Geographic Distribution and Habitat
The guereza colobus (Colobus guereza) monkey—also known as the mantled colobus, eastern black-and-white colobus, and magistrate colobus, as well as by several other variations of the common name—occupies a widespread geographic area that stretches across central Africa. The species’ distribution extends in an easterly direction from Nigeria, east and west of the Niger River and the upper Donga River tributaries, to the Yabassi District of Cameroon, to Equatorial Guinea, through Chad, Gabon, and the Central African Republic, across the Oubangui River to the Republic of Congo, to northern Democratic Republic of the Congo, through southern Sudan to Ethiopia, Kenya, Uganda, and Rwanda; and, finally, south into northern Tanzania.
Guerezas are found in both primary (“old growth”) and disturbed, secondary forests (forests that have regrown after a major disturbance, such as fire or timber harvest, but have not yet reached the mature state of a primary forests). The monkeys appear to prefer the latter. Scientists speculate that this predilection is due to the high diversity of food trees that secondary forests offer. Deciduous and evergreen thicket forests, where the canopies of individual trees overlap to form a closed, continuous layer, provide the monkeys’ main habitat.
Additional habitats include swamp forests, wooded grasslands, wooded savannahs, riparian forests (forestland adjacent to a body of water), gallery forests dominated by Acacia trees, moist lowland, medium-altitude rainforests, and high montane forests at altitudes up to 14,764 feet (4,500 meters). They are less likely to be found in shrubland habitats. Guerezas are known to venture into agricultural tracts of land, including eucalyptus plantations (a foray that likely fulfills a nutritional need, scientists speculate).
Colobus monkeys belong to the primate family known as Cercopithecidae and to the subfamily known as Colobinae, to which the guereza belongs.
The guereza colobus monkey is recognized as the “parent” species to seven “children,” or subspecies. Each of these subspecies occupies a specific range and exhibits slight variations in appearance.
- Western guereza (C. g. occidentalis) is found from eastern Nigeria, Cameroon, south to Gabon, eastward to southwestern Sudan, and into Uganda, west of the Nile River.
- Omo River guereza (C. g. guereza) is found in forested areas of the Ethiopian highlands, west of the Rift Valley and within the lowland areas along the Awash River, the Omo River, and in the Blue Nile gorge.
- Djaffa Mountains guereza (C. g. gallarum), also known as Neumann’s black-and-white colobus, is found only in the Ethiopian highlands east of the Rift Valley.
- Dodinga Hills guereza (C. g. dodingae) is found only in the Didinga Hills of southeastern Sudan.
- Mau Forest guereza (C. g. matschiei) is found in Kenya, west of the Rift Valley, inhabiting forestland within the Rift, west to Mount Elgon (Kenya and Uganda), and south to the Ngorongoro Crater and the Grumeti River in Tanzania.
- Uarges guereza (C. g. percivali), also known as Percival’s Black-and-White colobus, is found only at Mt. Uarges, the highest peak in the Matthews Range of central Kenya.
- Kenya guereza (C. g. kikuyuensis) is found in central Kenya, from the Ngong Escarpment, Mt. Kenya, and the Aberdare Mountain Range.
Subspecies classifications for the guereza colobus are not definitive, however, with some scientists continuing to debate the level of distinction that qualifies each guereza as a subspecies—or as a full species. As further research is conducted on these monkeys’ geographic distribution, physiology, behavior, and other attributes, their taxonomic status may change. For example, until 2018 when scientists elevated it to full species status, the Mt. Kilimanjaro guereza (Colobus Caudatus; formerly C. g. caudatus) had been considered a subspecies of the guereza colobus.
Size, Weight, and Lifespan
Guerezas are sturdy, long-tailed monkeys and are the largest members of the Colobinae subfamily. Males are slightly larger than females. In the subspecies, males typically (but not always) have larger teeth than females. Scientists attribute this dental differential to a natural response to habitat and to social dynamics between the sexes.
Head-to-body length in males is between 21.4 and 27.5 inches (54.3–69.9 cm), averaging about 24.2 inches (61.5 cm). Average weight is between 20.5 and 29.8 pounds (9.3–13.5 kg).
