The San Martín titi monkey, whose scientific name is Plecturocebus oenanthe, belongs to the a highly most diverse group of primates with almost 40 described species, distributed in the forests of Argentina, Brazil, Bolivia, Peru, Ecuador, Colombia and Venezuela.
Previously it was grouped in the genus Callicebus that included all known species of titi monkeys. However, recent molecular studies have separated that genus into three. The new genus Cheracebus is composed of all titi monkeys that were originally know as the “widow monkeys”, a group of blackish titi monkeys with a white throat, distributed in the region where the borders of Peru, Ecuador, Colombia and Brazil join each other. The original genus-name Callicebus was used for the larger titi monkeys that live in the Atlantic rainforest of Brazil, isolated from all other titi monkey species. All other species, such as the Amazonian species (originally belonging to the moloch and the cupreus groups), and the brownish-grey species (originally belonging to the donacophilus group) are now known under the genus name Plecturocebus. The three genera show differences in their genetics, morphology and have different vocalizations. Plecturocebus oenanthe is considered to be closely related to the donacophilus titi monkeys of Bolivia, but genetic studies are needed to confirm is affiliation.
Although it was originally thought that the San Martin titi monkey as confined to the Alto Mayo valley, the studies from Proyecto Mono Tocón showed that its distribution range is much larger. The species is restricted in the western and northern part of its range by high mountain ranges; in the south by mountains and the Huayabamba River, and in the east (more or less) by the Huallaga river.
Unfortunately, this whole region is much appreciated by humans for agriculture and most of the original habitat has been destroyed, challenging the future of this and many other animal species.