Black-Headed Night Monkey, Aotus nigriceps
BLACK-HEADED NIGHT MONKEY
Aotus nigriceps
Geographic Distribution and Habitat
The black-headed night monkey (Auotus nigriceps), also known as the Peruvian night monkey, is a small, sturdy monkey native to South America. This adaptable little species is frequently found in the Amazonian and Madeiran forests that occur throughout Brazil, Peru, and Bolivia. Recent research has also found them in the forested river basins along Huallaga, Ucayali, Yavari, Purus, and Madre de Dios.
Habitat varies widely and is provided by both lowland rainforest as well as tropical forests. Within these habitats, they will utilize both the upper and lower canopy, and they are tolerant of seasonal flooding as well as temperature fluctuations within their habitat. These night monkeys also feed in both primary and secondary forests, making them incredibly important environmental engineers.
Size, Weight, and Lifespan
Black-headed night monkeys are monomorphic; this means that the males and females of the species are very similar in size and appearance. The average weight for males is 1.6 to 1.9 pounds (750-875 gms), with females averaging 1.6 pounds (750 gms). The length for both is between 9.4 to 14.5 inches (24-37cm), with the nonprehensile tail giving them an additional 12 to 15 inches (30.5-38.1 cm) of overall length.
Due to their nocturnal nature, their lifespan is not very well understood, but estimations place their average lifespan at 11 years.
Appearance
These little guys are adorably compact and are covered in a short, dense haircoat. The adults have a gray-black agouti pelage that ranges from the front legs to the upper back. A striking orange color covers their chest, belly, and the outside of their rear legs. Their long tails are a blend of gray and orange that makes up their primary color scheme.
The term black headed arises from the three black stripes that begin at their upper jaw, or maxilla, rise along their cheeks and between their eyes, and then converge at the top of their heads. Between these black stripes, they have white patches that arch over each eye and cover each tiny cheek. These white patches that frame each eye serve to enhance the large appearance of their already big eyes, giving them an endearingly surprised look.
Those big eyes serve an important purpose for this denizen of the jungle night. Night monkeys have no tapetum lucidem, that’s a very fancy name for the reflective tissue at the back of the eye that gives nocturnal animals their “eyeshine.” It’s also what is responsible for their enhanced night vision. As night monkeys lack this tissue, their overly large eyes serve to improve their ability to see at night.
The scientific name Aotus means “earless,” but they aren’t truly earless. Their ears are quite small and hidden in the dense fur of their round heads. Despite their small ears, they have exceptional hearing, which likely assists in locating insects.
Like all Latin American monkeys, night monkeys have a platyrrhine nose that is broad, flat, and has nostrils that open to the side. It’s a great addition to an already adorable face.
Diet
They are predominantly frugivorous and prefer the ripened fruit of the ficus trees (fig trees). They will also eat leaves, flowers, and moths. Recent studies have observed them eating the arthropods that they find in some flowers. Their dietary plasticity makes them highly adaptable to life in secondary forests.
Behavior and Lifestyle
Black-headed night monkeys are largely nocturnal, with the largest portion of their activity coming during crepuscular hours, that is the during twilight hours of sunrise and sunset. However, they have been noted to have short bursts of activity during daylight hours.
Their waking hours are likely spent foraging in the arboreal canopy and engaging in social behaviors. Grooming and playing are vital behaviors that strengthen group bonds.
During the day, they sleep in cavities found in trees or small nests of foliage within their territory. They are quite territorial and will defend their homes fiercely through vocalization and threat displays. Territories are defined by scent marking and glandular secretions.
Night monkeys are also called owl monkeys. This is because of their round, flat faces and their big, round eyes that resemble those of an owl.
Night monkeys are the only nocturnal monkey and often emit a low-frequency call that sounds like a hoot, which may also contribute to the term owl monkey.
Like many primates, these small monkeys live in a familial social group. The troops are small, generally consisting of only 6 members, and are comprised of a monogamous breeding pair and their offspring.
When offspring reach sexual maturity, they leave their natal group to establish their own territory and form a new pair bond. Occasionally, a group may experience mate displacement, in which an unfamiliar adult male or female enters the territory and challenges one member of the resident pair. If the newcomer is successful, they replace that individual and form a new breeding pair with the remaining partner, assuming responsibility for territorial defense and reproduction. In some cases, incoming males have been observed caring for offspring conceived by the previous pair.
