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. (An exception is the subspecies A. a. azarae, which is cathemeral, active during both daylight and nighttime hours.) Both male and female Night Monkeys weigh almost the same amount. For example, in one of these Night Monkeys, A. azarae, the male weighs 2.76 pounds while the female weighs 2.75 pounds.
Until 1983, all night monkeys were placed into only a few species. Some authors still believe that there are only two or three true species, the remaining types being subspecies of these. The most widely agreed distinction is an even split of the eight species between a grey-necked group (Aotus lemurinus, A. hershkovitzi, A. trivirgatus and A. vociferans) and a red-necked group (A. miconax, A. nancymaae, A. nigriceps and A. azarae); authors recognising only two species refer to these as A. trivirgatus and A. azarae respectively.
Night monkeys make a notably wide variety of vocal sounds, with up to eight categories of distinct calls (gruff grunts, resonant grunts, screams, low trills, moans, gulps, sneeze grunts and hoots), and a frequency range of 190-1,950 Hz. Unusual among the New World monkeys, they are monochromats, that is, they have no colour vision, presumably because it is of no advantage given their nocturnal habits. They have a better spacial resolution at low light levels than other primates which contributes to their ability to capture insects and move at night.
All night monkeys form pair bonds, and live in family groups of the mated pair with their immature offspring. Family groups defend territories by vocal calls and scent marking. Only one infant is born each year. The male is the primary caregiver, and the mother only carries the infant for the first week or so of its life.
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.