Aotus azarae

Geographic Distribution and Habitat

Azara’s night monkeys, also called Azara’s owl monkeys and southern night monkeys, are endemic to the Gran Chaco region of South America—a large area including many diverse ecosystems, ​from grasslands to savannas to xeric thorn forests and gallery forests. These monkeys occupy all strata of the forest and are found in Bolivia, Paraguay, and Brazil, with habitat extending into a small region of southern Peru and northern Argentina.

Azara's night monkey range, IUCN 2018

Size, Weight, and Lifespan

Azara’s night monkeys are small primates with no differences in size or coloration between males and females. Their body measures 9–14 inches (24–37 cm) and their tail is about as long as their body. They weigh approximately 2 pounds (1 kg).

Longevity in the wild is estimated at 16–20 years. Records indicate that captive Azara’s night monkeys can live up to 25 years or more.


Azara’s night monkeys have beautiful markings. Their belly has a lovely orange color that contrasts with the grayish-brown woolly hair of their body. Their ears are round, short, and hardly visible. Their expressive face is outlined by white hair. Their big round eyes are hazelnut in color with large black pupils and framed by light brown hair, which makes them look like owls. Their nose is small and flat with sideways-facing nostrils. They have a throat sack that allows them to produce loud vocalizations. Their long and thin tail is semi-prehensile. They have large digit pads on their hands and feet and opposable thumbs. Large muscular legs make them skilled leapers.

Although not visible, their eyes bear a physical characteristic found in few primate species—each retina has only a single type of cone, which results in a lack of full color vision.

Azara's night monkey (Aotus azarae) feeding on bananas in a tourist lodge garden, Puerto Maldonado, Peru

Azara’s night monkeys are primarily frugivorous. They consume a lot of fruit from trees in the ficus and the chrysophyllum family, as well as fruit and leaves from the bastard cedar (Guazuama ulmifolia). During the dry season—when fruit is less abundant—they increase their consumption of leaves, including those of epiphytes such as ferns or lianas. It is important to note, though, that dietary habits differ based on the locations in which the monkeys are found. For instance, Azara’s night monkeys in the tropical Chaco of Paraguay spend 40% of their foraging time feeding on leaves, which is considerably more than those living in other areas.

Behavior and Lifestyle

As a rule, night monkeys are nocturnal, being most active at dawn and dusk. The only exception is the Azara’s night monkey. They have switched their activity pattern from strict nocturnality to one that also includes regular diurnal (daytime) activity. These monkeys are cathemeral; that is, they are either active during the day or at night. Scientists posit that harsher climate, food availability, and the lack of daytime predators or diurnal competitors, have all been proposed as factors favoring evolutionary switches in their activity patterns. In the Paraguayan Chaco, for example, where Azara’s night monkeys are most active during the day, primatologists think this may be due to the fact that great horned owls hunt at night.

Upon waking, Azara’s night monkeys spend time interacting and playing before going out to forage. They have been observed resting and sunbathing on open branches, especially in June and July. When they are done with their daily activities, they retire to their sleeping sites—which are typically thickets of branches with dense foliage and vine platforms or tree holes, preferably in the middle strata of the forest. There is only one sleeping site per tree. They sleep about 30 feet (10 m) above the ground—never at the top of a tree—and the branches they use are typically 4 inches (10 cm) in diameter. The monkeys change sleeping sites more often during the rainy season. While they do not build their own nests, they do sometimes modify existing birds’ nests for their own purposes. It is likely that they select branches surrounded by many lianas so they are hidden from predators.

Fun Facts

This monkeys’ scientific name is composed of two words: aotus, which means earless, and Azara, in reference to a Spanish naturalist named Felix Manuel de Azara (1742–1821). Several animals are named after him. He spent 20 years in Latin America and documented his observations in a book published in 1801 entitled “Voyage dans l’Amérique méridionale.”

Genetic testing has revealed that males in Azara’s night monkeys mated pairs are the fathers of the infants they care for. This proves that, contrary to the behavior of many other monogamous mammals, Azara’s monkeys are truly faithful.

