Spix’s Night Monkey, Aotus vociferans
SPIX'S NIGHT MONKEY
Aotus vociferans
Geographic Distribution and Habitat
The Spix’s night monkey (Aotus vociferans) is also known as the noisy night monkey, Colombian gray night monkey, and Spix’s owl monkey. Like all night monkeys, they live in the Americas. Spix’s night monkeys are found primarily north of the Amazon River in an area that includes parts of Brazil, Colombia, Ecuador, and Peru. Their habitat is bordered on the north by the Guaviare River, the Andean Ridge to the west, and the Rio Negro to the east.
They live in primary and secondary forests, particularly in humid tropical and subtropical forests. The Spix’s night monkey can be found both in lowland forests that flood seasonally and those at higher elevations that do not flood each season. These non-flooding forests, also called terra firma, tend to support smaller populations. Spix’s night monkeys have been spotted in elevations as high as 6,560 feet (2,000 meters) above sea level. They’re an adaptable species and can thrive even in forests that have been disrupted by human activity.
There are 11 species of night monkeys. Those found north of the Amazon, like the Spix’s night monkey, are known as gray-necked monkeys. South of the river, red-necked monkeys are common. Until the 1980s, all night monkeys were grouped into one or two species, but chromosome evidence has helped scientists recognize the different species of night monkeys.
Size, Weight, and Lifespan
The Spix’s night monkey is relatively small, with males and females of similar size. Males weigh only around 1.55 pounds (0.7 kg) on average; females weigh slightly less. They measure between 9.8 and 17.7 inches (25-45 cm) from head to body. Their tails are longer than their bodies, measuring 12.2 to 18.5 inches (31-47 cm) long. Spix’s night monkeys can live up to 20 years in captivity.
Appearance
Like other night monkeys north of the Amazon, Spix’s night monkeys have gray-brown fur on their necks. Most of their back is also gray-brown, but their tail becomes dark gray or black at the tip. Their bellies are golden or cream colored. Their faces are framed with light fur, with a distinctive set of three vertical black stripes that meet at the crown of their heads. The Spix’s night monkey has long gray fingers. But their most striking feature is their large eyes, which are critical for their nocturnal lifestyle. Spix’s night monkeys are not sexually dimorphic—males and females look the same.
Diet
Spix’s night monkeys are frugivores—fruit makes up over 80 percent of their diet. They prefer small, ripe fruit, but they will also eat nectar, flowers, leaves, and small prey like insects. They search for insects primarily at dawn and dusk. Unlike most animals that look in crevices and under rocks to find insects, Spix’s night monkeys can actually grab flying insects from the air or from branches. They will often eat moths, beetles, spiders, and grasshoppers.
Behavior and Lifestyle
As the name implies, Spix’s night monkeys are nocturnal. Night monkeys are the only nocturnal monkeys, though they actually evolved from diurnal ancestors. During the day, Spix’s night monkeys sleep in holes inside trees or in dense foliage. Sometimes they share these holes with other mammals, including kinkajous, olingos, bats, and porcupines.
The Spix’s night monkey isn’t threatened by many nocturnal predators, but they can be prey to owls, snakes, and some species of cats. During the day, if they don’t hide well enough in their sleeping sites, they can be prey for diurnal raptors.
They move through the trees by quadrupedal motion, climbing and leaping through all levels of the tree canopy to forage for snacks. They almost always have some weight on one or both of their hands, even while resting. However, they can use their powerful hind legs to leap up to 13 feet (4 m) to cross gaps between trees.
Spix’s night monkeys don’t interact very much with other primates because most primates are fast asleep when they are awake and vice versa.
As one of the few monkey species that are monochromats, Spix’s night monkeys cannot perceive color at all. They live in a world of black, white, and gray.
The Spix’s night monkey lives in small groups of 3 to 5 individuals, generally comprising an adult pair and their offspring. The group shares territory and forages for food together. When they reach maturity, both males and females leave their birth group to find new companionship.
Spix’s night monkeys emerge from their sleeping sites after sunset. After foraging for a few hours, they rest for an hour or two around midnight before returning to foraging. On moonlit nights, they are more active than during the new moon. On darker nights, they travel along paths marked by scent trails to navigate the dark. They return to their favorite sleeping holes before sunrise.
