The Philippine tarsier measures only about 85 to 160 mm (3.35 to 6.30 in) in height, making it one of the smallest primates. The small size makes it difficult to spot. The mass for males is between 80–160 g (2.8–5.6 oz), usually lighter for females, somewhat heavier than other tarsiers such as the pygmy tarsier. The average adult is about the size of a human fist.
The female tarsier has multiple sets of breasts, but the only functional set is at the pectoralis. The other breasts are used as anchor points for the newborn tarsiers. The gestation period lasts 180 days, or 6 months, after which only one tarsier is born. The newborn tarsier is born with much fur and eyes open. Its body and head length is about 70 mm, and its tail is around 115 mm long.
Like all tarsiers, the Philippine tarsier's eyes are fixed in its skull; they cannot turn in their sockets. Instead, a special adaptation in the neck allows its round head to be rotated 180°. Their eyes are disproportionately large, having the largest eye-to-body size ratio of all mammals. These huge eyes provide this nocturnal animal with excellent night vision. In bright light, the tarsier's eyes can constrict until the pupil appears to be only a thin line. In darkness, the pupil can dilate and fill up almost the entire eye. The large membranous ears are mobile, appearing to be almost constantly moving, allowing the tarsier to hear any movement.
The Philippine tarsier has thin, rough fur which is colored gray to dark brown. The narrow tail, usually used for balance, is bald except for a tuft of hair at the end, and is about twice the body length. Its elongated "tarsus", or ankle bone, which gives the tarsier its name, allows it to jump at least 3 m from tree to tree. Its long digits are tipped with rounded pads that allow C. syrichta to cling easily to trees and to grip almost any surface. The thumb is not truly opposable, but the first toe is. All of the digits have flattened nails, except for the second and third toes, which have sharp claws specialized for grooming.
The dental formula is 2:1:3:31:1:3:3, with relatively small upper canines.
The Philippine tarsier, as its name suggests, is endemic to the Philippine archipelago. C. syrichta populations are generally found in the southeastern part of the archipelago. Established populations are present primarily on the islands of Bohol, Samar, Leyte and Mindanao. It has also been found on various isolated islands within its known range, such as Maripipi Island, Siargao Island, Basilan Island and Dinagat Island.
The Philippine tarsier's habitat is the second-growth, secondary forest, and primary forest from sea level to 700 m (2,300 ft). Its habitat also includes tropical rainforest with dense vegetation and trees that offer it protection such as tall grasses, bushes, and bamboo shoots. It prefers dense, low-level vegetation in secondary forests, with perching sites averaging 2 m above the ground.
Early studies showed that the Philippine tarsier has a home range of 1 to 2 hectares, but more recent research shows that home ranges averaged 6.45 ha for males and 2.45 ha for females , allowing for a density of 16 male and 41 female tarsiers per 100 ha.
While both male and female tarsiers are solitary animals, they cross each other's paths under the cover of nightfall as they hunt for prey. They travel up to 1.5 km across the forest and the optimal area is more than 6 ha.
Besides human hunters, feral cats banished from nearby communities are the species' main predators, though some large birds are known to prey on it, as well. Because of its nocturnal and arboreal habits, the Philippine tarsier is most likely to fall prey to owls, or to small carnivores which it can encounter in its canopy homes.
The Philippine tarsier is primarily insectivorous, its diet consists of insects, spiders, small crustaceans, and small vertebrates such as small lizards and birds. C. syrichta preys on live insects, particularly crickets and grasshoppers. Upon seizing its prey, the tarsier carries it to its mouth using both hands.
As predators, Philippine tarsiers may help to structure insect communities. To the extent that it is preyed upon by other animals, it may impact predator populations.
The Philippine tarsier is a shy nocturnal animal that leads a mostly hidden life. During the day, it sleeps in dark hollows close to the ground, near the trunks of trees and shrubs deep in the impenetrable bushes and forests. It becomes active only at night, and even then, with its keen sight and amazing ability to maneuver around trees, is well able to avoid humans.
It is arboreal, habitually clinging vertically to trees and capable of leaping from branch to branch.
The Philippine tarsier is solitary. However, it is found to have either monogamous or polygamous mating system.
The Philippine tarsier uses varied means of communication. Although less vocal than many primate species, it uses calls which are often associated with territorial maintenance and male-female spacing. Three different audible calls have been documented. One is its "loud call"—a piercing single note. The second sound is a soft, sweet, bird-like twill, a sound of contentment. When several tarsiers come together, the combined effect of this chirping is a locust-like sound.