Head-to-body length in females is between 20.5 and 26.5 inches (52.1–67.3 cm), averaging about 22.7 inches (57.6 cm). Average weight is between 17.2 and 20.3 pounds (7.8–9.2 kg).
The nonprehensile tail is greater than head-to-body length, to varying degrees among the subspecies.
Lifespan for this species is about 20 years in the wild and about 29 years for individuals in captivity.
Guereza colobus monkeys call to mind forest wizards. A thick, glossy, black fur coat that covers the body is contrasted by a long, white, fringed “magical” mantle (or cape) draped over the shoulders that extends the length of the back, reaching the hips—likely the thought behind this primate’s nickname of “magistrate.” White whiskers and a full white beard frame a gray, hairless face characterized by a long, narrow muzzle with closely set nostrils that sit above unassuming lips that conceal an underbite. (The guereza’s white beard distinguishes it from its cousin, the Angolan colobus (Colobus angolensis), whose dark face is framed by silky white strands of fur.) Imagination might suggest that a guereza’s countenance and brown eyes evoke a pensive mien (fitting for a magistrate, perhaps?). The long, thin, mostly black tail ends in a fluffy, white, beautiful plume.
At birth, the fur coats of guerezas are completely white and remain so for the first few weeks of the young primates’ lives. In what appears to be an anomaly, all-white adult guerezas have been sighted at high altitudes on the west side of Mt. Kenya. No scientific explanation for this “albinism” has been proffered.
The seven subspecies of guerezas are differentiated by the coloring of their mantles, which can vary from whitish-yellow to creamy white. Tail length and the degree of fluffiness to the plume further distinguish one subspecies from another.
Characteristic of the Colobinae subfamily, colobus monkeys have no thumbs—and the guereza colobus is no exception. Scientists speculate that this adaptation allows the guerezas to use their long, slender fingers as powerful hooks. Hind legs are longer than forelimbs, and an opposable big toe on each foot helps them stick their landing.
Guerezas also lack the cheek pouches found in their Cercopithecidae “cousins” of the subfamily Cercopithecinae. But Mother Nature has fitted the rumps of guereza colobus monkeys with generous pads known as “ischial callosities,” unconnected in females and contiguous in males. These natural “seat cushions” provide comfort to the monkeys as they sit on their high treetop perches.
Guerezas are mainly folivorous, which means that they eat a whole lot of leaves. Leaves of the hackeberry tree are their favorite. Young leaves comprise a larger portion of their diet than mature leaves. Fleshy, unripe fruits, leaf buds, tree bark, seeds, blossoms, and the occasional insect or spider complement their meal plan. The monkeys’ geographic distribution and seasonal fluctuations impact food availability and thus, may cause related fluctuations in their diet. To lessen changes in their menu, guerezas select leaves that are less susceptible to seasonal change. But in times of food scarcity, mature leaves and fruits provide the guerezas with sustenance. (In what almost sounds like an example of forest etiquette, scientists speculate that guerezas select fleshy unripe fruits, rather than fully ripened fruits, to avoid competition with other primate species who prefer ripe fruit.)
On occasion, guerezas descend to the ground to feed on termite clay and aquatic plants. Drops of dew and rainwater that have collected inside tree trunk hollows provide their daily water requirement.
A multichambered stomach allows for the digestion of large amounts of foliage. Microbes in the guereza’s gut work with fluids secreted by the monkeys’ salivary glands to efficiently break down leaf cellulose. This morphological adaptation allows the monkeys to obtain the necessary nutrition from their leafy diet.
Behavior and Lifestyle
Although active during daylight hours, guerezas are late risers as compared to other diurnal primates—leaving their sleeping trees one to several hours after sunrise. Traveling single file as a group, they follow one another along a known path.
Mostly arboreal creatures (spending the bulk of their time in trees), guerezas reside deep within the forest where they travel through the connecting tree canopies quadrupedally (on all four limbs), bounding through gaps between trees. Their elegant mantle acts as an ersatz parachute, allowing the guerezas to seemingly glide as they leap up to 50 feet (12 m) as their long, plume-like tail acts as a rudder to provide balance. When trees are not densely spaced, thus providing no arboreal passageways, guerezas descend to the ground to forage and travel.
