Sexual swelling is exhibited by this species which is rare for single male multi female groups of Colobinae monkeys. Classification Many researchers would like to reclassify this species into the Nasalis genus but phylogenic analyses suggest that these two groups should remain in their separate sister taxa. The Critically Endangered status of this primate gives even more reason for it to remain under the genus Simias since the grouping of these genera could affect conservation efforts. Diet It is an omnivore with a diet consisting of leaves, flowers, fruits and occasionally insects. The availability of these foods in this area are abundant especially in unripe and ripe fruits. Leaves are the most abundant in their diet followed by fruits. As with all other Colobinae primates, this species is diurnal. Distribution and social systems This primate is endemic to the Mentawai Islands, there are two subspecies, Simias concolor concolor also known as simasepsep which resides on the southern islands of Sipora, North Pagai, and South Pagai. The second subspecies is Simias concolor siberut also known as simakobou which lives only on the island of Siberut. It is a diurnal and arboreal rain forest dweller, often spending spending most of its time in the canopy but occasionally comes to ground. The activity budget of this primate is mostly resting and feeding with a small fraction of time devoted towards travel and social behavior. Some studies in the Siberut area show relatively low group sizes (mean group size 2.6). Other parts in Siberut show relatively high group sizes (mean group size 8.7). The reason for different group sizes is not factually known, however many researchers suggest that it could be due to anthropogenic factors such as hunting and deforestation. The pig-tailed langur has single-male, multi-female groups. The mating system of this species follows a polygynous structure although it was originally thought to be monogamous. Endangerment status and threats to conservation This species is classified as Critically endangered by the International Union for Conservation of Nature. It was classified as Endangered (IUCN status) up until 2008 were the endangerment status was changed to Critically Endangered. It is among The World's 25 Most Endangered Primates due to the many anthropogenic pressures it faces on the islands which include, destruction of forest for commercial use, hunting and illegal pet trade. Population size estimates have been conducted across many years with varying islands being chosen for research