Aotus lemurinus

Geographic Distribution and Habitat

The Colombian night monkey, also known as the gray-bellied monkey, the gray-necked monkey, the lemurine night monkey, and the lemurine owl monkey, is native to South and Central America. This monkey’s territory ranges from Ecuador and Peru to Guyana and Brazil, and from Panama to northeast Argentina. 

As very adaptable animals, Colombian night monkeys can live in various habitats, ranging from dry scrub to moist rainforest. Because they gravitate towards vines—where trees are dispersed evenly—you’re most likely to see them in dense forests. They’re rarely on the ground but can be found on any level of the forest, and will most likely be 3,300 to 10,500 feet (1,000 to 3,200 m) higher than sea level.  


The Colombian night monkey is a small Latin American (Platyrrhini) species from the family Aotidae and genus Aotus. Prior to 1983, all night monkeys were classified as one or two species (Aotus lemurinus and Aotus azarae). At that time, chromosome variations led to the proposal of nine species. Then, Aotus was split into two groups: a red-necked, southern group, and a gray-necked, northern group. Gray-bellied night monkeys belong to the latter group. 

Colombian night monkey range, IUCN 2023

Size, Weight, and Lifespan

Colombian night monkeys can weigh up to 2.86 pounds (1.3 kg), with males typically weighing a bit more than females. The species exhibits no real signs of sexual dimorphism. Night monkeys range in height from 13.4 to 13.6 inches (346 to 341 mm), with females being slightly shorter than males. This species’ tails are not prehensile, meaning that their tails have not adapted or hold or grasp objects. Monkeys with prehensile tails are typically found in the Atelidae family. 

Colombian night monkeys can live up to 33.8 years in captivity. 


Colombian night monkeys can be recognized by their large round brown eyes, owl-like faces, and small rounded heads. Their thick white eyebrows appear under three black stripes on their heads. There’s also a large black spot between the eyes, which shine a reddish-orange color when reflected by light. Their fur is wooly, dense, and very gray. The species’ undersides range in color from bright orange to pale yellow. The tails are mostly black, sometimes with dark orange or brown coloring on them, but always with a black tip. Their fingers are slender and long, and feature bigger pads on their tips. On the fourth digit of each foot, each monkey has a special claw for grooming.   

Close-up of a Gray-bellied night monkey

For much of the year, the bulk of the night monkey’s diet consists of foliage, flower nectar, and fruits. However, as omnivores, they will also eat insects and occasionally small birds and mammals. Night monkeys have a knack for quickly snatching insects from branches, leaves, and out of the air—although, females and younger monkeys seem to be better at this than males. They capture insects by using the palms of their hands to flatten prey against a branch. 

In the summer months, when most of their food is scarce, they rely on nectar. They feed at night, and usually in the canopy. 

Behavior and Lifestyle

Colombian night monkeys are nocturnal and are most active several hours after dusk and before dawn. They typically will be active during this time to search for food, as they’re less likely to run into competition from other species (who would be inactive at that time). 

Night monkeys have small bodies, low basal metabolic rates, and the ability to detect cues visually at low-light levels. They can easily move through the forest on all fours and can leap up to 16 feet (5 m). As creatures of habit, they take the same routes, leading scientists to believe that they memorize their foraging routes. 

During the day, these monkeys sleep in dense brush, tree hollows, or vine-covered trees. They live in small groups of families that consist of two to five members; this includes a pair of adults and their children. As monogamous beings, both parents help raise their offspring. 

Allogrooming is not common within this species, and couples may groom each other only before mating. Among groups, grooming occurs less often than once a month. The adult male will play with his offspring by chasing and wrestling, exhibiting patterns of “rough and tumble” and “pounce and retreat” type of play for up to 20 minutes four to five times a day. Adult females, on the other hand, typically don’t play with their offspring. This may be because lactating females lack the energy needed to play with infants. 

Fortunately, night monkeys have few predators. They could be preyed upon by wild cats and large owls, who would probably only go after infants. The mere fact that they are nocturnal helps protect them from both predators and hunting by humans. no

Fun Facts

Night monkeys’ precise sense of smell helps them know the difference between unripe and ripe fruits. Their large olfactory bulbs signify that a large portion of their brains are dedicated to smelling. 

They also may rely on olfactory cues to communicate and forage—more so than other primates.  

Their genus name (Aotus) means “earless,” and was chosen because their ears can be difficult to see. 

Daily Life and Group Dynamics

As mentioned, Colombian night monkeys live in small family groups, which essentially move together as one. They’re never more than 32 feet (approximately 10 m) from one another. 

Antagonistic and/or aggressive behavior between members of a group is quite rare. Night monkeys might show aggression towards members of other groups, usually at the boundary of their respective home ranges. Fortunately, these occurrences don’t last more than 10 minutes, at which point there is no real winner and the groups retreat into their territories.  

These monkeys forage at night because it is cooler and there’s less competition. This is in line with the “optimal foraging theory,” which involves minimal energy output for maximum energy input. Also, the lunar cycle seems to influence the nocturnal and foraging behaviors of night monkeys. Lower levels of moonlight may lead to decreased activity for these monkeys.

