GRAY-HANDED NIGHT MONKEY

Aotus griseimembra

Geographic Distribution and Habitat
Gray-handed night monkeys, also called gray-legged night or owl monkeys, are endemic to Colombia and Venezuela. They are found in the lowland forests on the banks of the Magdalena River and the valleys of the Cauca and São Jorge rivers. Some populations also live in the Sinú Valley, located at the low point of Magdalena River and the Gulf of Urabá in northwestern Colombia. The region is a mosaic of tropical and subtropical dry broadleaf forests, with temperatures in the low 80s Fahrenheit (26–28 C) and yearly rainfalls averaging 59 inches (1.5 m). The valley is located in the Sierra Nevada de Santa Marta, which is one of the highest tropical coastal mountain ranges in the world. Luscious greens and tall skinny palm trees against a backdrop of white clouds and mountain peaks make the scenery unique and enchanting. The Sierra Nevada de Santa Marta National Park was established in 1964 and is the second oldest national park in Colombia. In Venezuela, gray-handed night monkeys are restricted to the northwestern parts of the country and inhabit evergreen forests in the Lago de Maracaibo basin and in the seasonally flooded forests of the Eastern Andes.

CONTROVERSIAL TAXONOMY

All night monkeys had been considered to be members of one or two species until the 1980s, when DNA studies identified chromosome diversity in the various groups of night monkeys throughout various regions of South America. Scientists reviewed the traditional classification of the genus Aotus and determined that there are between eight and eleven night monkeys species that are divided in two groups—northern gray-necked night monkeys and southern red-necked night monkeys. The discussions regarding the taxonomy of night monkeys continues. In fact, some scientists consider the gray-handed night monkey to be a subspecies of the Colombian night monkey (Aotus lemurinus) while others consider it to be a separate species. The International Union for Conservation of Nature treats the gray-handed night monkey as a distinct species.

Gray-handed night monkey range, IUCN 2008

Size, Weight, and Lifespan
Gray-handed night monkeys weight about 2 pounds (1 kg) with females weighing slightly less than males. Their body is approximately 28 inches (71 cm) long.

No information was found on their life expectancy but other species of owl monkeys have been reported to live up to 25 years in captivity.

Appearance
Gray-handed night monkeys’ bodies are covered with short and dense coats that appear soft to the touch. The coloring on their backs and limbs is gray, which contrasts with the yellowish color of their bellies and the nutty brown of the fur on their hands and feet. Their legs and tails are long. Both their hands and feet have large digital pads. They have opposable thumbs and a grooming claw on the second digit of each foot.

Their large round eyes are accentuated by white markings and are perfectly adapted to night vision—even if they lack the reflective membrane that makes the eyes of other nocturnal species shine in the dark. Their vision is monochromatic, which means they perceive light intensity but cannot clearly distinguish all colors.

They have a pointy nose and ears so small they are barely visible. Night monkeys have several scent glands that secrete pheromones—located under the tail, near the muzzle, and at the sternum.

Photo credit: ©Lida Trujillo/iNaturalist/Creative Commons

What Does It Mean?

Endemic:
Native or restricted to a certain area or country.

Lowland forest:
An equatorial evergreen rainforest, commonly known as a tropical rainforest, which receives high rainfall (80 in; 2 m) throughout the year.

Monochromatic: 
Having or appearing to have only one color, or hues of one color.

Scent gland:
Any of various specialized skin glands, occurring in many kinds of animals, that emit an odor commonly functioning as a social or sexual signal, or a defense weapon.

Zonation:
A system used in ecology to plan the conservation of a species by distributing plants and animals in specific zones according to various parameters.

Visit the Glossary for more definitions

Diet
Gray-handed night monkey families forage in groups at night. They eat fruit, flowers, young leaves, seeds, nectar, and insects. They also occasionally supplement their diet with bird eggs and small birds. When capturing an insect, they grab it with their hands and squish it.

They detect food visually but also through smell. For instance, they may scent-mark an area that contains many of the foods that they like so that they are able to find it again later. Smell also allows them to distinguish between ripe and unripe fruit.

Behavior and Lifestyle
Night monkeys are arboreal and most active at dawn, at dusk, and during the night, especially when the moon is full and when ambient temperatures range between 60 and 85 degrees Fahrenheit (15–30 °C).

Daily Life and Group Dynamics 
Night monkeys live in small groups that include up to 6 individuals and usually consist of a monogamous adult pair and their offspring. Groups’ territories overlap, so males often engage in displays, screams, threats, scent-marking, and chases to defend their turf.

During the day, they sleep in tree holes and hollows covered by thick foliage that provides them protection against dramatic weather and predators. These nests are usually found in the low and mid-levels of the forest strata.

Unlike other primates, adults rarely groom each other.

Fun Facts

The Colombian night monkey belongs to the Aotus genus, the only truly nocturnal monkeys in the world.

Aotus translates to “earless”—not because the monkeys don’t have ears, but rather because their ears are small and difficult to visually detect in their thick fluffy fur.

Communication
Night monkeys communicate through a vast repertoire of vocalizations, olfactory behavior, visual cues, and tactile actions.

