Northern Night Monkey, Aotus trivirgatus
NORTHERN NIGHT MONKEY
Aotus trivirgatus
Geographic Distribution and Habitat
Northern night monkeys—also known as northern owl monkeys, three-striped night monkeys, or douroucoulis—are found north of the Amazon River, primarily between southern Venezuela and north-central Brazil. In Venezuela, they range south of Río Orinoco and east as far as the middle Rio Caroni; in Brazil, they reside near Manaus and along the banks of the Rio Negro.
Northern night monkeys spend most of their lives high in the trees, especially in large canopied fruit trees. They can be found from sea level to 3,200 feet (975 m) in elevation. Their habitats include rain forests, cloud forests, woodlands, and savannas.
Size, Weight, and Lifespan
Males and females are similar in size, measuring 9.5–18.5 in (24–47 cm) in length. Their tails are almost as long as their bodies and can measure 8.7–16.5 in (22–42 cm). Infants are large in comparison to their parent’s body mass and grow quickly.
Northern night monkeys weigh between 1.7 and 2.7 pounds (0.77–1.22 kg).
Their average lifespan is between 12 and 20 years. In captivity, they can live to be 25 years old.
Precocial:
Born in an advanced state and able to feed itself almost immediately.
Sympatric:
Occurring or living in the same area; overlapping in distribution.
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Appearance
In general, night monkeys’ coat color ranges from brown to gray; their back is reddish and their belly, underarms, and inner legs are off-white to orange. The thickness of their coat depends on the altitude at which they live: those living in higher elevations, like northern night monkeys, have thicker, shaggier coats than those living at sea level.
Their long legs facilitate leaping from tree to tree or from branch to branch. Their nonprehensile tails are incapable of grasping but are used for balance when traveling and leaping through the forest.
Northern night monkeys have specific facial markings that distinguish them from other aotus species. These include a triangular black patch between their eyes and black stripes on the side of their white faces. Like all night monkeys, they have large brown or orange eyes, contributing to their alternate name, “owl monkeys.”
Unlike many nocturnal animals, night monkeys’ eyes lack a tapetum lucidum. The tapetum lucidum is what gives nocturnal animals the typical nighttime “eyeshine.” Night monkeys’ retinas contain a greater number of rods (responsible for vision at low light levels) than cones (responsible for vision at higher light levels).
Typically, night monkeys’ vision is monochromatic, which means that they can only see in one color—usually shades of gray. There is some speculation, however, around whether or not northern night monkeys can see certain shades of color instead of just gray shades. A few studies have found that northern night monkeys may actually see in other forms of color, though their vision was classified as a “protanomalous trichromat” by Gerald Jacobs in 1977 (protanomaly means seeing weaker shades of red and trichromaty means seeing in three colors). This means that they could potentially see shades of black, red, dark brown, dark green, dark oranges, and some blues.
Further studies on color vision in northern night monkeys established that, although their color vision is distorted, they may not see in that many colors, but rather in grays, blacks, browns, or some oranges. More studies will reveal what colors they see other than gray. These studies are important because they reveal that loss of color vision is not directly connected to a nocturnal lifestyle, but other factors still in question, like predation pressure, resource competition, and temperature.
In addition, predation pressure and competition for resources have led to night monkeys having a very well-developed sense of smell. A well-developed sense of smell indicates that a large proportion of the brain is dedicated to smell. In fact, northern night monkeys have one of the largest olfactory bulbs (a neural structure that aids in smell) of all New World monkeys.
Diet
Northern night monkeys mainly prefer to eat small ripe fruits, but they also eat leaves, nectar, insects, and occasionally frogs, lizards, or eggs.
Foraging at night has its advantages, such as a decrease in competition with diurnal monkeys (those who are active in daytime) and an increase in larger nocturnal insects (to eat). Due to the large amount of insects in the forest, it is more common for northern night monkeys to consume them than frogs, lizards, or eggs.
