Aotus nancymaae

Geographic Distribution and Habitat

The soulful eyes of the Nancy Ma’s night monkey (Aotus nancymaae), also called the Ma’s Night Monkey or Peruvian red-necked owl monkeys, peer from the hollow of trees in the Amazonian neotropics of South America. They make their homes in rainforests that range from Northern Peru to Western Brazil. Their exact natural distribution in Colombia is uncertain as endemic populations reside in some regions and introduced populations reside in others.

While once considered members of the northern night monkey species (Aotus trivirgatus), the Nancy Ma’s night monkey was identified as distinct in 1983 based on chromosome research. They are named after the pathology scientist who mapped their genetics in the 1970s and ‘80s, Dr. Nancy Shui-Fong Ma.

These monkeys live in the lowland tropical rainforest and prefer areas that are seasonally flooded. Seasonally flooded areas have more tree hollows and softer trees and brush, which they use to make their nests.

Nancy Ma's night monkey range, IUCN 2020

Size, Weight, and Lifespan

Similar to other night monkeys, this species averages a little over a foot (31.5 cm) in length—and a little over two feet (63.5 cm) if you include the tail.

They weigh an average of 1.75 pounds (.79kg). Males and females are similar in terms of size and weight.

The Nancy Ma’s night monkey’s lifespan in the wild is not known, but other night monkeys are reported to live between 12–20 years.

What Does It Mean?

Active during daylight hours.

An animal that feeds on plants.

Active at night.

Visit the Glossary for more definitions


Night monkeys are often also called owl monkeys. One look at them and it is easy to see why—they definitely have an owl-like appearance. While a Nancy Ma’s night monkey is certainly a contender for the “cutest” of Latin American primates, her large, soulful eyes have the beauty of poetry. But no need to write a sonnet to the windows of her soul; her eyes are all about function. They are what allow her to be one of the rare nocturnal primates.

Her face is defined with a heart-shape of white fur around her eyes and mouth, outlined in dark brown or black. Reddish-orange fur complements her soft light brown and gray coat. Her red fur identifies her to humans as one of the “red-necked” night monkeys. Like other night monkeys, her tail is black and hangs straight down and is non-prehensile—meaning it is not used to grasp things. Her hands, however, are excellent at grasping. Her fingers are long, with wide pads on the fingertips. A special grooming claw occupies the fourth digit of each foot.


Fruit and flowers foraged from the top of the forest canopy make up the bulk of this night monkey’s diet. Even though she is mostly a frugivore (preferring fruits), she may also eat insects from time to time. Her hand-mobility helps her to be an expert at catching insects on branches. She has been seen holding an insect in place with one hand and picking it apart to eat with the other.

As a nocturnal animal, she benefits from her nighttime foraging hours. She does not have to compete with the diurnal herbivores when she is finding food.

Behavior and Lifestyle

Nancy Ma’s night monkeys, like all night monkeys, differ from other Latin American monkeys in being monochromats—meaning they are not able to see color. They see the world in shades of black and white. But this is no deterrent to their excellent night vision. They have keen visual perception and a quick visual response rate, which helps them to catch insects and move gracefully through the dark forest. They also have a strong sense of smell, which helps guide them to fruits and flowers without the need for perceiving color. 

They are arboreal, meaning they spend most of their lives in the trees and have only been observed moving on the ground in the case of an emergency. They depend on the upper canopy of the trees for their food sources. But while they tend to move up for food, they like to move down to sleep in the lower canopy or understory, where there are preferable places for nests. They are most active at twilight and dawn (making them crepuscular), but daytime is sleepy time.

As day-sleepers, they are very particular about their shelter. Having a good shelter mitigates the risk of diurnal predators. They prefer tree hollows, or branches and shrubs that have a built-in depression or can be moved to create a hollow for them. Their shelter needs to be big enough for their entire group to sleep together. It also needs to protect them from predators by hiding them from view and provide multiple access points to escape in case they are found.

Daily Life and Group Dynamics 

These social monkeys spend their lives in small family groups of two to five individuals. The groups consist of a mated pair and their children. They have territorial boundaries, particularly around their nesting sites. Groups will defend their territory against other night monkeys but may share tree spaces for food foraging.

Near their nesting sites, they will alert any other night monkeys to stay away from “their” side of the forest. They do this with scent-marking and vocal calls. Often a younger member of the group acts like a sentry, keeping watch and making warning vocal calls to any potential trespassers. They can get aggressive with other night monkeys who get close to their nests and give chase and even attack.


These night monkeys make plenty of night music, as acoustics are their primary form of communication. As they do their nightly foraging, choruses of vocal calls let a family group know exactly where each of them is at all times. Shrill whistle-type calls alert a family group of any potential intruders. A raise in pitch indicates a predator. A half hour of squeaks and trills that say “this is our place” tell a competitive group not to get closer than 25 feet.