About half their day is spent resting. Foraging and feeding occupy most of their remaining daylight hours. Nap time follows feeding, accompanied by gassy belches (burps), resulting from the gastric fermentation occurring in their gut. Then, if they have the energy, particularly the young ones, they may engage in social activities like play.
Before sunset, guerezas once again retire to their sleeping trees. They deliberately avoid sleeping in proximity to other guereza groups and usually occupy up to four trees, in close proximity to the other, that offer food sources. The monkeys take turns sleeping so that at least one individual, positioned on a high perch, can act as “lookout” for interlopers, including members of outsider guereza groups and potential predators, who include crowned hawk eagles (Stephanoaetus coronatus), Verreaux’s eagles (Aquila verreauxii), chimpanzees (Pan troglodytes), and leopards (Panthera pardus).
The word “colobus” comes from the Greek word “kolobos,” meaning “mutilated one”—a not so sympathetic reference to this monkey’s lack of thumbs.
Hindu and Buddhist religions regard guereza colobus monkeys as icons of sacred gods.
Social animals, guerezas live in cohesive, mixed-gender groups (known as troops) averaging 3 to 15 individuals; though groups as large as 23 have been reported. A group typically comprises one dominant adult male who is the alpha, several reproductive adult females, juveniles, and infants. Occasionally, a group might include several adult males; however, this is a temporary situation and the superfluous males eventually wear out their welcome and exit the group (or they are unceremoniously “booted out” by the alpha male).
While a group’s alpha male most often assumes leadership duties, the females form the group’s core. Because females remain with their birth group for life (unless the group dissolves), they are likely close relatives to one another. Sometimes, female relatives occur in neighboring groups. Males typically leave their birth group before they are considered fully mature. These young males may, for a time, lead a solitary life or associate with other wandering males in so-called “bachelor groups”—until they can eventually create, or overtake a group (and a harem) of their own.
Guerezas live in well-demarcated territories of about 32–40 acres (13–16.19 hectares), though territories sometimes marginally overlap. Encounters with outside guereza groups are not friendly, but guerezas show tolerance for other primate species who share their habitat, including mangabeys (cercocebus) and guenons (cercopithecus). Scientists have observed infant guerezas playing with infant vervets (chlorocebus).
These are vocal creatures. Guerezas’ most impressive vocalization is a loud, croaking roar, most often sounded by a group’s alpha male who simultaneously jumps and drops through the forest canopy, making a lot of racket in what scientists refer to as a “jump-roar display.” Male guerezas from neighboring groups often return the alpha’s roar. Scientists believe that these loud roaring bouts, which occur during the night and at dawn, play a role in male competition as guereza males try to “out-roar” one another. Their roars also help to maintain territorial distance between groups. All species of colobus monkeys engage in their own version of jump-roar display.
Other guereza vocalizations include snorts, purrs, honks, and screams.
In addition to vocalizations, guerezas use body language to communicate. When facial expressions and subtle postures fail to dissuade outsiders during hostile encounters, a group’s alpha male resorts to intimidation tactics. He strenuously defends his group’s territory by dramatically flapping his white fringed mantle, leaping, chasing, and engaging in physical combat with his adversary—all accompanied by threatening screams and roars, of course.
Allogrooming is a tactile communication activity that helps strengthen social bonds among group members. While adult male guerezas are not likely to groom one another, females engage in mutual grooming sessions. Youngsters learn about social bonds by playing and fighting with one another.
Female guerezas reach sexual maturity (reproductivity) at about 4 years old. Males lag behind a bit, attaining sexual maturity (able to sire young) at about 6 years old. They make up for this lag time by embracing their polygynous culture: the alpha male gets to breed with all the females in his harem. The female initiates sexual encounters, however. By coquettishly “tongue-smacking,” she indicates her interest to the male.
Breeding occurs year-round, with births occurring at the time of greatest food availability. After a gestation period of nearly 6 months, a single infant is born weighing a mere 14 ounces (.4 kg). Birth interval for the species is 20 months.
