Gray-Handed Night Monkey, Aotus griseimembra
GRAY-HANDED NIGHT MONKEY
Aotus griseimembra
Geographic Distribution and Habitat
Gray-handed night monkeys (Altus griseimembra), also referred to as gray-legged douroucouli, lemurine owl monkeys, or gray-legged night monkeys, are a primate species native to the tropical and subtropical regions of Colombia and Venezuela. Between the two countries, the geographical borders of their home range include Colombia’s Sinú River and Venezuela’s Sierra Nevada de Santa Maria mountain range and Sao Jorge river valleys.
In northern Colombia, gray-handed night monkeys reside within the lowland forests of the Magdalena River banks and in the valleys of the Cuaca and São Jorge Rivers. In Venezuela, they are restricted to the country’s northwestern regions. They reside in Venezuela’s evergreen forests, southwest of the Lago de Maracaibo Basin, and in the forests of the Eastern Andes mountains. In both regions, they can be found in the forest canopy, the dense layer of trees, vines, and leaves that forms a roof over the rainforest floor.
Depending on elevation levels, environmental temperatures can be very hot or very cold. During Colombia’s dry season, which occurs between December and March, temperatures may range between 82 to 85 degrees Fahrenheit (28 to 30 degrees Celsius). Rainy season temperatures, occurring between April and July, have an average of 90 degrees Fahrenheit (32 degrees Celsius). As the average home range for a gray-handed night monkey is around 0.5 miles (0.8 kilometers), this species also occurs sporadically across Argentina, Ecuador, and Panama.
The taxonomy of the genus Aotus, whose members are collectively referred to as the night or owl monkeys, is a matter of debate. Prior to 1983, the genus contained just one species (Aotus trivirgatus), with its ten subspecies being elevated to species level after genetic analysis. Today, there are two groups of night monkeys: the gray-necked owl monkeys, which include the gray-handed night monkey and live north of the Amazon River, and the red-necked night monkeys, which live south of the Amazon River.
Size, Weight, and Lifespan
Although males and females have slight weight variations, females tend to be lighter than males. Males weigh around 2.2 pounds (1.09 kg) and females weigh around 2 pounds (0.923 kg). Both sexes have a body length of roughly 28 to 48 inches (71 to 121 cm) long, and a tail length of between 26 to 39 inches (66 to 99 cm).
The average lifespan of a wild gray-handed night monkey is around 11 years old.
Appearance
Gray-handed night monkeys are small wooly creatures with dense, tight fur. Their overall color ranges between various shades of gray to reddish brown, while the fur of their undersides has a slight, yellowish tinge. Unlike other species within the same genus, these monkeys have dark hair on the backs of their hands and feet, and dark, geometric stripes on the tops of their heads.
Gray-handed night monkeys have large brown eyes that appear to glow orange when reflected by direct light. This is due, in part, to the reflective layer behind their eyes called the tapetum lucidum, which enhances night vision.
Interestingly, unlike many Latin American monkeys who are red-green colorblind, gray-handed night monkeys have a monochromatic vision that allows them to see the world in various shades of gray. Because of this, they have keen visual awareness and are able to detect visual cues in darkness.
Additionally, having retinas that are 50% larger than most monkeys of similar size, their ability to recognize objects can be extremely accurate, which can be incredibly useful when hiding from predators and hunters.
Gray-handed night monkeys have nonprehensile tails, which by design, help them maintain their balance while they move. This support can be especially important when they are climbing or jumping.
Diet
Gray-handed night monkeys are omnivorous primates with simple stomachs that enjoy eating fruits, seeds, leaves, nectar, flowers, and insects.
In Colombia, lowland forests are often characterized by dense, tree-climbing trees and plants. Plant species these monkeys may frequent include Christmas orchids (Cattleya trianae ), wild cashew trees (Anacardium excelsum), and kapok trees (Ceiba pentandra).
In the Venezuelan Andes’ evergreen forests, gray-handed night monkeys often eat from plants found at high elevations, typically between 2,624 to 6,561 feet (800 to 2,000 m) high. Species found at these elevations include the pino de pasto (Podocarpus oleifolius) and the Mexican alder (Alnus jorullensis).
Although uncommon, when supplementing their diets for protein, gray-handed night monkeys may consume eggs or small birds. Often, this choice is a direct response to changes in their environmental condition, such as when the overall variety of plant species within the ecosystem declines. Although uncommon, these primates may eat avian species such as the gray-capped hemispingus (Hemispingus reyi), white-fronted whitestart (Myioborus albifrons), or gray-naped antpitta (Grallaria griseonucha).
Behavior and Lifestyle
Often referred to as owl monkeys, gray-handed night monkeys are nocturnal primates that are most active during dawn and dusk. During this period, their daily activities may include foraging, grooming, playing, and other social behaviors.
As a quadrupedal and arboreal species, when not leaping across their home range in search of food, they spend some time searching for places to sleep during daylight hours. On average, gray-handed night monkey groups tend to stay at a sleeping site for 1 to 2 nights, so they must choose a location based on four important factors: accessibility, protection, tree age, and size. For these monkeys, ideal sleeping locations often occur within dense vegetation, or inside large tree holes. They typically leave their sleeping sites only when it is safe to do so, often right after sunset; they return to safety just before sunrise.
Because of their small frame, gray-handed night monkeys are more susceptible to predator attacks and must be aware of any potential threats at all times. Between Colombia and Venezuela, the largest threats to this species are wild cats and birds of prey.