These mammals can also vocalize in an ultrasound frequency range of 70 kHz and can pick up frequencies above 90 kHz. This form of vocal communication is used as a distress call made by infants when they are separated from their mothers. It is also the call made by males to their mates during mating season.
Tarsiers also communicate through a scent from the circumoral gland located around the mouth, which the female uses to mark her mate. The males mark their territory with their urine. Tarsiers perform tactile communication through social grooming, removing dead skin and parasites, a behaviour observed in females on adult males, as well as in females on their offspring.
The Philippine tarsier's pregnancy or gestation period lasts about 6 months. The female's estrous cycle lasts 25–28 days. Mating season begins in April to May. The males deposit a mating plug in the female's vagina after intercourse. The female gives birth to one offspring per gestation. The infant is born with hair and with its eyes open. The females carry their infants in their mouths. A newborn can already cling to branches and in less than a month after birth, it can start leaping. Newborns are breast-fed until 60 days after birth. After two years of age, the tarsier is able to take a mate.
The Philippine tarsier is related to the Horsfield's tarsier of Borneo and Sumatra and to several species of tarsier on Sulawesi and nearby islands in the genus Tarsius. Although all living tarsiers had been conventionally placed in the single genus Tarsius, Shekelle and Groves (2010) placed the distinctive Philippine tarsier in its own genus, Carlito.
The Philippine tarsier is related to other primates, including monkeys, lemurs, gorillas, and humans, but it occupies a small evolutionary branch between the strepsirrhine prosimians, and the haplorrhine simians. While it is a prosimian, it has some phylogenetic features that caused scientists to classify it as a haplorrhine and, therefore, more closely related to apes and monkeys than to the other prosimians.
The smallest primate is the Madame Berthe's mouse lemur (Microcebus berthae), at around one third the weight of this species. The superlative 'smallest monkey' often refers to the pygmy marmoset (Cebuella pygmaea), an animal with a larger body size. The Philippine tarsier is considered to be the mammal with the biggest eyes, 16 mm across, in proportion to its body size.
The Philippine tarsier was only introduced to Western biologists in the 18th century through the missionary J.G. Camel's description given to J. Petiver of an animal said to have come from the Philippines. Petiver published Camel's description in 1705 and named the animal Cercopithecus luzonis minimus which was the basis for Linnaeus' (1758) Simia syrichta and eventually Carlito syrichta, the current scientific name. Among the locals, the tarsier is known as mamag, mago, magau, maomag, malmag, and magatilok-iok.
Three subspecies are presently recognized: C. s. syrichta from Leyte and Samar, C. s. fraterculus from Bohol, and C. s. carbonarius from Mindanao. The IUCN taxonomic note lists two subspecies, but the non-nominate one is poorly defined at present, so the species is treated as a whole. Tarsius syrichta carbonarius and Tarsius s. fraterculus: Hill (1955) recognized these taxa as weakly defined subspecies. Niemitz (1984) found the differences to be insignificant based upon comparisons with museum specimens. Musser and Dagosto (1987) felt that the available museum specimens were insufficient to resolve the issue, but mentioned that Heaney felt that a single male tarsier from Dinagat might be distinct. Groves (2001) did not recognize any subspecies of C. syrichta, but Groves and Shekelle (2010) recognized the subspecies C. s. fraterculus, C. s. syrichta, and C. s. carbonarius when splitting the species of Tarsius into Carlito.
For the past 45 million years, tarsiers have inhabited rainforests around the world, but now they exist on only a few islands in the Philippines, Borneo, and Indonesia. In Bohol, the Philippine tarsier was a common sight in the southern part of the island until the 1960s. Since then, the number has dropped to around 700 on the island according to the Philippine Tarsier Foundation. Once protected by the humid rainforests and mist-shrouded hills, these primates struggle to survive as their home is cleared for crop growing.
Due to the quickly growing human population, which causes more and more forests to be converted to farmland, housing areas, and roads, the place where the Philippine tarsier can live its secluded life is disappearing. The dwindling of Philippine forests—the Philippine tarsier's natural forest habitat—has posed a grave and significant threat to the survival of the Philippine tarsier. Indiscriminate and illegal logging, cutting of trees for firewood, kaingin or slash and burn method of agriculture, and human urbanization have encroached on the habitats of the tarsier.
Paradoxically, indigenous superstition, coupled with relatively thick rainforest, particularly in Sarangani province, has apparently preserved this endangered species. Indigenous tribes leave the Philippine tarsiers in the wild because they fear that these animals could bring bad luck. One belief passed down from ancient times is that they are pets belonging to spirits dwelling in giant fig trees, known as balete trees. If people harm tarsiers, they need to apologize to the spirits of the forest, or they are thought to encounter sickness or hardship in life.