If a Nancy Ma’s night monkey is on a solitary quest near another group’s territory, they may squeak in a high pitched series which is thought to be a type of call for a mate. Maybe this is their version of what we human primates call crooning?

In addition to vocal calls, smell is an extremely important sense for them to relay information. As with other night monkeys, scent-marking may be used to communicate everything from physical territories to mating partners.

Reproduction and Family

Romantics, look no further for inspiration. When a Nancy Ma’s night monkey mates, they mate for life and become the pair-bond that begins a new group. Thus, the mother and father of this species become the core of a family group that will include their children, until their children reach adulthood (up to 2 years) and leave to find their own lifelong mate. A mother-father pair may have as many as three children in a group with them.

The bonded pair will mate all year long, but have only one or maybe two offspring per year. Gestation is 133 days (a little over 4 months). For the first week after birth, the mother and father share the role of caring for their baby.

After the first week, however, the father plays the major role in childrearing, with nursing being the only caregiving job the mother accepts. Babies nurse for 1 to 3 weeks. After the first week, mothers will bite their baby after they are done nursing until they return to their father! The father then does all the carrying of the baby until they can move about independently. The father also provides protection, grooming, nurturing, and any additional feeding after the baby is weaned. So, this monkey is not just a good inspiration for Valentine’s Day, but also a great choice as the image for your next Father’s Day card.

Photo courtesy of ©Seig Kopinitz. Used with permission.
Ecological Role​

​The health of the rainforest can be seen in the health of her primates, and this species is no exception. These monkeys are vital for dispersing seeds that help the forest grow. Their nocturnal nature gives them a unique position as a primate in their ecosystem, and they have a low impact as far as food consumption and complement rather than compete with diurnal species.

Conservation Status and Threats

Habitat loss due to increasing deforestation is the greatest threat to this species, and all rainforest species. The International Union for the Conservation of Nature classifies Nancy Ma’s night monkeys as Vulnerable (IUCN, 2021), with populations decreasing. Agriculture and logging continue to drive rapid rainforest destruction. In Peru, slash and burn of rainforests for the conversion to livestock pastures, rice fields, and, more recently, oil palm plantations have expanded. In Brazil, the sanctioned burning of the rainforests for more cattle pastures and soybean production continues. The IUCN (2021) notes that deforestation is officially underestimated so the deforestation rates are actually higher than reported. 

Populations are further depleted by human trafficking of these family-bonded monkeys. Trapped, kidnapped, and separated from each other, they are both legally and illegally bought and sold. The IUCN reports that Nancy Ma’s monkeys have been “regularly” found being trafficked by the illegal exotic pet trade.

As could be suspected of a species named for a geneticist, these night monkeys are in great demand for biomedical research. The U.S. leads in legal documented trade. Because of the unique evolution of the night monkey, they are a preferred species for biomedical research. Nancy Ma’s night monkeys in particular are noted as being the “ideal” for malarial research due to their chromosomal resistance to the disease. The IUCN reports that in the 1970s biomedical research drove “continuous extraction” of wild populations of night monkeys, with this species being in the highest demand. The post-research release of some of these monkeys back into the wild in Colombia during that time period has had problematic implications—distribution into new areas and encroaching on the native populations.

Their use in research continues today. Researchers note that capturing these monkeys in the wild legally has led to loss of enforcement and protection for the night monkeys and has eased illegal trade as well. This is a serious problem in addressing their conservation in the wild.

Conservation Efforts

There are protected places for Nancy Ma’s night monkeys, as well as trade controls and legislation. There are researchers asking for further protections and smarter protections: better regulations of biomedical facilities would be a more logical and effective way of controlling the illegal capture of these animals for biomedical research than dependence on problematic wildlife border policing.

As with all species of the rainforest, our human efforts can make a difference to their future. Better understanding of how our choices are impacting habitat loss and changing our behaviors, raising our voices, and demanding that we as a species do better are all key to protecting the futures of these special night monkeys.

  • Beolons, B, Watkins, M., Grayson, M, The Eponym Dictionary of Mammals, p.288
  • Maldonado, A., Guzmàn-Caro, D., Shanee, S., Defler, T.R. & Roncancio, N. 2020. Aotus nancymaae. The IUCN Red List of Threatened Species 2020: e.T41540A115578713. Downloaded on 21 September 2021.
  •, M S, Shanee, S, Shanee, , Bannister, F B, Cervera, L, Donati, G, Huck, M, Jerusalinsky, L, Juarez, C P, Maldonado, A M, Martinez Mollinedo, J, Méndez-Carvajal, P G, Molina Argandoña, M A, Mollo Vino, A D, Nekaris, K A I et al. (2017) Disappearing in the night: an overview on trade and legislation of night monkeys in South and Central America. Folia Primatologica, 87 (5). pp. 332-348. ISSN 0015-5713 

Written by Laura Lee Bahr, September 2021