Geographic Distribution and Habitat
The olive colobus, also called green colobus and Van Beneden’s colobus, is endemic to the western coast of Africa, living in Benin, Cote d’Ivoire, Ghana, Guinea, Liberia, Sierra Leone, and Togo. An isolated population can also be found in eastern Nigeria.
Olive colobus monkeys live in the rainforest habitat where they prefer the dense understory of the forest and locations near water sources. Their habitats include second growth tall forests, palm forests, and swamps. Sometimes they travel into the middle canopy to sleep, but they rarely venture into the upper stratum.
Size, Weight, and Lifespan
The olive colobus monkey’s head-to-body length averages about 20 inches (50 cm). Their tails add another 24 inches (60 cm) and, on average, they weight just over 9 pounds (4 kg). There are slight size differences between males and females.
The lifespan of olive colobus monkeys has not been reported. Based on other leaf-eating monkey species, however, one might assume that they can live up to 30 years, with a lifespan of 20 years in the wild.
The olive colobus is the smallest of the colobus monkeys. As their name implies, their pelage is olive-colored with a tinge of brown and gray. This coloration camouflages them within the trees and reduces the risk of predation. They have a small crest on top of their heads. Their long tails can measure longer than their entire body length.
Like all colobus monkeys, olive colobuses have short, nub-like thumbs with four extra-long fingers that are used to wrap around tree branches like a hook. A longer thumb would hinder their ability to wrap their fingers around branches and swing freely during locomotion. Olive colobuses have the largest feet of any colobine monkey.
As folivores (leaf-eaters), olive colobuses mainly consume leaves and leaf parts, preferring young leaves. They supplement their diet with flowers and unripe seeds. Olive colobus monkeys forage in the understory and middle canopy for food.
Due to the high amount of foliage consumed, the olive colobus has a multi-chambered sacculated stomach to help with the breakdown of cellulose. Their complex stomachs also allow them to digest mature or even toxic foliage that other monkeys cannot. Since a diet of leaves is low in nutrition, they spend a great portion of their day foraging to meet their nutritional requirements.
Behavior and Lifestyle
Olive colobuses are arboreal (tree-dwelling) and diurnal (active during daylight hours) and are skilled climbers. They most commonly occupy the dense growth in the lower and middle forest strata but occasionally descend and travel on the ground. They may ascend higher when feeding or traveling in the company of other species.
Some of the main predators of olive colobus monkeys are humans, crowned hawk-eagles, leopards, and chimpanzees.
Olive colobus monkeys have a sympatric relationship with Diana monkeys.
Olive colobus monkeys have a very complex communication system, often making combination calls that are similar to sentences.
Olive colobuses are shy monkeys. Coupled with their drab coloration, they are difficult to observe in dense forest. Thus, they are not well studied. They live in small groups composed of multiple males, females, and their offspring. Group sizes can range from 3 to 15 individuals. There is no one single dominant male who monopolizes the females in the group. At maturity, all juveniles leave their natal groups and venture out to join other groups. Some females have been observed visiting groups other than their own for short periods of time strictly for mating opportunities.
Olive colobus monkeys frequently associate with other forest monkey species, especially with Diana monkeys (Cercopithecus diana). When Diana monkeys sound alarm calls, olive colobus monkeys freeze in place, sitting very still in the trees. The drab color of their pelage camouflages them. Their relationship with Diana monkeys has the benefit of reducing the risk of predation and attracting mates.
In some cases, scientists observing social interactions among olive colobus monkeys have noted that some males maintain years-long associations with Diana monkey troops. They further observed that, in those cases, females visit those male olive colobuses more frequently, and some females opt to become permanent members of those males’ troops. Scientists concluded that the males not only use their association with Diana monkeys for protection, but also to obtain mating partners. The females feel safer around males who have close associations with Diana monkeys and the males reap the benefits as a mechanism for obtaining new mates.
Communication in olive colobus monkeys is not well understood. However, some studies have found that they use context-specific alert calls for predator avoidance, to alert troop members to food sources or dangers, and to defend their home ranges. The most common calls are “hoos,” “zihs,” and “zucs.” These calls are used as alarm calls and as a response to disturbances, the presence of predators, falling trees or branches, and inter-group interactions; they are also used in peaceful contexts, such as in response to Diana monkey calls. Olive colobuses may combine calls to convey several messages at once, like information about disturbances and direction about how best to react.
