Northern Night Monkey, Aotus trivirgatus
NORTHERN NIGHT MONKEY
Aotus trivirgatus
Geographic Distribution and Habitat
The northern night monkey (Aotus trivirgatus) is a primate species endemic to southern Venezuela and northern Brazil. This species is also known as the three-striped night monkey and northern owl monkey.
Brazil’s Rio Negro, a large tributary of the Amazon River, marks the southwestern edge of this species’ geographic range, while the Rio Orinoco marks the northwestern edge. The northern night monkey’s distribution is bordered by Venezuela’s Rio Caroni to the northeast and the Rio Trombetas, another tributary of the Amazon, to the southeast.
The northern night monkey lives in primary forests, sometimes called old-growth forests, and secondary forests, which are younger forests that have grown back after human disturbance such as logging. The tropical region where these monkeys live has a rainy season from September to May and a dry season from June to August.
The genus Aotus, known as the night monkeys, only contained one species—the northern night monkey—and ten subspecies until 1983. Genetic work helped elevate the ten subspecies to distinct species, and ongoing work continues to refine the taxonomy to this day. Some published research through the early 1990s continued to refer to all species in this genus as A. trivirgatus, the northern night monkey. This, combined with the difficulties of observing a nocturnal primate in the wild, means studies sometimes must make generalizations about species in this genus. Some believe that eleven species is an undercount, and that the genus actually contains much more genetic diversity yet to be uncovered!
Size, Weight, and Lifespan
Male and female northern night monkeys are around the same size, making this a sexually monomorphic species. Both sexes are about 11.8-14.9 inches (30-38 cm) long, measured from the top of the head to the end of the body. Their tails are as long, if not longer, than their bodies, coming in between 12.9-15.7 inches (33-40 cm).
There is, however, a slight difference when it comes to weight. Males weigh an average of 1.79 pounds (0.813 kg), while females weigh an average of 1.62 pounds (0.736 kg).
Due to the challenges of observing a nocturnal primate, we don’t yet know how long this species lives in the wild. In captivity, they have a lifespan of 20 years.
Appearance
When you see the northern night monkey, what catches your attention first? Likely the big amber eyes, an essential adaptation to let in more light and help this nocturnal species see at night. These piercing orbs are framed by light gray “eyebrows” and a trio of dark brown stripes that curve around the eyes and dip down the forehead.
Species in the genus Aotus are generally divided into two groups: first, there are the red-necked species that live south of the Amazon River, and then there are the gray-necked species that live north of the Amazon. The northern night monkey falls in this latter group, with neck hair that is gray-brown agouti. This term, “agouti,” means each individual strand is flecked with alternating bands of color, which breaks up their silhouettes to help with camouflaging.
The outer side of their limbs and their back are mostly gray agouti, similar to their necks. The inner side of their limbs and chest, however, is an orange-tan color. Their hands and feet are dark brown
Diet
This frugivorous species primarily eats fruit, supplemented by nectar, flowers, leaves, and insects. The northern night monkey prefers small, ripe fruit when it’s available and looks for it in trees with large crowns, spanning more than 32.8 feet (10 m). Because colors are duller at night, night monkeys rely on their sharp sense of smell to tell if a fruit is ripe enough to eat.
Scientists who followed one group of four northern night monkeys observed that they ate from eleven fruit trees, representing nine fruit species, during the 9-week study. During the dry season from June to August, when there is less fruit, the northern night monkey eats more leaves.
The northern night monkey eats insects that are also active at night, such as moths, crickets, and nocturnal beetles. With the help of their big eyes, northern night monkeys can see these insects moving around and grab them out of the air and snatch them off a branch.
Behavior and Lifestyle
As a nocturnal species, the northern night monkey sleeps during the day and is active at night, with the most activity at dawn and dusk. Northern night monkeys prefer to be in the high canopy of the forest—more than 32.8 feet (10 m) off the ground—and travel on all four limbs over branches and vines. They are also skilled leapers and have been seen jumping across a 13.1-foot (4 m) gap in the tree canopy.
To better navigate at night, their eyes have evolved to be bigger and rounder than those of primates that are active during the day, with more light-sensing rods and fewer color-sensing cones. This trade-off means that northern night monkeys can see fewer color details, but their ability to visualize textures is enhanced. After all, who needs color vision in darkness? Having a keener visual sense of textures allows them to clearly see food, predators, tree branches, and tree holes when settling in for the day.
Night monkeys have small ears, which is somewhat unusual for nocturnal animals, but their brain contains a larger auditory cortex relative to the total cortex in comparison to diurnal primates. Scientists speculate that this enhances their ability to hear that which they may not be able to see under the cover of darkness, especially in dense forests. They use sound to find noisy insects, such as crickets, to eat.
Observations are limited, but a study following one group for 9 weeks found that they stayed within a home range of 0.012 mi² (0.031 km²). The distance that night monkeys travel during a single night is higher when there’s more light, like when there’s a full moon, and during the wet season, when fruit is plentiful. Their nightly mileage decreases during the dry season, when limited resources make resting more important.
