Cercopithecus hamlyni

Geographic Distribution and Habitat

The Hamlyn’s monkey (Cercopithecus hamlyni) is also known as the owl-faced monkey or guenon. This elusive and vulnerable primate lives at high altitudes of 3,000–15,000 ft (900–4,500 m) in dense bamboo and primary forests of the Democratic Republic of Congo and Rwanda. These forests are surrounded by rivers and volcanoes, which prevent the owl-faced monkey from expanding its territory, so the overall population is fairly small.

Groups in eastern Congo are found in a restricted area of the lowland forests that is nested between the Congo River in the west, the Lindi and Nepoko rivers in the north, and the Virunga volcanoes in the east. Temperatures in the region average 60–68 degrees Fahrenheit (16–20 degrees Celsius) with an average yearly rainfall of 49 in (125 cm).

The population living in Rwanda is limited to a small area, 12.3 sq mi (32 sq km) of the Nyungwe National Park, close to the border with Burundi.

The species is no longer found in the Gishwati forest of northwestern Rwanda.

Hamlyn's monkey range, IUCN 2019

Size, Weight, and Lifespan

Hamlyn’s monkeys are sexually dimorphic in size; that is, males are larger than females. Males are about 20–26 in (50–65 cm) tall and weigh 15–22 lb (7–10 kg); females are 16–21 in (40–55 cm) tall and weigh 10–13 lb (4.5–6 kg).

They can live up to 27 years in the wild and up to 33 years in captivity.

What Does It Mean?

A method of managing forested areas or land in order to preserve it and protect the species it inhabits from poaching or illegal logging. A gazetted forest is a protected forest.

Visit the Glossary for more definitions


Hamlyn’s monkeys have a greenish-gray coat with black patches of fur on their underparts and forelimbs. They owe their nickname of “owl-faced” monkeys to their distinctive facial characteristics. They have very large eyes, a white vertical nose stripe, a diadem-like band on the eyebrow line, and a round face framed by gray-green hair highlighted by yellow specks of color.

There are two sub-species: Cercopithecus hamlyni hamlyni, found in the bamboo forests; and Cercopithecus hamlyni kahuziensis, found in the lowlands of the Ituri Forest and South Kivu in DRC. In the lowland population, the nose stripe is either reduced or missing and there is no diadem.

It is thought that the nose stripe may be useful as camouflage since predators could be misled into thinking these monkeys are cats. The skin on the face is dark (completely black on lowland populations) and covered in tiny dark hair, except for a patch of clearer hair around the upper and lower lips.

They have cheek pouches to hold food while foraging. Their hands and feet are different from those of other primates, with much longer phalanges (fingers). Both males and females have blue buttocks. The adult male genitalia are bright blue and pink; the young and adolescent male genitalia are not.

Babies and juveniles are a yellow-reddish color with a pink face, large pink ears, and pink hands and feet. They do not have any nose stripe or diadem. Their eyes are extremely large.


Hamlyn’s monkeys mostly eat bamboo shoots, which they neatly break off from the stems. They also eat leaves and shrubs of other plant species throughout the year. They consume some fungi, flowers, piths, insects, and lichens, as well as fruit, like blackberries. Fruit is a minor part of the diet of bamboo forests owl-faced monkey populations, but is more important for populations living in lowland forests. Those living in the Epulu area also feed on fallen seeds of the ordeal tree or sassywood (Erythropleum suaveolens) and on sprouting seeds of an evergreen tree called Gilbertiodendron dewevrei that many other mammals also feed on.

Behavior and Lifestyle

Hamlyn’s monkeys spend most of their time on the ground, walking quadrupedally while they forage—although they are perfectly capable of vertical climbing as their elongated phalanges allow them to easily grip wet bamboo.

Because of their remote and densely forested locations, Hamlyn’s monkeys are difficult to observe in the wild. Although not documented, it is likely that, like similar guenon species, they retire to the trees at night and sleep sitting on a branch alone or huddling with a few other group members.

They maintain social bonds through grooming; observations in captive populations indicate that they spend a few hours a day grooming one another.

Like other guenons, it is probable that the rank of a female in the group is determined by her age—the youngest sister is higher in rank than her older sisters.

Fun Facts

The Hamlyn’s monkey is named after John Daniel Hamlyn (1858–1922), an animal dealer who first brought an owl-faced monkey to the London Zoo in 1907.

Rwanda’s Nyungwe National Park is contiguous with Burundi’s Kibira National Park. Together they form the largest and most biologically important rainforest in East Africa.

Daily Life and Group Dynamics 

This diurnal (active during daylight) creature lives in groups consisting of one dominant male and several breeding females and offspring. Females stay in their natal group and males disperse as they come of age. They may change groups several times over the years. The average group size is 5 and the maximum is 11 individuals.

Although there is no literature specific to the species on the subject, it is probably safe to assume that—like most members of the guenon family—affiliated females maintain strong bonds with each other throughout their lives, whereas adult males are intolerant of each other. This assumption is corroborated by information found in zoos, where the introduction of unrelated Hamlyn’s monkey females to each other has only been occasionally successful and the introduction of two adult males has consistently resulted in fights—except in one case where the two males were father and son.


