Azara’s Night Monkey, Aotus azarae
AZARA'S NIGHT MONKEY
Aotus azarae
Geographic Distribution and Habitat
Azara’s night monkeys, also known as the southern night monkey or Azara’s owl monkey, inhabit the Gran Chaco region of South America, characterized by dry forests, savannas, and scrublands. Their range extends across several countries, including Argentina, Bolivia, Paraguay, and parts of Brazil, particularly in the states of Mato Grosso and Mato Grosso do Sul. These monkeys are commonly found near rivers, such as the Paraguay River, Rio Pilcomayo in Bolivia and Argentina, and south of the Rio Bermejo and Rio Negro in Argentina’s Chaco region. They also inhabit areas like the Cordillera Oriental and Bolivia’s Bañados de Izozog.
They are typically found in primary and secondary forests, including areas affected by human activity, lowland forests, and cloud forests. This diverse range reflects the variety of regions they inhabit, as noted in the first paragraph. Azara’s night monkeys are very adaptable and capable of surviving in a wide range of environments. For instance, those in the Chaco region live in dry forests that receive as little as 20 inches (500 mm) of rainfall annually.
Size, Weight, and Lifespan
There is still much research to be done on Azara’s night monkey, as size and weight measurements are currently based on a limited number of wild samples. These monkeys are not sexually dimorphic, meaning males and females of this species are generally the same size and color. Their average head-to-body length ranges from 9 to 14 inches (24–37 cm), with tails of roughly the same size. They typically weigh around 2 pounds (1 kg).
On average, Azara’s night monkeys live 16–20 years in captivity, so it’s reasonable to assume that those in the wild have slightly shorter lifespans due to natural threats and the pressures of wilderness survival. That being said, there have been accounts of these monkeys living up to 25 years.
Appearance
Azara’s night monkeys are a striking and beautiful species, defined by their unique markings. Their heads are covered in lighter gray fur, with a black line that outlines their faces and forms a diamond-shaped mark on their foreheads, tapering to a point between their eyes, which are large, rounded, and brown, with black pupils. Their ears slightly poke out on either side, though they are hardly noticeable. The rest of their fur is a darker shade of gray compared to their faces, except for their bellies and under-tails, which are a tan-orange hue. Their small mouths curve slightly upward at the corners, giving the impression of a coy smile. Their noses are flat, with nostrils that open to the sides.
Most striking is their large eyes, from which their nickname, “owl monkeys”, is derived. Night monkeys have large eyes relative to their skull size. This is an adaptation that helps them see well at night. Their eyes do not have the typical “eyeshine” of other nocturnal animals because they lack a tapetum lucidum, a reflective layer in the eyes of many animals that helps them see in low light. In addition, their vision is monochromatic. They see their world in shades of gray, allowing them to better detect movement and shapes as their eyes primarily focus on detecting differences in brightness rather than color distinctions, which require more light.
Diet
Primarily frugivores, fruit makes up a majority of a night monkey’s diet. They have been observed eating a variety of foods ranging from leaves to flowers and nectar to insects. Some of their preferred fruits (and leaves) come from ficus (figs), chrysophyllum, and bastard cedar trees. Their dietary choices often depend on what is most readily available in different seasons and forest regions. During the dry season, when fruit becomes less abundant, they consume more leaves, particularly those of ferns and lianas. Monkeys living in areas with limited availability of fruits, such as the tropical Chaco of Paraguay, rely more heavily on leaves.
Behavior and Lifestyle
Azara’s night monkeys are arboreal, spending most of their time in trees. When they travel from branch to branch, they do so by using their strong hind legs to leap. Their non-prehensile tails help them keep balance. When walking along the length of a branch, they do so quadrupedally.
Though night monkeys in general are known for being nocturnal, certain populations of Azara’s night monkeys found in the Gran Chaco are known to have bursts of activity during daytime. This is likely to avoid certain natural stressors, like climate and predators. Among the more nocturnal populations, activity levels tend to increase when the moon is brighter, probably because the additional light helps them see and navigate familiar paths in the treetops.
These monkeys spend much of their waking hours playing, foraging, and resting. When it’s time to sleep, Azara’s night monkeys definitely have their preferences. While they don’t necessarily construct their own nests, they choose branches surrounded by dense foliage to protect themselves from predators and are even known to modify bird nests to help further create safe spaces. They usually stick to one tree but may move occasionally, especially during the winter season. Besides leaf-covered branches, they have been observed sleeping on vine platforms and in tree holes. These sleep zones are typically around 30 feet (10 meters) above the ground
Azara’s night monkeys are named for the Spanish naturalist Felix Manuel de Azara (1742–1821).
These monkeys are monogamous, meaning a male and a female typically reproduce exclusively with each other, forming a family group and staying together until one of them dies. The most common family size observed in this species is three monkeys—a mom, a dad, and their offspring—but groups of up to six or seven have been recorded. In these larger groups, there is usually still a heterosexual ruling pair, with the others being juvenile monkeys, often their offspring.