Vocalizations include soft, low-pitched “whoops” (a “resonant whoop” is a series of grunts that build in intensity to communicate an individual’s claim of a fruit tree and may also be emitted as a threat display during hostile encounters); guttural rumblings, grunts, clicks, and owl-like hoots (a long, low hoot call communicates sexual advertisement, used by females or younger males when looking for mates, and may also be used by a juvenile who has been separated from his group); and squeaks, moans, and high-pitched screams.

Olfactory behavior includes urine washing, performed by an individual when he wishes to convey sexual attraction. After urinating on his hands, a male will rub his hands on a tree branch or other surface, leaving an olfactory “love note” to the female whom he hopes to lure.

To demarcate their territory, they will mark the boundaries with brown, oily secretions from the base of their tail.

Besides being a prelude to copulation, individuals perform social sniffing as a kind of greeting or a method to perhaps gain information about one another. The most commonly sniffed body areas are the armpits and the genital region.

Visual cues are postures that convey specific information. An arched back conveys aggression. Swaying back and forth with hands free and palms facing outward precedes a calculated escape from a predator. Rubbing one’s genital region against a tree or other surface is performed by both sexes, usually in the context of hostile encounters or during pre-copulation. Females engage in this behavior more often than males, leading scientists to believe that rubbing may be a submissive display; in an instance where a female was dominant over a male, scientists observed that the male engaged in rubbing upon encountering the female.

Tactile communication includes a mother Colombian monkey’s rejection bite, which she administers to her infant in a most non-maternal rebuke.

Parents use their mouths to groom the face and genitals of their newborns. Other than this parental predisposition, social grooming occurs only between a pair of adults engaged in copulation.

Reproduction and Family
Gray-handed night monkeys achieve sexual maturity at about two years of age but tyically do not breed until the age of five. Grown offspring usually leave the parental group to look for a partner of their own. Although the dispersal patterns of gray-handed night monkeys are not well known, they are probably similar to those of other night monkeys. Because they cannot disperse very far and partner availability is limited (i.e., they have to wait for one individual in a pair to die to replace him or her), some grown offspring—referred to as “adult floaters”—attach themselves to a mating pair and help them defend their territory until they are able to find a mate of their own.

Once a monogamous pair has bonded, the male guards the female—preventing other males from getting near her. He rubs her body with the glands under his tail to let other males know the status of their relationship. 

Females can breed throughout the year but usually don’t have more than three babies over the course of two years. They give birth to one offspring (twins are rare) after a gestation period of 133 days. They don’t have another infant for at least 270 days after giving birth. The father takes an active role in the rearing of the infant. He carries his child all the time and only hands the baby to the mother for nursing. He also supports the mother by sharing his food with her.

Ecological Role
​As fruit eaters, gray-handed night monkeys likely act as seed dispersers.  

Conservation Status and Threats
The International Union for Conservation of Nature lists the gray-handed night monkey as Vulnerable (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. The assessment is due to the alarming rate at which the species’ natural habitat is disappearing. Over 80% of the forests in the Magdalena River basin have been converted for agriculture, cattle, and mining. In Colombia, the gray-handed monkey’s habitat has shrunk even more dramatically. Mining activities are an important source of revenue but also contribute to accelerated deforestation and water pollution. Consequently, it is estimated that the population has declined more than 30% over the last 25 years. The lack of connectivity between forest patches also negatively impacts these monkeys who are already naturally limited in how far they can disperse from their native groups.

These small primates are also killed or captured—albeit to a lesser extent than other species—for subsistence hunting or illegal trade for the pet or medical industres. Night monkeys have the unfortunate ability to contract malaria and, as such, are used as test subject for medical research.

Finally, recent research measured the impact of fires in the Amazon and showed that primates are the most affected since they depend on trees for movement, food, and shelter.

Conservation Efforts
Local and international non-profit organizations are working hard at preserving biodiversity in various areas of Colombia and Venezuela. For instance, the Serranía San Lucas area was recently added to the national park system in Colombia. This is good news for the gray-handed night monkeys and other endemic species such as the fronted capuchin and white-footed tamarin. Core-area zonation priority sites—that is, fully protected areas within a protected geographic zone–have been established to protect the habitat of flora and fauna in various areas. However, drastic International and governmental initiatives to tackle the effects of climate change (such as the increasing number of wild fires) are crucial to ensure biodiversity is preserved in the region.

The gray-handed night money is listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), a global agreement among governments to regulate or ban international trade in species under threat.

​References:

  • IUCN Red List
  • Encyclopedia of life eol.org 
  • Aotus Diversity and the Species Problem – Defler, Thomas R, and Bueno, Marta L. Published by Conservation International
  • https://www.worldlandtrust.org/species/mammals/grey-handed-night-monkey/ 
  • Moonstruck Primates: Owl Monkeys (Aotus) Need Moonlight for Nocturnal Activity in Their Natural Environment – Eduardo Fernández-Duque, Horacio de la Iglesia, Hans G. Erkert
  • https://www.sciencedirect.com/topics/immunology-and-microbiology/aotus
  • Fires in the Amazon have already impacted 90% of plant and animal species – mongabay.com 

Written by Sylvie Abrams, October 2021