Black-headed night monkeys are arboreal, that is, tree-dwelling, and quadrupedal (they travel on all fours). Their small size and sturdy musculature make them very agile, and they can leap easily from branch to branch in search of food or sleeping nests.
Like many primate species, night monkeys use hoots and whoops during vocal communication. Recent studies have suggested that night monkeys may be more reliant on vocal communication due to limited visibility at night when they’re most active. These studies found three distinct sounds common to the black-headed night monkey. They frequently use squeaks, ch-ch, and trills. The purpose of these individual sounds isn’t well understood yet.
Whoops are often used in combination with visual displays and scent marking. This sound is most frequently emitted during conflict with other troops. The visual displays that accompany whoops can include arched backs, erect fur, urinating, and defecating. All of these signals combined serve to reinforce boundaries and display aggression.
Hoots are emitted in communication amongst the troop to reinforce bonding or across distances when searching for mates.
They also communicate through scent and chemical signaling. Scent glands at the base of the tail produce a unique scent that can be used to mark routes to food or sleep sites. In addition to chemical signaling, night monkeys use urine washing. This means that they will rub urine on their hands and feet, which is then rubbed off on the trees and areas where they walk, leaving behind a trail that can be followed. This information can also determine the gender of the monkey leaving the trail.
Black-headed night monkeys are monogamous. This means that they will stay with one partner or mate for several years. If one mate in the pair dies or is displaced by violence, the remaining mate will accept a new partner with whom they will then stay.
Breeding usually occurs between August and February and can be determined by available food sources and other environmental factors. Females give birth to one infant per year. After the birth, males are often the primary caretakers of the infants. This care involves carrying the babies, protecting them, and teaching them. Fathers show the youngsters how to forage, how to find nest sites, and how to interact with troop members through play behavior. If the father dies, older members of the troop take on the care of the infant.
Weaning begins at about 8 weeks old, and the males will then begin to drive the young away from them, forcing them to become independent. At 2-5 years of age, they are considered to be sub-adults and will begin to leave their natal group to form their own family groups
As with many frugivorous species, black-headed night monkeys are essential environmental engineers. By eating fruits in one area and then traveling to another area where they defecate seeds, they are vital in the generation of new tree growth. Their consumption of insects that they find in flowers may help to keep the population of insects low enough to prevent damage to pollinating plants.
The International Union for Conservation of Nature lists the black-headed night monkey as Least Concern (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Recent studies have shown population declines in areas where the forest cover has been lost to deforestation and habitat degradation. In Brazil, agriculture and ranching are at the forefront of habitat loss. And in Peru, habitat loss can be attributed to infrastructure development for the ever-expanding human population.
Black-headed night monkey is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
There are extensive areas of protection that exist in Peru, Brazil, and Bolivia, but the level of protection varies throughout each area, and deforestation still occurs.
And, while it may be difficult to assess populations of a nocturnal species such as the black-headed night monkey, its presence in areas highly prone to deforestation should indicate a need for closer monitoring. Perhaps, with improved study, programs, and policies can be established to protect this small but mighty species.
- https://primate.wisc.edu/primate-info-net/pin-factsheet-owl-monkey/
- https://www.iucnredlist.org/species/41542/190455194
- Helenbrook WD., Wilkinson ML., Suarez JA. 2020. Habitat use, fruit consumption, and population density of the black-headed night monkey, Aotus nigriceps, in southeastern Peru. Acta Amazonica. 50 (1): 37-43.
- Helenbrook WD., Valdez JW. 2020. Species distribution modeling and conservation assessment of the black-headed night monkey (Aotus nigriceps) – A species of Least Concern that faces widespread anthropogenic threats. 1-26.
- Chaves dos Santos, J. V., de Sousa Lima, A. H., da Costa Tavares, A., Pommer Barbosa, R. A., Shanee, S., & Alvares Oliveira, M. (2022). Observations on the diet of Aotus nigriceps (Primates: Cebidae) in an urban and peri-urban area in Rondônia state, Brazil. Mammalogy Notes, 7(2), 270.
- Helenbrook WD., Linck NA,. Pardo MA., Suarez JA. 2019, Spatial variation in black-headed night monkey (Aotus nigriceps) vocalizations. bioRxiv
- https://www.tropicalconservationfund.org/nightmonkeys.html
Written by Jennifer Farris, January 2026