Behavior and Lifestyle
One might assume that a nocturnal lifestyle would be disadvantageous to monkeys. After all, monkeys generally rely on vision for finding their way during travel, while seeking out food, and for locating other resources. Cooler night temperatures can bring additional challenges. Night monkeys, however, have uniquely adapted to be highly successful in this niche.
Because they prefer warmer temperatures, night monkeys avoid foraging on colder moonless nights and are more active when the moon is bright. They generally forage starting 15 minutes after sunset and return to their nesting site right before sunrise. They travel over the same routes each night, so they may memorize their routes when the moon is bright. While hunting for insects, night monkeys listen for insects’ rhythms and calls, which expose insects’ locations. The monkeys are adept at snatching flying insects out of the air or grasping them off branches.
After spending the night eating and socializing, they continue to forage on their way back to their daytime sleeping site, which usually consists of thickets of dense foliage or tree holes. If large enough, other nocturnal animals, like bats, may share the sleeping site.
Play is common among infants, juveniles, sub adults, and adult males. During months when fruit is most abundant, playing occurs more frequently.
Night monkeys travel through the forest quadrupedally. They are avid leapers and often leap from tree to tree during the night.
The structure of night monkeys’ eyes suggests that their ancestors may have been diurnal.
Northern night monkeys have a grooming claw on the fourth digit of both feet.
Family groups usually consist of a mated pair and their offspring. Northern night monkey infants have high energy demands; thus, older siblings usually stay in the group to help take care of newborns. The young stay in their birth group from 1.5–2.5 years old before they disperse.
The home range of night monkeys is small. There are about 1–6 groups per square mile. On average, they travel under 0.5 miles (708 m) per night. They expand their travel range slightly to 0.515 miles (829 m) during the wet season and travel as little as 0.157 miles (252 m) during the dry season. This means that they spend more time resting and reserving energy during the dry season, due to less availability of resources.
The brightness of the moonlight on any given night is also linked to how far they travel. On nights when the moon is full, night monkeys travel twice as far as they do on darker nights. Even on dark nights, they still have to travel to find food. In addition to memorizing their routes when it’s brighter out, they use chemical cues, like scent-marking with urine and secretions from scent glands in the chest and base of the tail, to find their way in the dark. This helps them navigate between food sites and sleeping sites.
Northern night monkeys are sympatric with other diurnal primates that use the same food sources. There is no competition between them due to their alternate schedules (nighttime vs. daytime). There can be competition between neighboring night monkey groups if they come in contact with one another. Night monkeys are very territorial and defend their territory with loud calls and jumps and sometimes chase or wrestle the opposing group. These conflicts rarely last more than 10 minutes, since one group usually retreats.
Northern night monkeys are known for their load calls including hoots, whoops, and trills.
Hoot calls are used when sub-adult males are looking for mates or when a juvenile is separated from the rest of the group. This is a long and low call that can be heard up to 0.31 miles (500 m). Each hoot is made up of intervals alternating between two long sounds and two to five short sounds. They may repeat this call up to four times.
Another common call is the whoop call, which is used in defending resources or in hostile situations. This call consists of 10–17 low grunts that build up in intensity. The call starts out low and soft, then builds in volume. The cadence of this call is monosyllabic. While making a whoop call, the night monkey’s back arches and the elbows bend and move in a pumping motion, similar to doing a push up. This position helps with sound production by increasing air flow in and out.
Squeaks and trill calls are used when an infant or juvenile is distressed or separated from a parent. Trills sound like three or four squeaks that are emitted rapidly.
Olfactory forms of communication include urine washing and social sniffing. Urine washing is when an individual puts urine on his or her hands and then rubs the hands on a branch or other substrate. This is used to communicate sexual attraction. Social sniffing occurs when individuals meet and greet each other. They start out sniffing nose to nose, then move to the armpit and perineal region.
