Night monkey

Night monkeys, also known as owl monkeys or douroucoulis, belong to the genus *Aotus*. They are members of the family Aotidae. Most importantly, they are the only truly nocturnal monkeys in the world. This means they are active during the night and sleep during the day.

While some populations, like Azara's night monkey, show irregular activity during the day, most stay strictly nocturnal. They live in diverse habitats across Panama and much of South America. These include primary forests, secondary forests, tropical rainforests, and high-altitude cloud forests.

To survive in the dark, these monkeys have developed specialized physical traits. Their genus name, *Aotus*, actually means "earless" because their ears are mostly hidden from view. They possess large brown eyes that significantly improve their nocturnal vision. Unlike many other nocturnal animals, they lack a tapetum lucidum made of riboflavin crystals. Instead, some sources suggest they have a reflective layer composed of collagen fibrils. They also have monochromatic vision, meaning they see in black and white. This lack of color vision is thought to be an advantage for detecting visual cues in low light.

The taxonomy of night monkeys has changed significantly over time. Until 1983, scientists classified them into only one or two species. However, researcher Hershkovitz used morphological and karyological evidence to propose nine different species. Karyological evidence refers to the study of chromosomes, which are the structures inside cells. This evidence showed that there was much more variety than previously thought. Today, the genus is generally split into two main groups. The northern group is the gray-necked group, which includes species like *Aotus lemurinus*. The southern group is the red-necked group, which includes species like *Aotus azarae*.

These two groups are often separated by geography. Gray-necked night monkeys are typically found north of the Amazon River. In contrast, red-necked night monkeys are localized south of the Amazon River. The distribution of these species spans many countries, including Brazil, Peru, Colombia, and Argentina. Some species, like the black-headed night monkey, prefer cloud forests. These are areas with high moisture and consistent low clouds. Other species, like the Peruvian night monkey, live at higher elevations in the Andes.

Night monkeys have specific dietary needs and social structures. They are primarily frugivorous, which means they mainly eat fruit. They use their sense of smell to find food in the dark. Some species, such as *A. azarae*, also eat leaves and insects. When catching insects, they use their palms to flatten the prey against a branch. Socially, they are monogamous and live in small family groups. A group usually consists of one mated pair and their immature offspring. Interestingly, the male is the primary caregiver. He carries the infant to reduce the metabolic costs on the mother.

During the day, these monkeys must find safe places to rest. They use several types of tree nests to protect themselves from heat, sunlight, and heavy rain. They might sleep in holes in tree trunks or in concave sections of branches. They also use dense areas of vines, climbers, or thick foliage. These nests are usually found at low to mid-level vegetation rather than high in the canopy. To protect their territory, they use scent marking and a wide variety of vocalizations. They can make up to eight distinct types of calls, including grunts, screams, and hoots.

Conservation is a major concern for many *Aotus* species. They face threats from habitat loss, the pet trade, and hunting for bushmeat. They are also affected by biomedical research. Because they are one of the few monkey species affected by the malaria parasite *Plasmodium falciparum*, they are used in malaria research. The International Union for Conservation of Nature (IUCN) classifies their status differently. The Peruvian night monkey is listed as Endangered. Other species are classified as Vulnerable, Least-concern, or are data deficient.