Their home range is estimated at around 22,720 feet (4.3 miles/829 m) per night during the wet season and 827 feet (252 m) per night during the dry season. Since there is less food available during the dry season, Spix’s night monkeys spend more time resting and conserving energy. These monkeys are very territorial and don’t generally interact with other groups, though they do occupy overlapping territories with other groups.
Spix’s night monkeys deserve both the name ‘noisy night monkey’ and ‘Spix’s owl monkey’. They can emit loud, low-frequency calls that sound like owls. They inflate a sac in their throats to increase the volume of their vocalizations—often when they want to be aggressive. Spix’s night monkeys have a host of other vocalizations, described as trills, grunts, hoots, squeaks, and chirps. Young Spix’s night monkeys use specific sounds to ask for food.
Spix’s night monkeys also communicate with body language, such as arching their backs to show aggression. They will groom one another occasionally, but it’s not a frequent behavior. They use a lot of scent-based communications. Spix’s night monkeys have special glands under their tails that secrete scents. They use social sniffing to assess potential mates and show sexual interest. They can even use scents to point them in the direction of food or back to their sleeping sites—they will urinate on their hands and rub them on surfaces to leave these scent trails.
Spix’s night monkeys are monogamous and usually live with the same mate for their entire lives. However, there are instances of a new adult coming in and fighting to usurp the place of one of the bonded pair. These fights can be ferocious and may even be fatal. Research has shown that when one of the pair is ousted, the new pairing produces fewer offspring. In other words, being faithful is a better evolutionary strategy for Spix’s night monkeys.
These monkeys reach sexual maturity in 2-4 years, and both sexes leave their birth group to find partners of their own. The average age of a first-time mother is 4 years. The gestation period lasts around 4 to 5 months. They generally only produce one infant at a time, and only once per year. For the first 4 or 5 months of their lives, infants are carried by the male Spix’s night monkey. Males provide the vast majority of infant care, with females only taking over during feeding times. The infants begin to eat solid food when they are around 1-2 months old and are weaned completely by 7 months.
As a frugivore, Spix’s night monkeys likely play a vital role in dispersing seeds away from the ‘mother’ plant. They also control the insect population. Because they’re nocturnal, they generally do not compete with other primates in their area. Other nocturnal animals in their habitats, such as bats, do not present much of a competitive threat in the Spix’s night monkeys’ search for food.
The Spix’s night monkey is classified as Least Concern by the International Union for Conservation of Nature (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. These monkeys are distributed widely and are thought to have a large population, though additional studies are needed to know more about their numbers. They seem to be a highly adaptable species, thriving even in forests disturbed by logging and other human activity.
The biggest threat to Spix’s night monkeys is being captured for biomedical research, often for malaria-related studies. They are one of the few monkey species susceptible to malaria, a disease that kills nearly 600,000 people per year. Monkeys are taken for research along the shared borders of Brazil, Colombia, and Peru. For example, Colombia permitted the capture of 400 Spix’s night monkeys per year from 2020-2022 for use by a single biomedical laboratory. The monkeys are often released back into the wild after experimentation. There is the risk that these released monkeys disturb wild populations and habitats.
Other threats include the illegal pet trade and extensive habitat loss from farming, expanding human populations, mining, and logging. Deforestation has increased considerably in Ecuador and Peru in the last 20 years, disrupting prime habitat for Spix’s night monkeys and other forest-dwelling species in the area.
Spix’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. They’re also classified as ‘near threatened’ in Ecuador’s red book of mammals.
Spix’s night monkeys live in many protected areas across their habitats. They’re found in national parks and reserves in Colombia, Brazil, Peru, and Ecuador.
Many scientists and nonprofit organizations are working to protect night monkeys and other primates in the areas where Spix’s night monkeys live. Neotropical Primate Conservation in Peru has worked to help create 10 officially recognized reserves in night monkey habitats. They also work with local communities on education projects and train locals to survey and map protected lands.
Colombia has seen violence against environmental leaders and a rise in environmental crime. But, despite the great personal risk, scientists and conservationists continue to work in Colombia. One such organization, Entropika Foundation, works toward developing protected areas, completing scientific research, establishing community-based tourism, and fighting animal trafficking in Colombia.
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- https://www.alltheworldsprimates.org
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- https://asp.org/conservation/files/Aotus%20Conservation%20Action%20Letter%20March%202021.docx.pdf
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- https://www.owlmonkeyproject.com/
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- https://www.entropika.org/
Written by Courtney Daniels, Sep 2025