The Rio Mayo titi monkey (Plecturocebus oenanthe) is a species of titi monkey, a type of New World monkey, endemic to Peru. The Rio Mayo titi, was thought previously to have a small range of origin in the Alto Mayo valley, but research has proven that the range extends southward and reaches the Huayamba River, as well as Bajo Mayo. It had been classified as vulnerable but due to major habitat loss and restricted living space, it is now classified as Critically Endangered. In October 2012, it was included in The World's 25 Most Endangered Primates list. An increase in deforestation is leading to the decrease in available living space for this titi monkey, forcing it to live in sympatry with another species of Callicebus. Yet in some areas, such drastic deforestation has resulted in extremely high population density. The Rio Mayo titi is better adapted to moderately populated areas, thus overpopulation negatively impacts the species. The forests the Rio Mayo titi lives in are being destroyed for agricultural purposes, leaving little forest for the monkeys. They were only seen a few times and featured in museums until 2003 when more research was done on them. In order for this species to survive, their forests need to be protected to avoid overpopulation. Different conservation groups are working to help P. oenanthe survive. Neotropical Primate Conservation, Proyecto Mono Tocón and Amazónicos para la Amazonia are working in the more southern areas to protect the monkey. The Rio Mayo titi is a fairly inconspicuous creature, making observation and research difficult to obtain. Therefore, the traditional use of transect observation to monitor the monkey's population, is less effective. Instead, other methods of calculating the titi monkey's density in certain areas have been taken, such as research into the species-specific calls endemic to a certain area. Location, habitat and activity Rio Mayo titis usually live in dry woodland areas with a lot of vines and plants. They are found in the Atlantic Coast of Brazil and in tropical areas of the Amazon, the upper Paraguay basin, in Boirio, and Santa Cruz. They are found in Peru in the northern part of the Department of San Martin, at altitudes of 750–950 meters and also in the foothills of the Andes. One P. oenanthe family was studied by scientist DeLuycker from October 2004 to September 2005. The studied family consisted of a mother and father, two