The black-headed night monkey (Aotus nigriceps) is found in Peru, Bolivia and Brazil. Night monkeys are also known as owl monkeys, and as members of the genus Aotus, they are the world’s only nocturnal monkeys. It is not just their nighttime habits that have given night monkeys the additional name owl monkey: as darkness falls in the forests of South America, they can be heard calling to each other with a three-part hoot not dissimilar to the hooting of owls.
Among the family of New World night monkeys, the black-headed night monkey’s facial pattern, composed of broad black stripes and white areas, makes it easy to distinguish. The coloring of the rest of its fur, ranging from dark gray to brown with whitish to orange underparts, is less readily distinguishable from that of other South American night monkey species.
While night monkeys have developed a range of adaptations to facilitate their nocturnal existence, such as a low metabolic rate which enables them to survive with less energy consumption, the most distinctive feature of their appearance is the night monkey’s enormous orbit, or eye socket, which is the largest of any simian species. Their enormous brown eyes blink so quickly that to the human eye they appear to stare endlessly. While night monkeys are color blind, they respond very quickly to rapid movements, like those made by insects, and uniquely among anthropoid species they are able to see at extremely low light levels.
Night monkeys are small creatures, and across their different species typically they weigh just one kilogram or less (around 35 ounces). They are monogamous and live in small groups composed of up to four individuals. During the daytime, night monkeys will retire to sleep sites, using tree hollows, lianas, or epiphytes as refuges.
The night monkey’s diet is essentially frugivorous, although they will also supplement their diet by feeding on foliage and insects. In common with other small monkey species, night monkeys tend to feed at small trees that are evenly spaced throughout the forest and produce fruit on a regular basis. One particularly important advantage of their nocturnal existence is the opportunity it gives them to feed at large trees at night, when larger and more dominant New World monkey species are sleeping.
Within their natural territorial range, night monkeys can be found in different types of forest habitats and do not appear to have a preference for specific canopy levels. When moving through the forest canopy, they use all four limbs. While their tails are not prehensile, they are also skilled at leaping from branch to branch, and from tree to tree. Night monkeys have been recorded leaping up to four meters (13 feet) between trees.
At our own Tambopata Ecolodge, situated in the remote forests that form the buffer zone of the government protected Tambopata National Reserve in southeastern Peru, a troop of night monkeys can often be seen moving through the trees above our lodge facilities, particularly around the dining room, bar area, and boot building. They have occupied the area of forest we are working to conserve since at least the late 1980s, before we began construction work on our Ecolodge facilities. Our guests often spot night monkeys during their evening excursions to observe caimans, at the jetty before boarding one of our boats to explore nocturnal Amazon forest life on the Tambopata River that flows past our lodge.