|A night monkey in Panama|
Poche, 1908 (1865)
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). The lifespan of a wild night monkey is unknown; however, they have a lifespan of 20 years in captivity. The total population of night monkeys is similarly unknown, as they live near the canopies of deciduous forests, making counting them very difficult. 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). Chromosome variability showed that there was more than one species in the genus and Hershkovitz (1983) used morphological and karyological evidence to propose nine species, one of which is now recognised as a  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.
Aotus lemurinus (gray-necked) group:
- Gray-bellied night monkey, Aotus lemurinus
- Panamanian night monkey, Aotus zonalis
- Gray-handed night monkey, Aotus griseimembra
- Hernández-Camacho's night monkey, Aotus jorgehernandezi
- Brumback's night monkey, Aotus brumbacki
- Three-striped night monkey, Aotus trivirgatus
- Spix's night monkey, Aotus vociferans
- Aotus azarae (red-necked) group:
Night monkeys have large brown eyes; the size improves their nocturnal vision, thus increasing their ability to be active at night. Their ears are rather difficult to see; this is why their genus name, Aotus (meaning "earless") was chosen. There is little data on the weights of wild night monkeys. From the figures that have been collected, it appears that males and females are similar in weight; the heaviest species is Azara's night monkey at around 1,254 grams (2.765 lb), and the lightest is Brumback's night monkey, which weighs between 455 and 875 grams (1.003 and 1.929 lb). The male is slightly taller than the female, measuring 346 and 341 millimetres (13.6 and 13.4 in), respectively.
Night monkeys can be found in Panama, Colombia, Ecuador, Peru, Brazil, Paraguay, Argentina, Bolivia, and Venezuela. The species that live at higher elevations tend to have thicker fur than the monkeys at sea level. The night monkey can live in forests undisturbed by humans (primary forest) as well as forests that are recovering from human logging efforts (secondary forest).
The name "night monkey" comes from the fact that 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). 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. Night monkeys live in family groups consisting of a mated pair and their immature offspring. Family groups defend territories by vocal calls and scent marking.
The night monkey is socially monogamous, and all night monkeys form pair bonds. 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. This is believed to have developed because it increases the survival of the infant and reduces the metabolic costs on the female. Adults will occasionally be evicted from the group by same-sex individuals, either kin or outsiders.
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- FactsheetAotusPrimate Info Net