BLACK-HEADED NIGHT MONKEY

Aotus nigriceps

Geographic Distribution and Habitat
Black-headed night monkeys are native to neotropical South America. They are found in areas of the Amazonian and Madeiran forests in Peru, Brazil, and Bolivia. They are also found in several forest areas near river basins such as Huallaga, Ucayali, Yavari, Purus, and Madre de Dios.

Black-headed night monkeys prefer lowland and hillside tropical forests. They dwell high in the trees often hiding behind large leaves and a thick canopy cover.

Black-headed night monkey range, IUCN 2020

Size, Weight, and Lifespan
The weight of male black-headed night monkeys averages 1.6–1.9 pounds (750–875 g). The average weight of females is about 1.6 pounds (750 g). Head-to-body length in both males and females averages 9.4–14.5 inches (24–37 cm). Their non-prehensile tails can measure between 12 and 15 inches (30.5–38.1 cm).

Not a lot is understood about the longevity of black-headed night monkeys in the wild, but they are estimated to live up to 11 years. In captive populations they have lived 15–20 years.

Appearance
Black-headed night monkeys are about the same size as squirrels. Their small but sturdy bodies are covered in short and thick fur. Adults have a dark gray and black agouti pelage on their upper back and forearms. The lower back and outer rear legs are tinted with orange-tan fur. Orange fur is also present on the sides of the neck, lower neck, and belly. Their tails have orange and gray fur throughout.

The name “black-headed” comes from the three conspicuous black stripes on their forehead. Two of these stripes are present on the side of the face to the maxilla (the upper jawbone). The third and central stripe extends from the forehead to the bridge of the nose. A small patch of white extends over each eye, on the cheeks, and under the chin.

Hidden behind their dense fur are tiny ears that just barely peek out of either side of their head. In fact, the scientific name Aotus means “earless.” However, do not let their ear size fool you, because their hearing is superb—they can pick up the sounds of even the slightest movements.

The eyes of the black-headed night monkey may look disproportionately large to the size of their head. The size of their eyes improves their nocturnal vision, which increases their ability to be active at night. Unlike many nocturnal animals, the black-headed night monkey lacks a tapetum lucidem in the eye, which is a reflective layer of tissue behind the retina. This gives nocturnal animals the typical “eyeshine,” like a cat. In addition, their vision is monochromatic, in which they only see in one color, typically shades of gray.

Their hind limbs are slightly longer than their forelimbs. Mainly quadrupeds, their strong hind limbs make them adept leapers.

In addition, black-headed night monkeys have long slender digits with flat nails and thick fingertip pads. The second digit on each foot has a long curved nail, known as a toilet claw, for grooming. Their long, nimble fingers enable them to grasp onto tree trunks and branches while foraging.

What Does It Mean?

Lowland forest:
An equatorial evergreen rainforest, commonly known as a tropical rainforest, which receives high rainfall (80 in; 2 m) throughout the year.

Monogamous:

Having only one sexual partner. 

​Toilet claw:

A specialized grooming claw common to prosimians and certain other primates.

Visit the Glossary for more definitions

Diet
Fruits, specifically ripened fruits from ficus trees (fig trees), are black-headed night monkeys’ favorite foods. They also enjoy eating a variety of other fruits, making them frugivores. When fruit is unavailable they feed on leaves, flowers, and moths.

Behavior and Lifestyle
Black-headed night monkeys are primarily nocturnal, though they have been sited awake and active during daylight hours for short periods of time. They live in small family groups consisting of a pair and their immature offspring. They are very territorial and defend their home range with vocalizations and aggressive behaviors. Males usually attack other males and females attack other females when another group comes within their range. These confrontations can last up to 30 minutes and typically end when one group leaves.

When a male or female enters a new territory and fights with a resident group member, a technique called “mate displacement” may occur. If the resident male or female loses, the new male or female takes over the rival’s territory and mate. A male who has successfully intruded and taken over may care for the children of the previous pair.

Home ranges are typically small and average about 211 feet (64.3 m). Depending on the amount of moonlight, as well as favorable environmental conditions, their nightly path length varies between 1,115 and 3,363 feet (339–1,025 m).