That said, there are instances of single adults living on their own, called floaters. This usually happens when they leave their natal group and haven’t yet found a mate, or when they’ve been chased away by competitors. Monkeys that leave their natal group voluntarily tend to do so around four years of age, regardless of gender.
The size of their territory depends on the region where a population of Azara’s night monkeys resides, ranging from 10 to 44 acres (4–18 hectares). These territories often overlap with those of other families, as most foraging is done within a smaller core area of their overall range so competition is less of a problem in the overlapping areas. While this species is generally cooperative with neighboring families, violent encounters can still occur. In such cases, they typically start with charging and vocal behaviors before escalating to physical conflict.
Azara’s night monkeys are especially communicative, using specifically defined vocal calls, olfactory communication (scent-based), grooming practices, postural communication, and shared parental care as tools to strengthen social bonds and convey specific messages. When grooming, it has been observed in captivity that males in a partnership tend to groom their female counterparts more than the other way around.
Many of their audible sounds are used to ensure safety regarding territory and threats. Loud “hoots” serve both as mating calls and as a means of marking territory. Certain calls made by Azara’s night monkeys are exclusive to either males or females. Females can produce a more harmonic or “tonal” hoot, which often lasts longer and is more likely used for mating purposes than territory defense. Males, on the other hand, have a more abrupt and “gruff” hoot.
Mating pairs will often perform a 10-20 note “resonant whoop” call, audible up to 160 feet (50 meters) away, when they sense other families encroaching on their territory. These loud calls are made possible by the monkeys’ throat sacs.
A variation of this “whoop” call is used during moments of distress, such as when an infant is in danger and needs to alert its parents for help.
Like many primates, Azara’s night monkeys use scent glands on their chest and tails to mark their territory. The unique scent from these glands can communicate a monkey’s gender, age, rank, and reproductive status—imagine if someone could tell all that about you just because you forgot to wear deodorant one day! This scent-based information also helps “floater” monkeys locate potential mates.
Lastly, a monkey’s posture and demeanor convey much about its current state. When angry or aggressive, they will arch their backs, jerk their heads, jump with stretched-out legs, and sometimes their fur will stand on end.
After a gestation period of around 133 days, a female Azara’s night monkeys typically give birth to one infant (twins are rare). During the first week of the infant’s life, the mother takes on most of the caregiving responsibilities. Mothers carry their infants on their back, belly, or side for about three weeks when the family needs to travel. During these times, the mother always stays in the middle or back of the group, depending on how many members are in it, while the father leads the troop as a form of protection. After that, the father assumes the majority of parental duties.
At around two months old, babies begin exploring more independently and start eating foraged foods alongside their family, no longer relying solely on their mother’s milk. By the time they are three months old, they become fully independent. However, they do not reach sexual maturity until around four years of age, at which point they leave their natal groups. Sexually mature females can conceive 25 days a year, with the birthing season typically occurring in the spring months from October to December.
If a father is chased out by a more dominant “floater” male, the new male usually assumes most of the caregiving responsibilities for any infant that was previously under the care of the birth father.
Like many frugivores, Azara’s night monkeys play a pivotal role in seed dispersal throughout the forest. Not only do they drop seeds while eating fruit, but their fecal droppings also act as fertilizer, further stimulating the germination of seeds that pass through their digestive tracts. Additionally, these monkeys serve as forest gardeners, pruning foliage as they consume leaves.
Lastly, though unfortunate for them, Azara’s night monkeys are prey for many predators, including wild cats, larger arboreal snakes, and raptors, like the great horned owl.
Azara’s night monkeys are classified as Least Concern by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. Despite this, their population size is declining. Many of the same human-led ailments that plague other primates also negatively affect this species, including deforestation to create room for residential and commercial development, agriculture (livestock farms and ranches), energy production, mining, and quarrying. As a more specific example, those occurring in the Brazilian Cerrado, the Bolivian Chaco-Chiquitano forest, and the Paraguayan Chaco are falling victim to habitat loss for soybean farming and cattle ranching. They are also targeted for hunting and trapping, especially for the pet trade.
Azara’s night monkeys are 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.
Since the species is classified as Least Concern, there are few active conservation efforts. However, a number of national parks, indigenous territories, conservation units, and reserves help protect this species. These include Brazil’s Municipal Natural Park of Piraputangas, Argentina’s Plicomayo National Park, Bolivia’s Kaa-Iya Gran Chaco National Park, Otuquis National Park, San Matías Natural Area of Integrated Management, and Beni Biological Station, and Paraguay’s Tinfunque National Park and Defensores del Chaco National Park.
The Owl Monkey Project (Azara’s night monkeys are also known as Azara’s owl monkeys), established in 1996, is a broad initiative focused on advancing research and conservation efforts for this species.
The IUCN recommends conservation actions related to land and water management, site and area management, and harvest management. Additionally, they suggest that further research is needed on topics such as taxonomy, population size and trends, and the species’ life history and ecology.
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Written by Hannah Broadland, January 2025