Oustalet’s red colobus, Piliocolobus oustaleti
OUSTALET'S RED COLOBUS
Piliocolobus oustaleti
Geographic Distribution and Habitat
The Oustalet’s red colobus, also known scientifically as Piliocolobus oustaleti, is a monkey present in the Democratic Republic of Congo, the Republic of Congo, the southwest regions of the Central African Republic, as well as in small numbers in South Sudan. The borders of their habitat lie in the eastern forest above Lake Albert, the northern Uélé River, the southern Aruwimi (Ituri) River, and the Congo River until it connects with the Sangha River, and the western Sangha River until it reaches the savannah forest.
Their habitat elevations range from 1148.3 feet (350 m), where the Sanhga and Congo Rivers meet, to 6069.5 feet (1850 m) in the forests north of Lake Albert. They inhabit three distinct environments. The largest is the mature primary lowland tropical moist forest in Ituri, Rubi Tele, and Aketi in the DRC, and in northern Congo. Within these forests, Oustalet’s red colobuses find home in primary evergreen forests and prefer to reside next to bodies of water. Primary evergreen forests refer to older, undisturbed forests that have not been significantly altered by human activity. Outside the primary forests, the monkeys are not found in younger secondary forests, which are forests that have endured human logging or deforestation. They prefer primary evergreen forests and older secondary forests. The monkeys also inhabit swamp forests in Ngotto Forest in the Central African Republic and frequently enter the water in this moist environment. Easternmost Oustalet’s red colobus live in high-elevation isolated forest islands or in the savanna woodland.
The identity of the Oustalet’s red colobus has been fluid since its inception. With its many names, the species has currently been debated to be a full species, a subspecies, or a local population of a subspecies. Most recently, scientists have either separated Oustalet’s red colobus and the Olive colobus (Procolobus verus) by genus into Piliocolobus and Procolobus, respectively, or into the same genus Procolobus and separated into two subgenera, Procolobus for the Olive Colobus and Piliocolobus for Oustalet’s red colobus. Taking these into account, this profile will operate using the former option and place this species in the genus Piliocolobus.
The reason for the confusion about the identity of Oustalet’s red colobuses lies in coat color. The red colobus has high variation in coat color depending on the environment, leading scientists to try to classify these physical differences. Differences do not stop there, though, with Oustalet’s red colobus also having high mitochondrial genetic diversity. Mitochondrial DNA tells scientists a bit about how related individuals are in a group, and high variation signals possible separations on species and subspecies lines.
Size, Weight, and Lifespan
Oustalet’s red colobus is a sexually dimorphic monkey with males slightly larger than their female counterparts. Males’ head to body length measures 18-26.7 inches (45.9-68 cm) while females measure 20.47-25.20 inches (52-64 cm), slightly smaller with a smaller range. Their tails show a similar difference, with male tails at 21.65-28.74 inches (55-73 cm) and female tails at 26.77-28.74 inches (68-73 cm). Males weigh 28 pounds (12.5kg) and females weigh about 18 pounds (8.2kg).
Based on other red colobus species, Oustalet’s red colobuses likely live from 25 to 30 years.
Appearance
Oustalet’s red colobus varies in coat color. Most of the monkeys have dark hands and feet with a darker coat on their backs compared to their lighter bellies. Some monkeys in the Lobaye region along the Ubangi river in the Central African Republic are described as raw sienna brown, with golden underparts and a tail root that has a coppery red patch. Those from the Uele River forest region in northeastern Congo are lighter brown with even lighter colored fur on the limbs. Lastly, there are deep red-colored monkeys in both the southwestern and far eastern areas of their overall habitat.
While we see an extensive array of coat colors for Oustalet’s red colobus, their coloring makes them extremely difficult to observe in the field, making documented research observations few and far between.
Diet
Like many other red colobus species, Oustalet’s red colobuses are primarily folivorous, meaning their main source of food comes from leaves. While they primarily eat young and mature leaves, they also consume fruits, flowers, buds, and occasionally seeds. Since their habitat can include wetlands, some Oustalet’s red colobuses enter water to collect the bulbs of aquatic plants. In the Okapi Wildlife Reserve, they are found to feed on flower buds and immature seeds as well.
Red colobus monkeys have multi-chambered (sacculated) stomachs that facilitate the digestion of cellulose-heavy leaves, as are found high in the canopy, where colobus monkeys generally reside. They also have specialized saliva that helps break down tougher leaves. This unique digestive system gives them access to food sources that might make many other animals, including other primates, ill.
At the same time, red colobuses’ digestive processes make consuming overripe fruits nearly impossible. They can only digest unripe fruits. Due to their complex and prolonged digestion, introducing overripe fruit can create too much gas, leading to extreme pain or even, in some cases, death. Some red colobus species consume charcoal or clay to help detoxify their body of cyanide from some toxic plants in their diet. We do not know if this is the case with Oustalet’s red colobuses.
Behavior and Lifestyle
Since Oustalet’s red colobuses live in three different environments, or different ecological modes, their behavior varies between them. Living in different environments means slightly different food sources, things to play with, sleeping behavior, and activity in general. Due to this variation, scientists are unable to get a full understanding of the breadth of the monkeys’ behavior.