Grooming has rarely been observed in wild groups, but studies done in captivity show that monogamous pairs often groom each other. They do so in brief bouts, during which they take turns, although males tend to groom females more often than females reciprocate. The sessions consist of tugging the hair or skin of one’s partner with the mouth or flexed fingers.

Daily Life and Group Dynamics 

Groups are typically composed of one monogamous adult pair and their offspring. Large groups can include up to 6 or 7 individuals—usually the adult parents, one infant, and juveniles or subadults. The pair stays together for an average of 9 years or until one of them dies.

Their territory, which can be up to 24 acres (10 hectares), slightly overlaps with that of other family groups. Within that territory, each group forages in a core area of 5 acres (2 hectares), which remains pretty stable year after year. Even though the territory of a group can overlap with that of another, members of both groups generally cohabitate peacefully. However, occasionally inter-group encounters end up in fights that can be pretty violent. Group members exhibit territorial behavior through displays—mostly chases and vocalizations.

As they reach maturity (2 to 4 years old), both males and females abandon the natal group to live on their own as “floaters.” It is estimated there are about 5 floaters for every 10 breeding pairs. Joining a group is the only option floaters have to breed; for this reason, they remain in proximity of neighboring groups while, at the same time, avoiding the core area of their territories. Indeed, getting too close could potentially cost them their lives. Intruding floaters can have a negative impact on established groups—by creating a need for a larger territory, disrupting pregnancies, or even causing the death of a group resident. If a resident is expelled and becomes a floater, it is unlikely he or she will be able to rejoin that group.


Azara’s night monkeys use vocalizations in a variety of contexts and, interestingly, the structure and frequency of their calls vary by gender and age.

Loud calls, or “hoots,” play an important role in maintaining spatial cohesion within a group. These calls are used to defend territory and attract mates. There are two types of hoots: narrow tones with occasional harmonics or “tonal hoots” that only females produce; and noisy “gruff hoots” that only males produce.

Females call twice as many times as males and their calls have a longer duration. It is possible these differences are valuable when loud calls are used during mating season.

There are other types of calls, such as distress calls, that sound like “whoop whoop whoop.” Those can be heard when a monkey is in danger or when infants need help. In the latter case, the distress calls prompt the adult pair to come to the rescue.

During close-range intergroup encounters, the mated pair utter a series of 10–20 notes referred to as “resonant whoop” calls that can be heard 160 feet (50 m) away.

They use glands on their chest and the inside of their tails to mark their territory. Markings can provide important information to other monkeys about the gender, age, rank, and reproductive status of the individual that left them. Scent marking is important for individuals to find a partner. The secretions from the pectoral glands are different from those of the caudal glands and probably serve a different communication function.

In addition to vocalization and scent marking, Azara’s night monkeys’ posture can speak volumes. For example, jerky movements of the head and upper body and back arching are indicative of aggression. So are stiff-legged jumps, piloerection (hair-bristling to appear larger), urination, and defecation.

Reproduction and Family

Males and females become mature at 4 years of age. Females are able to conceive 25 days a year. Births are seasonal and generally occur in the spring (October–December); this is also the time when subadults disperse from their natal group. Fertility is impacted by weather patterns. After a drought, interbirth intervals (which are normally a year apart) can last up to two years.

After a gestation period of approximately 133 days, mothers give birth to one infant. They carry the newborns on their belly, back, or side until the infant is three weeks old. At that point, the father becomes the main carrier and caregiver. Siblings may help out, but babies are only carried by adults. While carrying an infant, the parent always travels in the middle of the group, sometimes in the back, but never in the front, to ensure maximum protection of the little ones. Babies start eating solid foods when they are two months old. They also start exploring their surroundings at that time, but they are not independent until they are 12 weeks old. They become pubescent at 15 months of age.

Since males play such a crucial role in infant care, one might expect that floaters who integrate into a group by chasing away the father would not care for an infant they did not sire. However, this is not the case. They still provide about 70% compared to 90% of the care duties that natural fathers provide.

Ecological Role

Azara’s night monkeys play an important role in maintaining the health of the forest they live in by dispersing seeds and clearing trees of branches.

