The night monkeys, also known as the owl monkeys or douroucoulis, are the members of the genus Aotus of New World monkeys (monotypic in family Aotidae). They are widely distributed in the forests of Central and South America, from Panama south to Paraguay and northern Argentina. The species that live at higher elevations tend to have thicker fur than the monkeys at sea level. The genus name means "earless"; they have ears, of course, but the external ears are tiny and hard to see. Night monkeys have big brown eyes and therefore have increased ability to be active at night. They are called night monkeys because all species are active at night and are in fact the only truly nocturnal monkeys. Night monkeys constitute one of the few monkey species that are affected by the often deadly human malaria protozoan Plasmodium falciparum, making them useful as non-human primate experimental models in malaria research.