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 Aotus azarae azarae, which is cathemeral). 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.
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 spatial 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.
Until 1983, all night monkeys were placed into only one (A. lemurimus) or two species (A. lemurinus and A. azarae). Some authors still believe that there are only two or three true species, the remaining taxa being subspecies of these. An often used distinction is an even split of eight species between a northern gray-necked group (A. lemurinus, A. hershkovitzi, A. trivirgatus and A. vociferans) and a southern red-necked group (A. miconax, A. nancymaae, A. nigriceps and A. azarae). It has been argued that the taxa otherwise considered subspecies of A. lemurinus – brumbacki, griseimembra and zonalis – actually should be considered separate species, whereas it has been argued that A. hershkovitzi is a junior synonym of A. lemurinus. A new species from the gray-necked group was recently described as A. jorgehernandezi. As is the case with some other splits in this genus, an essential part of the argument for recognizing this new species was differences in the chromosomes. Chromosome evidence has also been used as an argument for merging "species", as was the case for considering infulatus a subspecies of A. azarae rather than a separate species. Fossil species have (correctly or incorrectly) been assigned to this genus, but only extant species are listed below.
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