Tarsiers in the Philippines have been sought out as pets or sold for trade, despite their low survival rate outside of their natural habitat, where they feed on live insects. Displays by private people in Loboc, Bohol tend to diminish the lives of tarsiers. Further, some feel that the display of captive tarsiers might encourage tourists to acquire them illegally as pets.
Tarsiers do not do well in captivity. Life expectancy decreases by 2 to 12 years (if taken from the wild), as compared to the 24 years the tarsier can live to in the wild. The tarsier can develop sore eyes, which is an indication of a poor diet. Also, the lighting usually used in captivity can cause long-lasting damage to the eyes. Another danger of captivity is the creature's tendency to commit suicide. Because the tarsier is often shy and nervous, many activities associated with captivity (such as camera flashes, being touched, and being kept in an enclosure) stresses the animal. Such stress leads to the tarsier hitting its head against objects, thus killing it because of its thin skull.
No negative impact is known of the Philippine tarsier on humans, as long as it is in its native environment. However, when kept as pets, the species may spread worms and other parasites to its human owners.
In 1986, 1988, and 1990, the Philippine tarsier was assessed as endangered by the IUCN Conservation Monitoring Centre. On September 13, 1991, the Department of Environment and Natural Resources (DENR), per DENR Administrative Order Number 48 (DAO 48), also listed the Philippine tarsier as endangered.
In 1996, it was assessed as lower risk/conservation-dependent by Baillie and Groom-bridge. In 2000, the IUCN assessed the Philippine tarsier as data deficient, which means that inadequate information was available to make a direct or indirect assessment of its risks of extinction based on its distribution and/or population status.
The most recent IUCN red list assessment, in 2008, classified the Philippine tarsier as near threatened. This classification is based on an estimated significant decline over the last three generations (about 20 years), but less than 30%, due to habitat loss and because of poaching for the pet trade.
The Philippine tarsier is listed in Appendix II of CITES, and the U.S. ESA classifies it as threatened.
A Tarsier sanctuary is maintained in the town of Corella (Bohol). Run by the Philippine Tarsier Foundation, it has a visitor centre and habitat preserve/sanctuary of 7000 m2 in a natural forest.
Several laws have been passed to protect and conserve the Philippine tarsier. DENR Administrative Order No. 38, Series of 1991 (DAO No. 38) included the Philippine tarsier among the national protected wildlife species and proposed its listing under Appendix 1 of the Convention on International Trade in Endangered Species (CITES). Moreover, the IUCN/SSC Primate Specialist Group had given the species Conservation Priority Rating 4, which means that the species is highly vulnerable and threatened by habitat destruction and/or hunting.
Republic Act No. 7586, otherwise known as the National Integrated Protected Areas System (NIPAS) Act of 1991 mandates the establishment of appropriate sanctuaries to preserve and protect the Philippine tarsier.
Proclamation 1030 was signed by then President of the Philippines Fidel V. Ramos on June 23, 1997, declaring the Philippine tarsier a specially protected faunal species. 
Also legislation at other local levels include provincial ordinances and proclamations (Bohol Province), municipal ordinances (Corella), and Barangay ordinances (Canapnapan, etc.).
On July 30, 2001, Philippine President Gloria Macapagal-Arroyo signed Republic Act No. 9147, also known as the Wildlife Resources Conservation and Protection Act, that provided for the conservation and protection of wildlife resources and their habitats, including the Philippine tarsier, and its inclusion as a flagship species.
Conserving biological diversity involves the protection of natural or seminatural ecosystems, the restoration and rehabilitation of degraded lands, and ex situ conservation techniques. In situ conservation is the maintenance of plant and animal genetic material in their natural habitats. The aim of in situ conservation is to allow the population to maintain itself within its community and in the environment to which it is adapted so that it has the potential for continued evolution. Protected areas are among the most valuable in situ conservation tool and cost-effective means for preserving genes, species, and habitats and for maintaining various ecological processes of importance to humanity.
Two groups are involved in the conservation of the Philippine tarsier: Endangered Species International (ESI) and the Philippine Tarsier Foundation. ESI works in Mindanao Island where the conservation group created a tarsier sanctuary, planted endangered trees to reforest tarsier habitat, and conducts research and educational activities. In partnership with local groups and government, ESI established the tarsier trail including a view point on habitat. Interpretative boards about plants and animals found in the sanctuary are displayed.