Fun Facts

The large size of the black-headed night monkey’s eyes makes up for the lack of a reflective eye layer found in many nocturnal mammals for night vision.

Night monkeys are also called owl monkeys because of their round, flat face and eyes that resemble those of an owl.

Main predators of night monkeys are wild cats, large snakes, hawks, and eagles.

Daily Life and Group Dynamics 
As arboreal and quadrupedal monkeys, they are very agile when navigating the forest canopy throughout the night. When needed they can leap remarkable distances between branches.

About 15 minutes after sunset, black-headed night monkeys leave their sleeping sites to forage and return just before sunrise each day. With all this late night/early morning travel, a break is necessary, which occurs at midnight each night.

Communication
Whoops and hoots are the two main vocalizations black-headed night monkeys use. When there are intergroup encounters or aggressive interactions with other groups, whoop calls are emitted. Whoops are usually accompanied by visual displays and scent marking territory (right before the call).

Hoots are used in distance communication and mating rituals. A single male may call out to a female and vice versa. This call is also a form of social bonding.

Other crucial forms of communication are visual cues. Conflicts between neighboring groups tend to occur when the moon is bright, perhaps because they can better see physical signals. When conflict happens, visual signals are displayed as a form of aggression. Some of these signals include: arching of back, erecting the fur, urinating, and passing wastes.

Reproduction and Family
Similar to other night monkeys, black-headed night monkeys are monogamous. They stay with one mate for many years, unless one dies or they are displaced by violence from an intruder male or female.

After about a year, and depending upon the season, environmental factors, and food availability, a newly mated couple conceives. This typically occurs between August and February when conditions are favorable. A female night monkey gives birth to one infant per year. When the infant reaches sub-adulthood (between 2–5 years), they disperse from their natal range. Once they disperse, they venture off to find a mate to make their own family group. Sometimes they even take over the territories of their parents.

Males are the primary caretakers of their young. They carry them, protect them from predators, play with them, and show them foraging and eating techniques. If the father dies, an older sibling, not the mother, assumes infant care. Mothers nurse their young every 3 hours, then drive them away by nibbling on their feet and tail. At the time of weaning at about 8 weeks old, males also start to drive away their young so they become more independent.

Photo Credit: MIguel Rangel JR/Creative Commons

Ecological Role
Due to the high amount of fruit consumed, black-headed night monkeys assist in seed dispersal throughout their range.

Conservation Status and Threats
The black-headed night monkey is listed as Least Concern by the International Union for Conservation of Nature’s Red List of Threatened Species (IUCN, 2015). Throughout Brazil, populations have decreased mainly due to habitat fragmentation by infrastructure development. Similarly, in Peru, widespread deforestation due to infrastructure development and the expansion of human populations have reduced quality habitat for black-headed night monkeys.

They are also found in the wildlife and pet markets in the Amazonian region of Peru and Brazil.

Conservation Efforts
Black-headed night monkeys are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES). They are found throughout several protected areas in Brazil such as Amazonia National Park, Serra do Divisor National Park, Juruena National Park, Pacáas Novos National Park, Abufari Biological Reserve, and more. They are also found in Peru in Manu National Park and in Bolivia in Manuripe National Reserve.

Collecting information on the ecology and the population trends of black-headed night monkeys can be difficult given their nocturnal lifestyle. However, with increased technologies future research can provide better conservation techniques and policies that can be put into action to protect night monkeys and their habitats.

​References:

  • ​​https://primate.wisc.edu/primate-info-net/pin-factsheets/pin-factsheet-owl-monkey/
  • https://animaldiversity.org/accounts/Aotus_nigriceps/
  • 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.
  • Khimji, SN., Donati, G. 2013. Are rainforest owl monkeys cathemeral? Diurnal activity of black-headed owl monkeys, Aotus nigriceps, at Manu Biosphere Reserve, Peru. Primates. 1-6.
  • Kinzey, WG. 1997. ed. New World Primates: Ecology, Evolution, and Behavior. New York: Aldine de Gruyter.

Written by Tara Covert, April 2021