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Although Azara’s night monkeys are listed as Least Concern on the International Union for Conservation of Nature’s Red List of Threatened Species (IUCN, 2015), the population is declining. These primates can survive in disturbed forests, but their habitat is rapidly disappearing and is already severely fragmented. Deforestation is the main reason. Land is cleared to make room for agriculture, especially soy bean plantations, but also for cattle ranching, mining and quarrying, and of course urban expansion. For instance, out of the 15,000 square miles (39,000 km2) of territory the species inhabits in Argentina, only 60 square miles (156 km2) in Chaco Province and 252 square miles (654 km2) in Formosa Province, were estimated to be suitable habitat 20 years ago. More recent numbers are not available.

Natural predators include arboreal snakes, cats, and raptors, such as the great horned owl.

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Because the species is not considered at risk, there is no specific conservation program for it. However, in 2012, the species was declared a natural treasure in the Formosa province. 

Night monkeys are also referred to as “owl monkeys.” The Owl Monkey Project, which began in 1996, is a multi-disciplinary program that can help identify areas of improvement—such as the re-evaluation of the species distribution due to deforestation. It can also provide data on the impact of environmental change on these monkeys’ health, especially regarding new pathogens they may be exposed to.

The number of individuals per square mile is higher in gallery forests than in discontinued patches, so it is important to reduce deforestation and provide corridors for animals to move from one forested area to another.

  • ​IUCN red list assessment – 
  • Size, site fidelity, and overlap of home ranges and core areas in the socially monogamous owl monkey (Aotus azarae) of northern Argentina – Wartmann, Flurina M; Juarez, Cecila P; Fernández-Duque, Eduardo
  • Sleeping Sites and Lodge Trees of the Night Monkey (Aotus azarae) in Bolivia – Juan E. Garcia and Francisco Braza.
  • Rearing without Paternal Help in the Bolivian Owl Monkey Aotus azarae boliviensis: a case study – B. Jantschke, C. Welker, A. Klaiber-Schuh
  • Density and population structure of owl monkeys (Aotus azarai) in the Argentinean chaco – Eduardo Fernández-Duque, Marcelo Rotundo, Carrie Sloan.
  • Dry Season Resources and Their Relationship with Owl Monkey (Aotus azarae) Feeding Behavior, Demography, and Life History – Eduardo Fernandez-Duque, Griëtte van der Heide – International Journal of Primatology (2013) 34:752-769
  • Infant Development and Parental Care in Free-Ranging Aotus azarai azarai in Argentina – Marcelo Rotundo, Eduardo Fernández-Duque, Alan F. Dixon – International Journal of Primatology 26, 1459-1473(2005)
  • Adult male replacement and subsequent infant care by male and siblings in socially monogamous owl monkeys (Aotus azarai) – Eduardo Fernández-Duque, Cecilia Paola Juárez, Anthony di Fiore – Primates (2008) 49:81-84
  • Anti-Predator Strategies of Cathemeral Primates: Dealing with Predators of the Day and the Night – Ian C. Colquhoun 
  • Sexual dimorphism in the loud calls of Azara’s owl monkeys (Aotus azarae): evidence of sexual selection?  –  Alba Garcia de la Chica, Maren Huck, Catherine Depeine, Marcelo Rotundo, Patrice Adret & Eduardo Fernández-Duque 
  • Monogamous Cebids and Their Relatives: Intergroup Calls and Spacing – John G. Robinson, Patricia C. Wright and Warren G. Kinzey
  • Azara’s Owl Monkey in the Humid Chaco: Primatological Long-Term Studies in Argentina – Cecilia P. Juárez, Maren Huck, Eduardo Fernándex-Duque
  • The floater’s dilemma: use of space by wild solitary Azara’s owl monkeys, Aotus azarae, in relation to group ranges – Maren Huck, Eduardo Fernández-Duque
  • Owl monkeys don’t cheat: Intensive fathering plays a role –
  • Chemical composition of glandular secretions from a pair-living monogamous primate: sex, age, and gland differences in captive and wild owl monkeys (Aotus spp.) – Andrea Spence-Aizenberg, Bruce A. Kimball, Lawrence E. Williams, Eduardo Fernández-Duque
  • Aotus azarae – La Senda Verde Wildlife Sanctuary – 
  • Aotus – an overview –

Written by Sylvie Abrams, April 2020