1. Scientists are in agreement that the tarsier lineage may have been in existence at least 40 million years ago during the Middle Eocene. This is based on a 40 million year old material from Fissures A & C at Shanghuang of Jiangsu Province, China. A fossil tarsier, Tarsius eocaenus, possesses a dentition similar to the teeth of modern Tarsius. Another fossil tarsier, Xanthorysis tabrumi, was also found in a Late Middle Eocene Heti formation, Yuanqu Basin, Shanxi Province, China. The next oldest fossil tarsiid is the Afrotarsius chatrathi from an early Oligocene sediments of Quarry M of the Jebel Qatrani Formation of the Fayum Province of Egypt. A. chatrathi exhibits a fused tibiofibula which indicates a mode of leaping locomotion the same as that of modern tarsiers. An early Miocene form was also found on northwestern Thailand, the Tarsius thailandicus (Simons 2003; Jablonski 2003).
2. Primatologists posit that Tarsius syrichta is perhaps the most recent among the modern tarsiers. Dagosto et al (2003:246) argue that given the geologic history of the Philippine Islands and the amount and placement of emergent land during the Cenozoic, dispersal of tarsiers to the Philippines is very unlikely to have occurred before the late Miocene, and may have been much later. In the Philippine context, islands inhabited by tarsiers today were located farther east and south of mainland Asia during the early Cenozoic, with no evidence of land bridges connecting these islands to the mainland. Dagosto et al (2003) believe that tarsiers originated from mainland China and later on dispersed overwater to Sulawesi and the Philippines. The Philippine tarsiers may have migrated from Borneo through the Sulu archipelago, arriving sometime in the late Miocene to mid-Pleistocene.
3. Tarsiers have been an enigma for scientists because they share characteristics with prosimians as well as with anthropoids. For example, when in estrus, females have red swollen vulvas like Old World Monkeys, give birth to one large infant, but on the other hand, they have multiple (four to six) nipples, similar to lemurs and lorises. After a six-month gestation, newborn infants can weigh up to 25-30 percent of the mother’s weight; males provide little paternal care, unlike other primate species that have large infants. Females “park” their infants on branches, while they forage nearby (Wright et al. 2003). Tarsiers can turn their head 180 degrees in both directions (Ankel-Simons 2000), they have very long legs, their tarsal bones are elongated (hence their name) and their tibia and fibula are fused (Wright et al. 2003).
4. Behavioral and ecological data on tarsiers in the wild is rather difficult to obtain due to their nocturnal activity, small size, lack of tapetum lucidum, fast locomotion, and social organization (Gursky and Nekaris 2003). The increasing use of radio telemetry has facilitated the collection of data on these nocturnal and “cryptic” prosimians (Gursky 1998a). According to Gursky and Nekaris (2003), there is a new group of scientists who think it is important to document species-level differences among nocturnal prosimians so that broader correlations concerning ecology and behavior can be discerned.
5. Although there is a debate concerning the number of tarsier species, most researchers agree that tarsiers are represented and formally recognized by five species: Tarsius bancanus, the Bornean tarsier; T. dianae, Dian’s tarsier; T. pumilus, the pygmy tarsier; T. spectrum, the spectral tarsier; and T. syrichta, the Philippine tarsier (Dagosto et al. 2003; Gursky 2002; Neri-Arboleda et al. 2002; Dixson 1998). These species are grouped into two distinct phenotypic groups: the Philippine-Western group, from the Philippines and Borneo; and the Eastern group, from Sulawesi (Brandon-Jones et al. 2004). Tarsiers have a limited geographical distribution in a few Southeast Asian islands (Neri-Arboleda et al. 2002). Tarsius bancanus is found in Borneo and some parts of Sumatra. T. dianae, T. pumilis, and T. spectrum are found in Sulawesi. Dian’s tarsier is restricted to Sulawesi’s central lowlands, the pygmy tarsier to the central part of the island, and the spectral tarsier to the northeastern part of the island (Neri-Arboleda et al. 2002). The Philippine tarsier is restricted to the Philippine islands of Bohol, Leyte, Samar, Maripipi, Biliran, Dinagat, Siargao, and Mindanao (Neri-Arboleda et al. 2002).
6. According to Neri-Arboleda et al. (2002), there are several field studies of T. bancanus, T. spectrum, and T. dianae, but very few of the behavior and ecology of T. pumilus and T. syrichta (see also Dagosto 1998; Dagosto et al. 2003; and Wright 2003b). There is not sufficient data to precisely determine the social organization of the Philippine tarsiers (Dagosto et al. 2001). Because they have not been well studied in the wild and there are limited published observations regarding their ecology and behavior, T. syrichta is currently classified as “data deficient” by the IUCN-Red List of Threatened Species (IUCN website 2004). Neri-Arboleda et al (2002) suggest that because of this lack of information on the Philippine tarsier, its status as “data deficient” should be maintained until further studies are done.
7. A nocturnal habit makes possible the exploitation of uniquely nocturnal food resources and avoidance of diurnal predators. Modern tarsiers lack a tapetum lucidum but have a well-developed fovea in the center of an all-rod retina, where visual acuity is concentrated and visual image is intensified because of the dense arrangement of visual receptor cells (Jablonski 2003). Their dentition, which is almost similar in structure to that of the fossil tarsiers, favors a strictly insectivorous or carnivorous diet. In the wild, tarsiers prefer large bodied coleopterans and arthropods. The molars of the tarsiers are suited to breaking the exoskeletons of insects and the skeletons of small vertebrates, efficiently consuming the fats, protein, and carbohydrate-rich tissues of their prey by digestion. Energy expenditure for the tarsiers is minimized by its low basal metabolic rates (which is 65% for the Philippine tarsiers) and low body temperature. Although this has not been studied yet, I also suspect that energy expenditure is minimized by daytime torpor, also observed among spectral tarsiers (Dagosto 2003; Gursky 2003).
Wright, P.C., Simons, E.L., and Gursky, S. 2003. Tarsiers: Past, Present and Future. New Brunswick, NJ: Rutgers University Press.
Neri-Arboleda, I., Stott, P. and Arboleda, N.P. 2002. Home Ranges, Spatial Movements and Habitat Associations of the Philippine Tarsier (Tarsius syrichta) in Corella, Bohol. J. Zool., London 257:387-402.
Gursky, S. 1998. Conservation Status of the Spectral Tarsier Tarsius spectrum: Population Density and Home Range Size. Folia Primatologica 69: 191-203.
Gursky, S. 2002. The behavioral ecology of the spectral tarsier, Tarsius spectrum. Evolutionary Anthropology 11: 226-234.
Gursky, S. and Nekaris, K.A.I. 2003. An introduction to mating, birthing, and rearing systems of nocturnal prosimians. Folia primatologica. 74: 241-245.
Beard, K. C. 1998. A new genus of Tarsiidae Mammalia: Primates from the middle eocene of Shanxi Province, China, with notes on the historical biogeography of tarsiers. Bulletin of Carnegie Museum of Natural History 34:260-277.
Tarsier-viewing in the Philippine Tarsier Sanctuary, Corella, Bohol