Geographic Distribution and Habitat
The Dryas monkey (Cercopithecus dryas), also known as Salonga monkey, ekele, inoko, the Dryad monkey, Dryas guenon, and Salonga guenon, is one of the least known monkeys in Africa. Virtually everything about this monkey—from their range to their reproductive habits to their behavior—is not fully understood, and in some cases, not understood at all. What is known is that Dryas monkeys are endemic to the rainforests of the Democratic Republic of the Congo in the heart of the Congo Basin, the second largest tropical rainforest in the world. They live in at least two widely separated areas, the Kokolopori-Wamba area and the Lomami-Lualaba area, in 14 specific sites within these areas. It is unlikely that individuals from each broader area have direct contact with those from the other area.
Dryas monkeys favor secondary lowland forests, riparian forests, and swamp forests. They seem to avoid primary forest, preferring instead forests that have been disturbed by wind, flood, and elephants. That has worked out in their favor, as they have easily adapted to their new landscape dotted with gardens and fallow fields. Although they are very cryptic, they actually often live in close proximity to humans, even living up to the edges of people’s gardens in some areas.
There is a great deal of taxonomic confusion for Dryas monkeys, largely due to the fact that they are classified based on just a few hunted individuals. The first mention of a Dryas monkey in scientific literature was an individual captured in the early 1930s from an unknown population in the Democratic Republic of the Congo. Two more individuals were collected in the 1970s. One was originally thought to be a new species called the Zaire Diana monkey (C. salongo), although in the 1990s they were both determined to be Dryas monkeys, and C. salongo was combined with C. dryas. However, some researchers disagree, believing Zaire Diana monkeys to be a valid classification. In 2014, more examples of the species came to light.
Most recently, Dryas monkeys’ placement in their genus has been disputed. While the International Union for Conservation of Nature (IUCN) currently lists them as belonging to the genus Cercopithecus, the genus of the guenons, recent evidence suggests that they may be more suited to the genus Chlorocebus. This is being hotly debated in the current literature. It is possible that they may even belong to their own genus.
Size, Weight, and Lifespan
The average size and weight of Dryas monkeys are not known, but based on the closely related red-tailed monkey (C. ascanius), females are most likely about 15 inches (38 cm) long and weigh about 6.4 pounds (2.9 kg) on average. Males are a bit larger at 18 inches (46 cm), and weigh 9 pounds (4.1 kg) on average. Lifespans of guenons in captivity range from 20 to 30 years.
Dryas monkeys are on the smaller side, with the long arms, legs, and tail that one might expect of an arboreal (tree-dwelling) primate. They have a beautiful coloration. Their black, mask-like face is ringed with white, surrounded by an orange or yellow neck and chest. Their back is dark brown, and their legs are black. Their bellies, the insides of their arms and legs, and the bottom side of their tail is white. By far their most striking characteristic, however, is the males’ vivid blue scrotum and anogenital region, which is a characteristic shared with other species in Cercopithecus and other genera. Not all males have this, however, and it’s thought to be related to age or hormones, or a signal of health or status. Females have a similar blue rear, but it’s not quite as vivid as the males’. The aquamarine patch is further highlighted by a ring of white hair, which contrasts brilliantly with the otherwise brown and black fur of their backside. Females and juveniles are colored similarly to males, but the white areas around their shoulders and rear are smaller than males’.
Dryas monkeys eat fruit, both young and mature leaves, shoots, piths, seeds, insects, and mushrooms. Young leaves are notoriously difficult to digest, and a leaf-packed diet is unusual for small monkeys, whose diet is usually composed of more energy-packed foods like fruits and insects. It’s unknown what adaptations Dryas monkeys may have to be able to extract nutrients from young leaves.
Behavior and Lifestyle
Dryas monkeys are mostly arboreal and are usually found in the lower regions of the forest. Between four camera trap surveys of different strata of the forest, an overwhelming majority (97%) of Dryas monkeys were found between 6 and 33 feet (2–10 m) above ground. Dryas monkeys also forage on the forest floor. Based on the very limited observations that have been made of the monkeys, they seem to prefer dense, wet vine thickets in the understory, especially areas where a tree has fallen and allowed the understory vegetation to flourish. Their preference for such densely vegetated habitats—with plenty of spiky, spiny plants—may explain why they have evaded the notice of scientists for so long and why there are so few observations of them in the wild. Camera traps have been critical to understanding even the basic behaviors of these monkeys. When spooked, Dryas monkeys vanish silently into the dense thickets of vegetation that they live in. They emit no alarm calls and they don’t even run away. One scientist had an encounter with a Dryas monkey where the monkey was spooked, and hid, perfectly still, among the vines for over an hour.
