soft, velvety fur, which is generally buff, beige, or ochre in color.

Unlike many nocturnal vertebrates, tarsiers lack a light-reflecting layer (tapetum lucidum) of the retina and have a fovea.

The tarsier’s brain is different from that of 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.

Behavior

Tarsiers are the only extant entirely carnivorous primates: they are primarily insectivorous, and catch insects by jumping at them. Their favorite prey are arthropods like beetles, spiders, cockroaches, grasshoppers, and walking sticks. 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 and are sensitive to bright lights, loud noises, and physical contact. They have been reported to behave suicidally when stressed or kept in captivity.

Due to their small size, tarsiers are prey to snakes, owls, lizards, and cats. When a predator is present the tarsiers surround the threat vocalizing and attacking it. While tarsier groups only contain one male, when confronting a threat other groups will join, meaning there are multiple alpha male tarsiers attacking.

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