While studying this species of red colobus has proven difficult, researchers have found that across all environments, Oustalet’s red colobuses are arboreal and spend most of their time in trees. In the wetter regions of the Ngotto Forest in the Central African Republic, they spend 30% of their time less than 32.8 feet (10 m) above the ground and the rest of the time investigating the rest of the canopy. Since they live near bodies of water, monkeys will come down from the canopy to forage for bulbs from water plants as a food source.
Oustalet’s red colobuses are also diurnal, meaning they sleep at night and are active during the day. During this time, they interact with other group mates, forage for food, and rest. Since they have complicated digestive processes, many red colobus species take time out of their day just to process food. This extensive rest time has not been directly observed in the Oustalet’s red colobus, but is very likely given their similarities to other red colobus species.
The Oustalet’s red colobus may be named after a French zoologist who studied the animals of Africa, Jean-Frédéric Émile Oustalet (1844-1905).
This species of red colobus’s identity has been highly debated for many years. Due to their complicated genetics, they are hard to clearly delineate from other species of red colobus.
Even with the 17 species of red colobus present in Africa, the red colobus genus is the most endangered genus in Africa. Their health is indicative of the health of the entire ecosystem.
Oustalet’s red colobus form groups of 3-18 individuals and, like other red colobus monkeys, live in female-male mixed groups with strict social hierarchies. Other red colobus monkey species groups can reach 80 individuals, but groups of that size have not been recorded for the Oustalet’s red colobus. Not much is known specifically about Oustalet’s red colobuses’ group dynamics, but generally, red colobus females are philopatric, meaning that they stay in the troop they were born into and move in tight-knit groups within their larger community. Males, on the other hand, leave their home group and travel to find other groups to join once they reach sexual maturity. Like most monkey species that participate in this practice, this can be a testing time for a male monkey. Male red colobuses have to endure bullying and violence as a new group member until they are fully accepted, not to mention the hard journey of getting there all alone. To combat this, there are accounts of red colobus monkeys taking refuge with green monkey (Chlorocebus sabaeus) troops to spy on the red colobus troops they wish to move into. Bachelor groups may also occur. Lone males will sometimes form bands with other males, creating a small group, commonly seen in other red colobuses but not yet identified with Oustalet’s red colobuses.
Red colobus troops or groups also overlap in their home ranges. Many red colobus species can live in close proximity and sometimes overlapping areas, causing intergroup interactions. These interactions can sometimes be hostile, but they can also just be passive. When violent interactions do break out, males and females alike will fight, and females tend to form groups with each other during such altercations.
Since Oustalet’s red colobus monkeys are not well-studied, we can assume what their communication might be like by looking at other red colobus species. Since red colobus monkeys live in groups with many individuals, communication within groups is very important. Barks and squawks can be helpful indicators of a nearby threat or food source.
Red colobuses also communicate with their body as much as with their voices. Red colobuses groom each other to communicate affiliation, called allogrooming. Allogrooming is the act of grooming another monkey by methodically combing through hair to remove parasites or debris. Aside from grooming, red colobuses also “present” to each other. To present is to use specific body language to show their behinds and genitalia in order to indicate their place in the social hierarchy.
Little is recorded about the reproduction of the Oustalet’s red colobus, but using other red colobus species as a model can be helpful in understanding how they reproduce. In red colobus monkeys, when a female is fertile, or in estrus, genital swelling will occur, attracting mates. Males will copulate with many females during the fertile period, so knowing who an infant’s father is can be difficult. The exact duration of gestation, or pregnancy, is unknown but likely to be around 5 to 7 months. When the baby is ready to be born, the mother will remove herself from the group to give birth and return very closely bonded to her infant. Like many other nonhuman primates, the mother-infant bond is very strong. Mothers do not let other individuals of the group tend to their baby early in life, but may allow some help as they grow older. When they do, this is called alloparenting.
Since Oustalet’s red colobus diet consists of organic matter, their feces can help fertilize the forest floor. In the cases where they eat seeds, they are seed dispersers.
Red colobus monkeys are also animals of prey. They are preyed upon by chimpanzees and bonobos. There is no direct record of Oustalet’s red colobus and great ape interactions, but their habitats do overlap in areas where predation could occur.
The Oustalet’s red colobus is listed as Vulnerable by the International Union for Conservation of Nature (IUCN, 2018), appearing on the IUCN Red List of Threatened Species.
The biggest threats to Oustalet’s red colobuses are habitat loss and hunting. Habitat loss in the northern and eastern regions of their habitat has caused devastating losses to the population. Logging, charcoal production, oil palm plantations, and even smaller-scale timber harvesting have had a profound impact on their natural environments.
Oustalet’s red colobuses are also hunted, most commonly using shotguns, for bushmeat outside of protected areas. The bushmeat trade is active in the region, and bushmeat markets have been recorded to sell Oustalet’s red colobuses.
With these different threats, the Oustalet’s red colobus population is predicted to decline by more than 30% over the next 30 years.
While there is no direct record of Oustalet’s red colobuses being predated upon by great apes, a natural threat could be chimpanzees and bonobos, as their diet consists of red colobus monkeys.
The Oustalet’s red colobus is listed in Appendix I 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 addition to the multiple countries that list red colobus monkeys as protected, a large number of Oustalet’s red colobus populations live in protected national parks and reserves.
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- iucnredlist.org — Oustalet’s Red Colobus
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Written by Nami Kaneko, Jun 2025