All members of a group help care for the newborn, particularly a group’s females (males begin lending their support when the infant is a few weeks old). In a phenomenon known as “infant transfer,” several females handle the infant soon after its birth, carrying it as far as 82 feet (25 meters) away from the mother. In another biological marvel, a mother might suckle the infant of another female while simultaneously allowing her own to suckle her. For their first 20 weeks of life, infants cling to their mother’s chest; toward the end of this period, they begin to grow more independent. At about one year old, young guerezas are considered weaned. Because females remain with their birth group, mothers and daughters forge life-long bonds.
Infanticide has been reported in the species, and the murderers are usually outsider guereza males who have either infiltrated the group or are newly immigrated males.
Guerezas’ folivorous habits help to prune the trees of their forest ecosystem. Their consumption of fruits helps to regenerate new plant growth by dispersing, via their feces, the seeds of the fruits they eat. Guerezas also play an important, but unwitting, role in the food web of their ecosystem: they are a meal to other species.
Because of their widespread distribution and large populations in certain areas, guerezas are classified as Least Concern by the International Union for Conservation of Nature (IUCN 2016), appearing on the IUCN Red List of Threatened Species. Nevertheless, their populations are declining in many localities.
Of the seven subspecies, both the Mt. Uarges guereza and Dodinga Hills guereza are classified as Endangered by the IUCN (2016 and 2017 respectively). The Mt. Kenya guereza, Omo River guereza, Mau Forest guereza, and Western guereza are all classified as Least Concern (2017, 2019, 2017, 2018, respectively). The Djaffa Mountains guereza is classified as Data Deficient (2019), meaning not enough studies have been conducted on this subspecies to reveal significant knowledge.
Hunting poses a significant threat to these monkeys. The animals are killed and eaten for their flesh, known as bushmeat. In what was once a lucrative business for the trade and fur industries, guerezas have been killed and skinned, and their pelts have been sold to be turned into “fashionable” fur coats for human primates to wear. Sometimes, these pelts have been used in traditional ceremonies and the monkey’s body parts are worn as ornaments.
Habitat loss is another threat, as it is for the many wildlife species who share the guerezas’ range. Forestland is routinely razed and transformed into agricultural tracts of land. Should the guerezas venture onto these new farmlands, they are regarded as crop pests and killed. Commercial logging, charcoal harvesting, and gold mining also contribute to habitat loss. Extraction of plant foods for human use and stripping bark from trees to be used in human medicines rob guerezas of important sources of sustenance. While guerezas are more resilient than other primates in coping with habitat degradation, the species has a breaking point—and we are seeing this now with the steady decline of the population.
Then there’s the direst of all anthropogenic consequences—the climate crisis—looming over guereza colobus monkeys, as it looms over all of earth’s citizens.
Captive guerezas are popular zoo inhabitants. Others are used in biomedical research, though they are not the most favored nonhuman primate test subjects (macaques hold this ignoble distinction).
Guerezas 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. The species also appears as Class B of the African Convention on the Conservation of Nature and Natural Resources.
Protected areas, such as national parks, offer a modicum of safety to those guerezas residing within the parks’ boundaries.
Conservationists call for local government to protect guereza natural habitat, create additional protected areas for the species, and work to create awareness and appreciation for the species among the people living in the region with guerezas as their neighbors.
Research is currently underway in Kenya to collect baseline data on the endangered Mt. Uarges guereza for the purpose of establishing appropriate conservation initiatives.
African Wildlife Foundation (AWF) is one organization working to protect the guereza. Projects include implementing Geographic Information System (GIS) technology to identify key conservation threats and to pinpoint areas with the most potential for wildlife. AWF also works with local livestock farmers to help them develop sustainable agricultural practices while conserving guereza habitat.
- Jensz, K. and Finley, L. (2011) Species profile for Colobus guereza. Latitude 42 Environmental Consultants Pty Ltd. Hobart, Tasmania: https://nre.tas.gov.au/Documents/Colobus-guereza_Species-Profile.pdf
Written by Kathleen Downey, July 2023