These studies also found that olive colobuses rely heavily on Diana monkeys for alarm calls, especially to alert to the dangers of chimpanzees on the hunt. Diana monkeys emit a few quiet alarm calls upon spotting a chimpanzee. Olive colobuses respond to this call by staying in the lower canopy and sitting in a frozen posture. All other forest monkeys silently flee to the upper canopy.
Tactile communication includes grooming and playing. Visual cues, such as facial expression and body posture, are used as additional forms of communication.
Females reach sexual maturity at around 3–4 years of age, and males around 5–6 years. During estrus, females’ perineal regions swell significantly to signal readiness to mate, more so than in other colobus species. At the same time, olive colobus males have unusually large testicles. Since males do not dominate a harem of females, scientists speculate that these unusually enlarged sexual swellings may be related to the need to attract mates across multiple troops in densely forested habitats that may be interspersed with other forest monkeys. In other words, it may be a means of recognizing one another for mating.
Olive colobus monkeys are polygynandrous, a mating system in which both males and females have multiple mating partners. There is no specific breeding season. After a gestation period of 5 to 6 months, one offspring is born. Females generally reproduce every two years, typically bearing one offspring at a time.
After an infant is born, the mother carries him or her in her mouth for the first few weeks. Carrying an infant in the mouth is unusual among primate species. As the infant matures, he or she clings to the mother’s abdomen. Mothers provide milk, grooming, and protection to the young. The male’s role in infant care is not known.
Because their supplemented diet includes seeds and flowers, olive colobus monkeys disperse seeds throughout their range.
The International Union for Conservation of Nature lists the olive colobus as Vulnerable (IUCN, 2018) appearing on the IUCN’s Red List of Threatened Species. It was recently changed from the lesser threat of Near Threatened to the greater threat of Vulnerable due to a decline in population from habitat destruction and hunting. From 1990–2015, there was a 22.5% reduction of forest across eight West African countries where the olive colobus resides. When combined with data from hunting pressure, especially the bushmeat trade, it can be suspected that there has been at least a 30% reduction in population size over the past 27 years.
The cryptic nature of the olive colobus monkey—and their ability to survive in riverine forests and small forest fragments, and to adapt to some anthropogenic disturbances—has enabled them to persist in many parts throughout their range. However, their populations in Western Africa are expected to continually decline given the accelerating loss of habitat (in a region which has one of the highest rates of deforestation in the world) and continuing hunting pressure. Both of these threats are connected to an increase in human population growth.
Additional threats include logging, mining, and quarrying, as well as war, civil unrest, and military exercises in parts of Western Africa that are throughout the range of the olive colobus monkey.
The olive colobus monkey is listed as Class A under the African Convention and under Appendix II of the Convention on International Trade in Endangered Species (CITES). They also occur in a number of protected areas. In addition, the African Wildlife Foundation (AWF) works with pastoralists to develop sustainable solutions for agriculture and settlement growth by providing training on best practices and providing conservation when appropriate. Scientists of AWF use Geographic Information System (GIS) technology to identify key threats and to pinpoint which areas have the most potential for animal populations to thrive. Once these areas are identified, scientists can work with local communities and governments to save these areas for wildlife.
- Bene JCK., Zuberbueler, K. 2009. Sex Differences in the use of Vocalizations in Wild Olive Colobus Monkeys. European Journal of Scientific Research. 25(2): 266-279.
- Bene JCK., Ouattara, K., Bitty, EA., Kone, I. 2012. Combination calls in olive colobus monkeys (Procolobus verus) in Taï national Park, Côte d’Ivoire. Journal of Asian Scientific Research 2(9):466-477.
- Oates, J., G. Whitesides. 1990. Association between Olive Colobus, Procolobus verus, Diana guenons, Cercopithecus diana, and other Forest Monkeys in Sierra Leone. American Journal of Primatology, 21(2):129-146.
- Amanda H. Korstjens & Eva Ph. Schippers, International Journal of Primatology, Dispersal Patterns Among Olive Colobus in Taï National Park.
Written by Tara Covert, December 2020