The six wild cat species that are most threatening to gray-handed night monkeys are jaguars (Panthera onca), pumas (Puma concolor), ocelots (Leopardus pardalis), oncillas (Leopardus tigrinus), margays (Leopardus wiedii), and jaguarundis (Herpailurus yagouaroundi). Avian predators include hawks, owls, and other diurnal bird species. The most prolific avian hunters for this species are harpy eagles (Harpia harpyja).
Did you know gray-handed night monkeys belong to the genus Aotus, meaning “earless” in Greek? Although they have fully functioning ears, their ears are very small and are often hidden behind their coronal hair. Talk about noise canceling!
Troops of gray-handed night monkeys consist of between 2 to 6 individuals: one male, one female, one infant, and a few juveniles. Adults are socially monogamous, meaning the same parental pair lives with their offspring for extended periods. In this case, the parental pair will share territories, resources, and offspring with one another. Regardless of age, these animals rely on social bonds formed within their troops for protection and support. That being said, although both males and females parent the young, males tend to take on larger parental responsibilities than females. Unless nursing, males care for infants throughout their entire weaning process and most of their young adult lives.
In general, gray-handed night monkeys are territorial, regardless of sex. Although territories overlap, because of the differences in daily activity patterns with other monkeys, there is little to no competition. For reference, primates that traditionally share territories with gray-handed night monkeys include the brown spider monkey (Ateles hybridus) or ornate titi (Plecturocebus ornatus). Furthermore, because these animals are the world’s only nocturnal monkeys, they rarely compete with other monkey species for food resources or sleeping locations because of their differing schedules. In the case of gray-handed night monkeys, opposites do not attract!
In general, night monkeys make many different vocalizations, some of which include clicks, guttural rumblings, and hoots. The hoots and grunts of gray-handed night monkeys are more distinct because they utilize the strength of their laryngeal sacs. Located in the mucosal membrane pouch between the mouth and the trachea of primates, the laryngeal sacs act as a sound resonator and modify pitch. Under circumstances of distress, gray-handed night monkeys may also produce high-pitched shrieks, which act as a siren call to warn nearby troop members.
Gray-handed night monkeys also mark their territory using an oily brown secretion, typically produced by their scent glands, which produce specific pheromones that can be transferred onto vegetation or other territorial boundaries. As they are also covered by greasy hair patches, the combination of sweat, hair, and grease makes for an effective tool to ward off predators. Additionally, gray-handed night monkeys participate in a practice known as urine washing. Urine washing is a behavior in which primates urinate on their hands or feet and rub it on their bodies, particularly to communicate an individual’s social and reproductive status. Gray-handed night monkeys may leave these scent trails throughout their home range, as a unique form of olfactory communication.
As stated previously, gray-handed night monkeys are a socially monogamous primate species. Unlike monogamy, which involves having a relationship with one individual at a time, social monogamy refers to two individuals who are cohabitating, maintaining a sexual relationship, and sharing basic resources such as shelter, food, and parenting responsibilities. When sexual maturity is reached between the ages of 2 and 4 years old, individuals begin searching for a single partner in order to establish their own troop.
On average, the gestation period (length of pregnancy) is around 133 days. Once their gestation period is completed, female gray-handed night monkeys give birth to a single infant, often weighing around 0.25 pounds (4 ounces). Immediately after birth, nearly all parental responsibility is passed to the troop’s adult male. These males dutifully raise infants until the infants become self-sustaining juveniles or sub-adults. For this species, after birth, the adult female’s only parental responsibility is to nurse her infant, which she will do for 1 to 3 weeks.
The interbirth interval, or the period of time between the birth of one offspring and the next, for these monkeys, is around 271 days.
In general, primates play a critical role in maintaining the biodiversity of various forest types. Because they are foragers, gray-handed night monkeys are natural seed dispersers. These primates eat fruit and seeds, making them important dispersers in a particularly frugivore-dependent ecosystem. By distributing seeds away from parent plants, primates promote tree growth throughout different areas of a forest. This ecological distribution prevents overcrowding and helps facilitate the regeneration of diverse plant communities. This is especially important when considering long-term forest conservation.
The International Union for Conservation of Nature lists gray-handed night monkeys as Vulnerable (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
According to the IUCN Red List of Threatened Species, this species is listed as Vulnerable based on the substantial rate of habitat loss across its geographical range. This loss has caused a presumed decline at a rate greater than 30%, over the past 24 years. This species’s largest threats include large-scale cattle ranching, palm oil production, hunting, and mining. In Colombia, amongst the wetlands of the Magdalena River basin, slightly less than 20% of their habitat remains.
The gray-handed night monkey is 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.
In Colombia, the gray-handed night monkey occurs in three protected areas across the country: the Sierra Nevada de Santa Maria National Park, Tayrona Natural National Park, and Ciénaga Grande de Santa Marta Fauna and Flora Sanctuary. In Venezuela, the two protected areas this species occurs in are Sierra de Perijá and Ciénagas de Juan Manuel.
In order to safeguard the future of this species, research and conservation actions are desperately needed. Considering research, it is necessary for primatologists and ecologists to study this species’ taxonomy in more detail, particularly by analyzing their population size, distribution trends, ecology, life history, resource harvests, and daily behaviors. In particular, the most important conservation actions for gray-handed night monkeys include land management, water management, species management, and educational awareness.
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Written by Brooklynne Mitchell-Arno, September 2024