Night monkeys’ big eyes can improve their vision at nighttime, as they have monochromatic vision. This means that their color vision is poor. And, because mostly see only in shades of grey, they can be considered color blind. Also, night monkeys’ eyes lack the so-called nighttime “eyeshine” that you’d see on many nocturnal animals. 


Colombian night monkeys can be quite vocal; their calls include eight different types: gruff grunts, gulps, hoots, low trills, moans, resonant grunts, screams, and sneeze grunts. The frequency range for these noises can be 190–1,950 Hz. When they sense danger, they will make high-pitched shrieks to alarm others. 

As mentioned, night monkeys of different groups might be aggressive towards one another. They can be found exhibiting loud vocalizations, stiff-legged jumping displays, wrestling, chasing, and other related behaviors. They possess special scent glands under their tails that secrete certain chemical signals. This allows them to convey information to other monkeys, such as sexual identification and route marking. Night monkeys can also be found engaging in “urine washing,” which is when they urinate on their hands and feet while walking over branches to leave a scented path. Males will do this to leave “love notes” to females they’re hoping to entice.

It’s also not unusual for night monkeys to sniff each other as a sort of greeting or to learn more about each other (similar to how dogs greet each other). They often sniff the genital areas and the armpits. These monkeys also use body language to convey things visually. For example, if a monkey’s back is arched, he/she is conveying aggression. If you see a night monkey sway back and forth without using their hands, this could lead to a preplanned escape from a predator. Tactile communication can occur when a mother bites her infant as a sort of rebuke or rejection.

Reproduction and Family

Colombian night monkeys reach sexual maturity when they’re about two-and-a-half years old. The females’ ovarian cycles last anything from 13 to 19 days. After the male initiates reproduction, the female undergoes a gestation period of 133 days. The female will then give birth to one baby, with the occasional twin birth. There need to be at least five months between births. Even though both the mother and father share parenting duties, the male usually assumes the majority of the responsibilities for his young. The female essentially only nurses the infants, and she will refuse to take over caregiving duties even if the male dies. 

The infant survival rate for owl monkeys is quite high (estimated at 96%), most likely due to nearly constant parental care and low predation levels. Newborns weigh around 3.40 ounces (96.5 g) and remain attached to their mothers for three to four weeks of their lives, although the fathers will start carrying offspring on the day of birth. 

When males and females of the species reach sexual maturity, they disperse from their familial groups in search of mates and start their own groups. While males begin puberty at about one year old, females mature a bit later, yet can reproduce as young as two years old. However, because most owl monkeys haven’t obtained full adult body mass and adult characteristics until about four years old, first reproduction typically occurs at around five years old. 

Gray-bellied night monkey
Ecological Role

Night monkeys help nourish and protect trees in their native habitats, primarily by eating insects that could do damage to the trees. Also, because the seeds from the fruits they eat are dispersed through their feces, they help keep their habitat naturally replenished.

Conservation Status and Threats

The International Union for Conservation of Nature lists the Colombian night monkey as Vulnerable (IUCN, 2020), appearing on the IUCN Red List of Threatened Species.

According to the most recent assessment, the Colombian night monkey’s biggest threats include commercial and residential development, mining and quarrying, and various other human-caused disturbances and intrusions. In addition, deforestation has occurred in their habitats, primarily due to cattle ranching, logging, and wood harvesting. Also, more recently, illegal crop expansions have affected populations. 

Unfortunately, Colombian night monkeys are widely used in medical research and are viewed as an ideal model for research on malarial vaccines. Scientists will inject these monkeys—who are kidnapped from the wild and forced into labs—with anti-malaria compounds and vaccines because they are susceptible to malaria. These monkeys have also been hunted (and then eaten) by local communities, and this species has also been sold via the illegal pet trade. 

Conservation Efforts

The Colombian 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. However, as is often the case with endangered species, laws created to protect them are largely ignored and difficult to enforce.

Fortunately, these monkeys reside in numerous protected areas: the Cofán Bermejo Ecological Reserve, Llanganates National and Sumaco Napo Galeras National Parks in Ecuador, and Puracé and Tama National Parks in Colombia. This hopefully helps protect native populations from the threat of deforestation.

New England Primate Conservancy, the New England Anti-Vivisection Society, the National Anti-Vivisection Society, and Animal Defenders International (ADI) are among those organizations advocating to keep primates out of research labs, proposing instead scientific methods that are humane and superior to experimenting upon nonhuman primates.

ADI celebrated a 2012 ruling by Colombia’s Administrative Tribunal of Cundinamarca to revoke permits to capture wild night monkeys for malaria experiments, calling the decision a “breakthrough for animal protection and conservation.” The Tribunal ruled that research on these night monkeys breached Colombia’s commitment to CITES. Had the permits not been revoked, 4,000 of these primates would have been trapped and sent to research labs for experiments related to malaria vaccine at the Institute of Immunology Foundation of Colombia (FIDIC).


Written by Alyssa Christian, March 2024