Bale Monkey, Chlorocebus djamdjamensis
BALE MONKEY
Chlorocebus djamdjamensis
Geographic Distribution and Habitat
The Bale monkey goes by many different names. They are most frequently called the Bale monkey but have also been called the Bale Mountains grivet, Bale Mountains vervet, Djam-djam, and scientifically as Chlorocebus djamdjamensis. This small monkey can be found in, or is endemic to, the bamboo forests of the Bale Massif and Hagere Selam regions of the Ethiopian highlands. Their home ranges in an area of about 6,757 square miles (17,500 square km), with elevations ranging from 7,595 feet (2,315 m) to 10,663 feet (3,250 m).
Historically, all the monkeys in the Chlorocebus genus were classified under Cercopithecus. DNA research revealed that Chlorocebus monkeys are genetically distinct from guenon monkeys in the genus Cercopithecus. Since this distinction, all Chlorocebus monkeys have been colloquially called vervet monkeys, grivets, or green monkeys, interchangeably. With some researchers once calling the Bale monkey a subspecies of the grivet (Chlorocebus aethiops), Bale monkeys are a genetic enigma. Today, the general consensus is that the genus Chlorocebus has six distinct species: The Bale monkey (Chlorocebus djamdjamensis ), the green monkey (Chlorocebus sabaeus), the grivet monkey (Chlorocebus aethiops), the malbrouck monkey (Chlorocebus cynosuros), the tantalus monkey (Chlorocebus tantalus), and the vervet monkey (Chlorocebus pygerythrus). These six species appear somewhat similar but differ significantly in their habitat, appearance, and diet.
Currently, there is some debate among researchers that the Bale monkey can be separated into two subspecies: The Djam-Djam monkey (Chlorocebus djamdjamensis djamdjamensis) and the Bale Mountains monkey (Chlorocebus djamdjamensis harennaensis). Primarily driven by geographical differences in habitat, these two subspecies differ in their mitochondrial DNA, a special type of DNA that allows researchers to understand similarities and differences across species. Not only is their geographic separation causing this genetic divide, but researchers also note possible gene flow between Bale monkeys, vervet monkeys, and grivet monkeys, suggesting that the subspecies differentiation is caused by hybridization. In other words, since these Bale monkeys, vervet monkeys, and grivet monkeys live in similar areas and because they are closely related, these three species could possibly reproduce, creating Bale monkey hybrids. In fact, in the areas where habitats overlap, the monkeys appear to have similar physical characteristics that imply some gene flow. However, the three monkeys have not been observed living side by side, implying that the mixed-looking monkeys may instead be a product of hybridization more than a century ago. Due to this intermixing and the confusion on subspecies, more research is required to fully grasp the complexity of how Bale monkeys fit into the Chlorocebus genus.
Size, Weight, and Lifespan
Bale monkeys are sexually dimorphic, meaning that males are differently sized than their female counterparts. Males head to body length is 16.5-23.6 inches (42-60 cm) with an added tail of 18-30 inches (46-76 cm). Females, on the other hand, measure about 11.8-19.7 inches (30-50 cm) with tails adding an extra 16.1-26 inches (41-66 cm). Their tails are considered short in comparison to other Chlorocebus monkeys.
Males weigh in at 6.8-14.1 pounds (3.1-6.4 kg) while females weigh 3.3-10.8 pounds (1.5-4.9 kg).
There are no direct accounts of the Bale monkey lifespan, although similar species can live up to 25 years or even up to 30 years of age.
Appearance
Bale monkeys have long, thick fur. The pelage on their upper body is grizzled brown in color with a creamy white underside closer to the belly, which gives way to gray closer to the chest. Their limbs are grayish, with hands and feet a dark gray. The tail is grayish with an obvious brown-red tuft at the base. The face is black except for a bright white mustache and scruffy white beard. Their brows are made up of barely visible white hairs. Their appearance looks similar to other species that are closely related to them, but the Bale monkey is distinguished by their comparatively shorter tail, hard-to-see eyebrows, and shorter facial hair.
