Grey-legged night monkey (Aotus griseimembra)

Infant grey-legged night monkey
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Grey-legged night monkey fact file

Grey-legged night monkey description

GenusAotus (1)

The grey-legged night monkey belongs to the nocturnal primate genus Aotus, which is also one of the most widespread primate genera in the neotropics. This species is a relatively small monkey, and there are no differences in appearance between males and females. This monkey has a slightly astonished look due to its large eyes, but unlike many nocturnal species, the eyes do not shine in torch light. This is because they do not have a ‘tapetum lucidum’ – the reflective layer behind the retina of night-time animals that enhances the amount of light they can receive. The lack of this layer betrays the grey-legged night monkey’s relatively recent evolutionary split from diurnal monkeys. The coat is thick and two-tone; the tips of the fur are greyish-brown with much lighter bases on the back (3), and pale yellow or orange on the underside (4). The face has white patches above and below the eyes, the cheeks are grey and there is a thin grey stripe down the centre of the forehead. The thick tail is only a little shorter than the body and acts to balance the monkey when walking on all fours along branches. The fingers have true nails and obvious pads to aid grip (3).

Also known as
grey-handed night monkey, yellow-bellied night monkey.
Aotus lemurinus griseimembra.
Mico De Noche Caribeño.
Head-body length: 44 – 48 cm (2)
Tail length: 37 – 44 cm (2)
780 g – 1.1 kg (2)

Grey-legged night monkey biology

With unusually small territories for their size, grey-legged night monkeys are relatively sedentary primates, but will defend their range with vigour. Males display and call to intruders, particularly during the breeding seasons, and mark their territory by rubbing a gland at the base of the tail to release a brown, oily and smelly substance. However, although this species is highly monogamous, and exists usually in pairs with up to three dependent young, aggregations of these family units have been seen together, both feeding and sleeping, numbering up to around 30 individuals (3).

Each female will only give birth once a year after a gestation of about 130 days. A single infant is born and is lavished with parental care. However, the mother will only have contact with the infant to suckle it. The male is responsible for carrying, defending, playing with and instructing the young, and the mother will actively pull an infant off her and bite it should it try to clamber aboard. The young stay with their parents for two or three years, after which time they become temporarily nomadic as they search for a mate (3). Both males and females urinate on their hands and rub it on branches in order to track down a mate and two individuals will sniff each other for an extended period upon meeting (5).

The grey-legged night monkey feeds most on moonlit nights, leaving the sleeping sites shortly after sunset and returning before sunrise (4). It takes fruits, insects and leaves for much of the year, but during July and August when most foods are scarce, it will mainly eat nectar (3). Night monkeys are extremely efficient at snatching insects from leaves, branches (3), and even out of the air with lightening speed (4). They may also feed occasionally on bats, small birds, eggs and lizards. They are rarely preyed upon, but potential predators include cats and large owls, who would likely only manage an infant (3).


Grey-legged night monkey range

The grey-legged night monkey is known for certain only in northern Colombia (4), roughly between the Rio Sinu and Rio Cauca (2).


Grey-legged night monkey habitat

Preferring older, undisturbed forests with high species diversity, the grey-legged night monkey is found only in primary forest and very old secondary forest, but never in young secondary forest (4). Their nest sites are inconspicuous to reduce the threat of predation. They are commonly found in holes in hollow trees, sometimes in cohabitation with bats, as well as in dense vine tangles (3).


Grey-legged night monkey status

The grey-legged night monkey is classified as Vulnerable (VU) on the IUCN Red List (1).

IUCN Red List species status – Vulnerable


Grey-legged night monkey threats

Being nocturnal not only reduces the threat of natural predation but also significantly protects the grey-legged night monkey from hunting by humans. Whilst many monkey species are killed and captured for sport, food, ‘traditional medicine’ and to become pets, night monkeys’ reclusiveness provides them with some sanctuary. Nonetheless, they are not entirely safe from human interference as they are the best primate model for medical research into malarial vaccines. Many humans are infected with, and die from, malaria each year. This species is particularly suited to malarial research due to its resistance, to both forms of malaria-causing Plasmodium parasites, and the similarity of its immune system with that of humans. It is also used to test anti-malarial drugs (6). There is much controversy as to whether it is justifiable to continue taking this threatened species from the wild for the protection of humans, and whilst it can be bred in captivity, individuals are occasionally taken from the wild to prevent inbreeding depression in captive populations (4).

Habitat destruction and disturbance are also threats, but the relatively high level of isolation in Colombia’s forests affords it some protection (7).


Grey-legged night monkey conservation

The listing of the grey-legged night monkey on Appendix II of the Convention on International Trade in Endangered Species of Fauna and Flora has placed limitations on the numbers allowed to be exported from Colombia to countries conducting medical research. Whilst this has implications on the efficiency and value of medical research into malaria vaccines, it may simply result in the development of malaria research facilities in Colombia, thereby preventing the need for export of grey-legged night monkeys, but continuing their removal from the wild (6) (8).

Colombia was the subject of a debt-for-nature swap coordinated by the WWF and The Nature Conservancy, and using funds from the United States Government and the Global Conservation Fund. Millions of U.S. dollars will be released to five conservation sites in Colombia over a 12-year period, in return for the cancellation of a large amount of national debt. This scheme prevents the inevitable loss of funding for environmental projects that occurs as governments slip further into debt, and puts the funds back into environmental enhancement programs, such as the creation of protected areas (9).


Find out more

For further information on debt-for-nature swaps see:



Authenticated (31/05/2006) by Matt Richardson, independent primatologist and writer.



Active during the day.
The state of being pregnant; the period from conception to birth.
Inbreeding depression
The decreased vigour in terms of growth, survival or fertility that follows one or more generations of interbreeding between closely related individuals.
Having only one mate during a breeding season, or throughout the breeding life of a pair.
Active at night.
Primary forest
Forest that has remained undisturbed for a long time and has reached a mature condition.
Secondary forest
Regenerating forest that has not yet reached the mature state of primary forest.
A population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.
Area occupied and defended by an animal, a pair of animals or a colony.


  1. IUCN Red List (January, 2009)
  2. Richardson, M. (2006) Pers. comm.
  3. MFOUs and Night Monkeys: Paternal care in Aotus sp. By Jonathan Greenberg (April, 2006)
  4. Primate Info Net (April, 2006)
  5. The Primata (April, 2006)
  6. Herrera, S., Perlaza, B.L., Bonelo, A. and Arevalo-Herrera, M. (2002) Aotus monkeys: their great value for anti-malaria vaccines and drug testing. International Journal of Parasitology, 32(13): 1625 - 35.
  7. Defler, T.R., Rodríguez-M, J.V. and Hernández-Camacho, J.I. (2003) Conservation Priorities for Colombian Primates. Primate Conservation, 19: 10 - 18.
  8. CITES (April, 2006)
  9. The Nature Conservancy (May, 2008)

Image credit

Infant grey-legged night monkey  
Infant grey-legged night monkey

© Rod Williams / Auscape International

Auscape International
PO Box 1024,
Tel: (+61) 2 4885 2245
Fax: (+61) 2 4885 2715


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