During the day, northern night monkeys sleep either in a hole in a tree or a dense mat of foliage—often made of woody vines or air plants. Northern night monkeys will always pick a spot in their home range, but may or may not return to the same exact spot every night.
The only truly nocturnal monkey species all belong to the genus Aotus and are only found in Latin America.
When the sun is away, the night monkeys come out to play! Like many other members of its genus, the northern night monkey usually gets out of bed 15 minutes after the sun sets. They then travel with their group to find food and eat until about midnight, when they take a break for 90 to 120 minutes. Throughout the rest of the night, they will travel, look for food, rest, and socialize until dawn approaches.
They are most active and do most of their insect foraging at dawn and dusk, when the insects themselves are also most active. Some of this foraging happens on their way back to the spot where they’ll go to sleep–now that’s a productive commute!
Northern night monkeys tend to live in small groups, consisting of an adult pair and offspring of varying ages, that travel as a unit, never going more than 32.8 feet (10 meters) from each other. Within the same group, night monkeys are hardly ever aggressive or antagonistic towards each other. Members of the same group groom each other fairly infrequently, but often play—especially when the moon is bright.
Once the young become old enough to reproduce, they will move out to find a mate to start a family. In the month before their departure, these newly mature night monkeys will start spending more time away from other members of their group, including sleeping in a different spot.
Species in this genus are territorial and will show aggression towards other groups along the perimeter of their home range. If they get close enough to each other, the opposing monkeys may also chase each other and wrestle. These confrontations, however, normally last only ten minutes. At the end of it, the groups each retreat into their respective areas, with no winners. These confrontations, along with most night monkey social activity, peak when the moon is brighter.
Species in the night monkey genus communicate using vocal calls, a few visual signals, and scent marking. To use this latter technique, they secrete chemicals from the scent glands under the base of their tails. Based on the smell, other night monkeys can tell whether the one who left the scent was male or female, and where they sleep and get food. Night monkeys also pee on their hands and feet as they move through the tree tops, leaving behind a scented path. This behavior is known as “urine washing.”
Night monkeys have special throat pouches that inflate to make their vocal calls louder and more powerful. Males and females that have recently left their birth family will call out a series of three to five hoots in search of a mate. They use “sneeze-grunts” to express alarm and purrs to direct members of their group to good food sources or indicate the friendliness of their relationship. Baby night monkeys squeak loudly when they either want food or want to be held.
To show aggression, night monkeys call out loudly, jump with stiffened legs, and bristle their hair. Both male and female monkeys make a call known as the “war whoop,” which is a series of several low notes that crescendo into powerful hoots. The “war whoop” is only emitted when different night monkey groups are confronting each other.
Although their reproduction has not been well-studied, it is believed, based on studies of other night monkey species, that the northern night monkey reaches reproductive age between two and three years. We also believe this species is monogamous, so one male and one female will exclusively mate with each other.
A female night monkey’s pregnancy is typically 4.5 months or 133 days long. She gives birth to just one offspring at a time, and will usually go one year before giving birth again.
The father is the primary caretaker and spends the most time carrying the young night monkey, only handing the infant over to mom to nurse. He will also play with the baby by wrestling and chasing him or her several times a day. The mother, on the other hand, rarely plays with her offspring. This arrangement, though it may seem heartless to our human sensibilities, offers the species a distinct advantage. Nursing a baby is very taxing on the body, so the mother would not have much energy to flee, let alone carry her baby, if there was a threat. The father, on the other hand, is in a much better position to run away from predators and other dangers with the baby.
Older siblings in the group might also take turns holding the baby, but this doesn’t happen often and stops once the baby is a few weeks old. At that point, the infant is too heavy for the siblings to carry. Young night monkeys can start moving on their own once they’re five months old, but aren’t completely weaned until they’re seven months old, and will still run back to their parents if something distressing happens.
As frugivores, northern night monkeys play an important role in supporting the health and diversity of their forest home. They ingest plant seeds with every fruity meal and spend their days moving around the forest. When it’s time to go to the bathroom, they poop those seeds back out, helping the plants spread and grow in new locations.
The International Union for Conservation of Nature lists the northern night monkey as Least Concern (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
This species is listed as Least Concern because it has a wide geographic range, a presumably large population, and faces no major threats such as hunting. Like many other primate species, it has experienced habitat loss, but is not significantly threatened by it at the moment.
The northern 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.
Fortunately, the northern night monkey’s range is covered by many protected areas. In Brazil, this includes the Pico da Neblina National Park, Rio Trombetas Biological Reserve, Uatumã Biological Reserve, Anavilhanas Ecological Station, Caracaraí Ecological Station, and Niquiá Ecological Station. In Venezuela, the northern night monkey is found in protected areas such as Jaua-Sarisariñama National Park, Yapacana National Park, Duida-Marahuaca National Park, Parima-Tapirapeco National Park, and Serrania de la Neblina National Park.
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Written by Jiayu Liang, Oct 2025