Owl-faced monkeys always vocalize at dawn to start the day. They use several vocalizations to communicate their location, keep the group together, attract a mate, and warn the group if predators are approaching. These vocalizations include boom calls. Compared to other primate species, their vocalizations are relatively quiet. The female’s call is a whimpering quaver, whereas the male’s is a deep boom. Infants use high-pitched chirps but learn quickly to use lower, quieter calls.

Both males and females have a scent gland on the chest that they use to rub trees and mark their territory. The scents allow them to detect the presence of non-group members trespassing on their turf.

These monkeys have a limited range of visual cues to communicate with each other. The only two communication gestures known are tilting of the head and flashing of the genitals.

Reproduction and Family

Females give birth to one offspring every two years, between May and October, after a six-month gestation. Babies are born with yellow coats and pink faces, lacking the facial patterns of adults. Juveniles—about four months in age—are easily recognizable. They have brighter colors than adults, with more yellow on the face, throat, upper chest, and side of the face. Their back and limbs are brownish. Juveniles’ coloring darkens gradually over time; the white stripe and diadem also appear when they become older.

Infants and juveniles are carried by their mothers. Because Hamlyn’s monkeys are difficult to observe in the wild, there is no documentation indicating that non-maternal females help care for infants in the group, as has been reported in other guenon species. Males do not interact much with offspring, but their vigilance protects them from predation and attacks from other males.

Females become adults at around 5 years old, males do so when they are 6 or 7 years old.

​Ecological Role

Like other primates, they play a role in their ecosystem as seed dispersers.

Conservation Status and Threats

The Hamlyn’s monkey species is classified as Vulnerable by the Internation Union for Conservation of Nature (IUCN, 2018), appearing on the Red List of Threatened Species. The species’ population is in decline. It is difficult to have an accurate count of the current population because the Hamlyn’s monkey blends into its surroundings, thanks to its pelage coloring.

Natural predators include leopards, golden cats, and humans (due to encroachment and land conversion, but also due to hunting for bushmeat).

Despite the gazetting of the Nyungwe park in 1933, people started to exploit the forest for gold mining as early as 1935. In the 1950s, there were about 3,000 miners working in the local watersheds. Illegal harvesting of bamboo, honey collection, tree cutting, and hunting never stopped and continue to be a threat to the local fauna and flora. The forest lost a quarter of its surface between 1958 and 1979, mostly due to agricultural land expansion. 90% of the local population relies on subsistence farming.

In addition, all species and the environment suffer from the consequences of the many human conflicts in the region and the resulting population displacements.

Conservation Efforts

The Nyugwe National Park in Rwanda is one of the few protected lands in the country. It is home to 1,200 plant species, 275 bird species, and many other animals, including 13 primate species—the most threatened of which is the Hamlyn’s monkey.

Over the years, several initiatives were put in place. The Swiss technical assistance program established buffer plantations of pine trees along the northern edge of the Nyungwe Reserve in 1967. This was followed by a NGO plan to fully protect the Nyungwe Forest as an International Biosphere Reserve. Unfortunately, that fell through.

In 1984, the forest was divided by the Rwandan Ministry of Agriculture and the Swiss government. They wanted to implement zones where timber harvesting was permitted and others where harvest was limited. The Wildlife Conservation Society also established its research station that same year. Unfortunately, the project was affected by the 1994 genocide. It did survive and there are studies conducted today, including some on the Hamlyn’s monkey.

A recent report shows that efforts to protect bamboo forests and other past initiatives have not yielded the expected results. There is hope though that with education on sustainable harvesting of bamboo and the implementation of the 2020 plan to increase access to electricity for 35% of the population in Rwanda, the dependance of the local population on wood and bamboo products will decrease.

  • The Britannica Guide to Predators and Prey — Primates – Edited by John P. Rafferty.
  • Status and conservation of the only population of the Vulnerable owl-faced monkey Cercopithecus hamlyni in Rwanda – Julian Easton, Nerissa Chao, Felix Mulindahabi, Nicolas Ntare, Louis Rugyerinyange and Innocent Ndikubwiman.​
  • The Rufford Foundation – Assessment of Conservation Efforts, Incentives, and the Current Status of Threats to the Hamlyn’s Monkey and Its Bamboo Habitat in Nyungwe National Park, Rwanda.
  • Review of the Economic Impacts of Climate Change in Kenya, Rwanda and Burundi – Brian Harding
  • Wildlife Conservation Society – Biodiversity Survey of the Nyungwe Forest Reserve in S. W. Rwanda – working paper no. 19
  • www.animaldiversity.org 
  • Mammals of Africa – vol 1-6 – Jonathan Kingdon, David Happold, Thomas Butynski, Michael Hoffmann, Meredith Happold, Jan Kalina
  • Sniffing Behaviors in Guenons – Anja Zschoke, Ruth Thomsen – Folia Primatol 201 4;85:244-251
  • www.factzoo.com 
  • The Kingdon Field Guide to African Mammals: Second Edition – Jonathan Kingdon
  • powo.science.kew.org – Kewscience – Plants of the World online
  • wildpro.twycrosszoo.org 
  • ​The Evolution of Primate Societies – John C Mitani, Josep Call, Peter M Kappeler, Ryne A Palombit, Joan B Silk   //  Chapter 5 – “The Behavior, Ecology and Social Evolution of Cercopithecine Monkeys” – Marina Cords

Written by Sylvie Abrams, March 2019