Tactile communication, such as biting or rejection biting, is done by the mother to the infant when the infant has finished suckling or when the infant tries to establish contact after the first week. The mother bites the tail, hands, or feet of the infant, which signals the infant to move away. After being bitten, infants emit a squeal sound. Mothers and sometimes fathers do this up to the first 8 weeks of the infants’ life.
Although less common among northern night monkeys, social grooming is also a type of tactile communication. When it does occur, individuals remove dead skin, hairs, and insects from each other’s fur. The most common places to groom are the back, the crown of the head, and the back of the head.
Forms of visual communication are important for displaying aggression or copulatory behaviors. When individuals are aggravated, they may arch their back while in a quadrupedal stance or they may stand on their hind limbs.
When a northern night monkey spots a potential predator, they move their body and head back and forth in a swaying motion. While doing this motion, they may make grunting sounds and grasp a tree branch. This behavior is followed by a quick escape. This entire display lets the predator know it has been spotted and it is discouraging to the predator. The predator may not go after the monkey in order to save energy since it now knows it has been seen.
Rubbing is another form of visual communication. This display is used by males and females, but more by females. Individuals have favorite spots that they rub their perineal region on, such as a tree branch or a similar substrate. It can either be used in antagonistic or copulatory contexts.
Northern night monkeys live in monogamous pairs or in long-term pair bonding. Mating takes place around August and September, so infants are born when fruit is highly available. Copulation typically takes place during the night, though in some instances it has been found to take place during the day.
Females give birth to one offspring a year; they rarely have twins. Infants are mostly precocial at birth, meaning they are born rather mature and are able to feed themselves. Males are primary caregivers of infants, and often carry them, guard them, and share food with them for about the first four weeks after birth. Mothers nurse their young every 2 to 3 hours. The mother does not participate in play with the infant and only interacts with him or her during nursing. After 5 months, infants travel independently, but they are not weaned until 6 or 7 months. Northern night monkeys reach sexual maturity at 3 to 4 years old.
By dispersing seeds of the fruit they eat through their feces, they help to naturally replenish and regrow their habitat.
The northern night monkey is listed as Least Concern by the International Union for Conservation of Nature’s Red List of Threatened Species (IUCN, 2018). They are widespread in their range, yet they are threatened by habitat loss. Continuous threats against this species include residential and commercial developments, such as construction of houses and urban areas as well as roads and railroads. In addition, agriculture and aquaculture, logging, and wood harvesting are major threats leading to habitat loss, disruption, and degradation. Northern night monkeys are sensitive to clear cutting and deforestation because it limits the amount of diversity in diets within groups.
Night monkeys are also a food source used in the neotropics. They are hunted for their skin, teeth, and bones. Most of the time they are hunted by subsistence hunters who have been forced to hunt smaller animals because bigger game is no longer available.
Trade of northern night monkeys as laboratory research animals (and even pets) in the U.S. and other countries has led to a decrease in population. They can carry the same malaria parasite that effects humans (Plasmodium falciparum). As a result, they have been used in many studies and experiments in testing human diseases and potential treatments.
Natural predators include arboreal snakes, cats, and hawks.
Northern night monkeys are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES). In Brazil, the northern night monkey is found in a few protected areas such as Pico da Neblina National Park, Rio Trombetas Biological Reserve, Uatumã Biological Reserve, Anavilhanas Ecological, Caracaraí Ecological Station, and (Niquiá Ecological Station. In Venezuela, they can be found in a number of national parks including Jaua-Sarisariñama National Park, Yapacana National Park, Duida-Marahuaca National Park, Parima-Tapirapeco National Park, and Serrania de la Neblina National Park.
Today, there are governmental bans in some South American countries limiting the export and import of goods to the U.S., thus reducing the impact of trapping night monkeys as a threat. Unfortunately, there are many economic and political issues in these areas, which means that bans on hunting, trapping, and deforestation are not regularly enforced.
Much more research is needed on population distribution and site management to better understand the effects of environmental changes specifically from the impact of humans.
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Written by Tara Covert, July 2020