Diet
Bale monkeys are picky. Compared to their other Chlorocebus counterparts, Bale monkeys have a very distinct diet, largely consisting of one specific species of highland bamboo. Their diet is 80-82% made up of the young leaves of the highland bamboo. Younger leaves are softer and easier to digest than mature leaves, but are still a very niche choice of food. They fill what is called a unique ecological niche. Their bodies are built to digest and glean nutrients from these young bamboo leaves, giving them a food resource no one else wants. This guarantees that they will have a stable food source without having to compete as much with other animals. This is a great strategy to stave off starvation until that one food resource goes away. So, to ensure they don’t put all their eggs in one bamboo basket, the rest of the Bale monkey diet comes from fruits and flowers (9.6% of their diet), animal prey (2.3%), and finally, small amounts of shoots, stems, mature leaves, and roots.
Across seasons, scientists have found that Bale monkeys eat more fruit in the dry season, and more shoots are consumed in the wet season.
Since bamboo is one of their key food sources, disruptions to natural bamboo growth impact the species greatly. One study found that populations of Bale monkeys that live in degraded bamboo forests change their diet significantly. Since the ecosystem can no longer provide an adequate amount of bamboo necessary to maintain the Bale monkey’s diet, the monkeys have become resourceful, turning to other types of leaves, fruits, seeds, grasses, stems, and insects. However, bamboo was still the top preference for most groups living in these fragmented forests, with the exception of one group living in a habitat where bamboo is especially scarce. This reliance and preference further reinforce the importance of bamboo in the Bale monkey diet, and provide a glimpse into a concerning future if the fragmented bamboo forests continue to deteriorate.
Behavior and Lifestyle
Bale monkeys are arboreal, meaning they live in the trees, but this depends on their environment. Bale monkeys that live in more abundant and continuous bamboo forests spend only 2% of their time on the ground, while individuals in more fragmented forests can spend up to 36% of their day on the forest floor. They are different from other species in the Chlorocebus genus because they spend much more time in the trees than other closely related species.
Bale monkeys have relatively small home ranges and are most likely restricted to the boundaries of their most important food source, bamboo.
These monkeys are diurnal, meaning they are active during the day.
Bale monkeys are named after the Bale Mountains.
The scientific name, Chlorocebus djamdjamensis, comes from chloro (green) and cebus (monkey), with djamdjamensis referring to the Djam-djam mountains.
Dr. Addisu Mekonnen is one of the top researchers of the Bale monkey, currently running his long-term Bale Monkey Bamboo Research Project alongside other wildlife projects in Ethiopia
Bale monkeys love to eat. One study found that Bale monkeys spend 65.7% of their day feeding. The rest of the day is spent moving 14.1% of the time, 10.7% resting, 7.1% engaging in social activities, and the last 2.4% for other activities. Scientists found that these monkeys also tended to spend even more time feeding in the wet season than in the dry season. At night, Bale monkeys sleep in dense leafy trees. More often than not, the monkeys will choose trees they have recently fed on and will reuse sleeping sites.
Bale monkey group size ranges from 9 to 29 individuals, with an average group size of about 25.5 monkeys. Little is known about the Bale monkey social structure, but similar species have multi-male and multi-female groups with males leaving the group at sexual maturity. Once males leave their home groups, they form temporary bachelor groups until they find a new group to integrate into. Females do not leave their natal, or home, group.
Among the least studied primates in Africa, little is known about Bale monkey communication. While social interactions have been recorded, from aggressive fighting to affiliative grooming, researchers have yet to record specific vocalizations or body movements. Since they are social creatures, it is expected that they communicate through calls, gestures, and facial expressions similar to other species in their genus. For example, vervet monkeys have distinct calls for different predators, and some calls overlap for aggressive interactions. They also differ by sex, with males emitting alarm calls with more energy but at a lower frequency than females.
Similar to communication, little is directly known about the Bale monkey’s reproduction and family. It can be assumed that it is somewhat similar to other species in the Chlorocebus genus, with births occurring once or twice a year of a single infant. These infants are generally weaned at 6 months and typically reach sexual maturity at around 4–5 years.
The main source of food for the Bale monkey is the fast-growing and extremely invasive bamboo. Their constant consumption of this plant keeps the growth of it in check, aiding the delicate balance of the ecosystem they live in. Eating fruits and dispersing the seeds through feces also aid in the biodiversity of the ecosystem.
Bale monkeys are prey to spotted hyenas, African wolves, and birds of prey (e.g., tawny eagles, martial eagles, and augur buzzards). As animals of prey, they get incorporated into the food chain and contribute meaningfully to the ecosystem.
The Bale monkey is classified as Vulnerable by the International Union for Conservation of Nature (IUCN, 2016), appearing on the IUCN Red List of Threatened Species.
