also recognized an additional species, Collins' squirrel monkey Saimiri collinsi that had previously been considered to be within S. sciureus. Some more recent taxonomies also recognize Saimiri sciureus macrodon as a separate species Saimiri macrodon, but others recognize S. macrodon to be a synonym of Saimiri cassiquiarensis. Genus Saimiri S. sciureus group Central American squirrel monkey, Saimiri oerstedii Black-crowned Central American squirrel monkey, Saimiri oerstedii oerstedii Grey-crowned Central American squirrel monkey, Saimiri oerstedii citrinellus Guianan squirrel monkey, Saimiri sciureus Saimiri sciureus sciureus Saimiri sciureus albigena Ecuadorian squirrel monkey, Saimiri sciureus macrodon Humboldt's squirrel monkey, Saimiri cassiquiarensis Bare-eared squirrel monkey, Saimiri ustus Collins' squirrel monkey, Saimiri collinsi S. boliviensis group Black-capped squirrel monkey, Saimiri boliviensis Bolivian squirrel monkey, Saimiri boliviensis boliviensis Peruvian squirrel monkey, Saimiri boliviensis peruviensis Black squirrel monkey, Saimiri vanzolinii Fossil species †Saimiri annectens, Honda Group, Kay and Meldrum 1997 †Saimiri fieldsi, Honda Group, Stirton 1951 Evolution The crown group of the extant squirrel monkeys appears to have diverged around 1.5 million years ago. Diversification of squirrel monkey species appears to have occurred during the Pleistocene Epoch, likely due to climatic changes associated with interglacial periods in South America at the time. The origin of squirrel monkeys in Central America is unclear, but a possible hypothesis is human transport. More genetic work needs to be done on the subject to reveal a true answer.S. boliviensis appears to be the first diverging species in the group. S. oerstedii and S. s. sciureus, are sister species. S. s. macrodon is the sister species to the S. oerstedii / S. s. sciureus clade. Description A squirrel monkey's fur is short and close, coloured black at the shoulders and yellowish orange on its back and extremities. The upper parts of their heads are hairy. This black-and-white face gives them the name "death's head monkey" in several Germanic languages (e.g., German , Swedish , Dutch ) and Slovenian (). Squirrel monkeys grow from long, plus a tail. Male squirrel monkeys weigh . Females weigh . Both males and females are equipped with long and hairy tails, flat nails, and pointed claws. Female squirrel monkeys have pseudo-penises, which they use to display dominance over smaller monkeys, in much the same way that the male squirrel monkeys display their dominance. Behaviour, ecology, and physiology Like most of their New World monkey relatives, squirrel monkeys are diurnal and arboreal. Unlike other New World monkeys, their tail is