Despite Dryas monkeys sometimes making their homes at the edges of villages and gardens, they are considered one of the most rare and elusive monkeys in Africa—a true testament to their stealthy skills.
Group size ranges from 2 to 15 individuals. At each of the 14 sites where Dryas monkeys are known to live, there are between one and six groups living there. About half of the individuals in each group are adults. Some groups have only one adult male, while others have several. They also interact closely with other primate species, such as red-tailed monkeys (C. ascanius). Based on very limited observations, their home range size may be 7 to 17 acres (2.8–7 ha), although it may be much larger than this.
Communication among Dryas monkeys is very poorly understood. No loud calls have ever been recorded, although it is known that they make quieter contact calls to keep in touch with their groupmates, emitting chirps, murmurs, and chuckles. They are believed to be rather quiet, with none of the loud booming calls of their close cousins.
As an exceedingly rare and elusive monkey, there is little to no information available on the reproductive habits of Dryas monkeys. Based on the composition of the few groups that have been observed, and from what is know about other guenons, Dryas monkeys are likely polygynous, with each male mating with multiple females. They most likely breed year-round and have one baby at a time. Gestation lengths among guenons range from five to seven months. Mothers provide all of the newborn care after the babies are born. Female guenons reach sexual maturity at about four or five years; males reach it a bit later at about six years.
As fruit eaters, Dryas monkeys may serve as seed dispersers to some of the plants they eat. They are likely preyed upon by predators of the Congo basin, such as leopards.
Dryas monkeys are classified as Endangered by the International Union for Conservation of Nature (IUCN, 2019), appearing on the IUCN Red List of Threatened Species. In 2008, they were classified as Critically Endangered, but their rank was changed following the discovery of Dryas monkeys in eight locations in Lomami National Park, suggesting that their range and population is larger than first thought. There are now 26 known groups of Dryas monkeys, with a total known population of fewer than 250 individuals. Genetic research suggests that, despite their low population size, there is little inbreeding among Dryas monkeys and they may be sufficiently genetically diverse for long-term survival. While their future is undoubtedly fragile, the recent trend for Dryas monkeys has been positive.
Because Dryas monkeys actually tend to prefer secondary forest, they are not at as much risk of habitat disturbance as many other primate species. Instead, the main threat facing Dryas monkeys is hunting pressure. Although illegal, Dryas monkeys are a target of hunting. Much of the information known about the morphology and distribution of Dryas monkeys is from individuals that were shot by poachers. Even if Dryas monkeys aren’t a main target of poachers, any amount of hunting pressure puts the species in significant peril due to their extremely small and tenuous population. As long as any number of Dryas monkeys are the target of poachers, they are faced with extinction.
Dryas monkeys are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES). Only three of the 14 known sites of occurrence for Dryas monkeys are within the Lomami National Park. One more is in the Luo Scientific Reserve, another is in the new Iyondji Community Bonobo Reserve, and four more are in the Kokolopori Bonobo Reserve. This leaves six sites unprotected—and for a species with a population of less than 250, this is a major gap in protection. The top conservation priority to protect Dryas monkeys is conducting research to better understand their overall population size and range.
- Alempijevic, D., Boliabo, E. M., Coates, K. F., Hart, T. B., Hart, J. A., & Detwiler, K. M. 2021. A natural history of Chlorocebus dryas from camera traps in Lomami National Park and its buffer zone, Democratic Republic of the Congo, with notes on the species status of Cercopithecus salongo. Am J Primatol. 83. https://doi.org/10.1002/ajp.23261
- van der Valk, T., et al. 2020. The Genome of the Endangered Dryas Monkey Provides New Insights into the Evolutionary History of the Vervets. Molecular Biology and Evolution. 37(1):183–194. https://doi.org/10.1093/molbev/msz213
Written by K.Clare Quinlan, April 2022