The pig-tailed langur (Simias concolor), monotypic in genus Simias, is a large Old World monkey, endemic to several small islands off the coast of Sumatra in Indonesia. Its face is black, its fur is black-brown and it has a relatively short tail. It is a diurnal species, feeding in the rainforest canopy on leaves, and to a lesser extent, fruit and berries. Little is known of its natural history, but it is heavily hunted, its populations have been declining rapidly and the International Union for Conservation of Nature has assessed its conservation status as being "critically endangered". It has been included on a list of the World's 25 Most Endangered Primates. As an arboreal primate the pig-tailed langur spends most of its time in the canopy devoting the majority of its activity budget towards resting and feeding. The structure of groups is single-male, multi-female. Group sizes range from around 3 to 9 individuals depending on locations and hunting pressures. There are two subspecies of this primate, Simias concolor concolor which lives in the southern Mentawai islands, and Simias concolor siberu which lives exclusively in the northern island of Siberut. Sexual dimorphism and sexual swelling is exhibited by this species. Population estimates from the 1980s compared to data from the 2000s show a decrease in population size. The northern island of Siberut shows a greater abundance of this species in comparison to the southern Mentawai islands. This can largely be attributed to the difference in conservation efforts as much of northern Siberut has been declared a national park whereas the southern islands have been historically devastated by commercial logging. This species is subject to hunting by local inhabitants. The reduction in forestry cover has allowed for greater levels of human activity and settlement which facilitates hunting, especially in the southern islands since most areas are unprotected. Description The pig-tailed langur is a large and rather heavily built Old World monkey, adapted to climbing with its long arms. Its fur is black-brown, and its hairless face is also black. It is the only monkey in the subfamily Colobinae to have a relatively short tail; the tail is only slightly furred and is only long. The short nose is pointed upward. The pig-tailed langur reaches a full grown length of approximately and a weight of . Sexual dimorphism is exhibited by this species with adult males being larger and heavier than adult females.