The current size of the overall Bale monkey population is unknown. In the past, Bale monkeys were abundant, inhabiting flourishing bamboo forests. In recent years, the population has noticeably declined, but has yet to be measured. A subspecies of the Bale monkey, the Djam-djam monkey, has been recorded to have less than 400 individuals remaining, classifying them as Endangered by the IUCN redlist (IUCN, 2022). The other possible subspecies, the Bale Mountain monkey, has been classified as Endangered as well (IUCN, 2022), with an uncertain number of monkeys remaining. The last known count was in 2007 at just below 1,500 individuals.
The largest threats to the Bale monkey are habitat destruction, hunting, and loss of the species through hybridization. Since the majority of their diet relies on young highland bamboo, human razing and harvesting of these forests has caused irreparable damage to the Bale monkey population. Bale monkey groups are thus fragmented and isolated in increasingly human-dominated areas.
There have also been recorded instances of human and Bale monkey conflict, most notably in regard to crop raiding. Landowners will drive Bale monkeys away with traps, spears, and dogs.
Lastly, there have been some studies that suggest that the Bale monkey is getting progressively more endangered because they are beginning to interbreed with the vervet and grivet species. Due to their reproductive compatibility and habitat overlap, it is possible for vervets, grivets, and Bale monkeys to reproduce, essentially erasing the Bale monkey as we know it. However, this has not been directly observed. The only evidence is some overlapping physical characteristics in regions where all three habitats coincide.
Bale monkeys 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 Bale monkey is also classified as Class B of the African Convention on the Conservation of Nature and Natural Resources.
The Bale monkey finds home in Harena-Kokosa National Forest Priority Area, Bale Mountains National Park, Bodie Mountain, Felada Mountain, Geramba Mountain, and Sucha Mountain, which offer general protection of the Bale monkey. However, the harvest of bamboo is still allowed in these areas, inevitably contributing to diminishing the Bale monkey habitat and food sources.
Kumara Wakjira of the Ethiopian Wildlife Conservation Department, in collaboration with the Darwin Initiative-funded Harenna project, the Bale Mountains National Park, and the University of Aberdeen, recently conducted the most up-to-date census of the Bale monkey and continues to provide more information on the species.
Scientists have recommended the end of bamboo harvesting in their habitat, as well as replanting initiatives to combat isolation and bring fragmented habitats back together.
- Mekonnen, A., Bekele, A., Fashing, P.J. et al. Diet, Activity Patterns, and Ranging Ecology of the Bale Monkey (Chlorocebus djamdjamensis) in Odobullu Forest, Ethiopia. Int J Primatol 31, 339–362 (2010). https://doi.org/10.1007/s10764-010-9389-3
- Mekonnen, A., Fashing, P.J., Bekele, A. et al. Dietary flexibility of Bale monkeys (Chlorocebus djamdjamensis) in southern Ethiopia: effects of habitat degradation and life in fragments. BMC Ecol 18, 4 (2018). https://doi.org/10.1186/s12898-018-0161-4
- Mekonnen, A., Fashing, P.J., Venkataraman, V.V. et al. Sleeping Site and Tree Selection by Bale Monkeys (Chlorocebus djamdjamensis) at Kokosa Forest Fragment in Southern Ethiopia. Int J Primatol 42, 915–932 (2021). https://doi.org/10.1007/s10764-021-00251-1
- Mekonnen, A., Rueness, E.K., Stenseth, N.C. et al. Population genetic structure and evolutionary history of Bale monkeys (Chlorocebus djamdjamensis) in the southern Ethiopian Highlands. BMC Evol Biol 18, 106 (2018). https://doi.org/10.1186/s12862-018-1217-y
- Price, T., Wadewitz, P., Cheney, D. et al. Vervets revisited: A quantitative analysis of alarm call structure and context specificity. Sci Rep 5, 13220 (2015). https://doi.org/10.1038/srep13220
- Mekonnen, Addisu & Fashing, Peter & Venkataraman, Vivek & Chapman, Colin & Stenseth, Nils Chr & Hernandez-Aguilar, R.. (2021). Sleeping Site and Tree Selection by Bale Monkeys (Chlorocebus djamdjamensis) at Kokosa Forest Fragment in Southern Ethiopia. International Journal of Primatology. 42. 10.1007/s10764-021-00251-1.
- IUCN redlist: Bale Mountains monkey
- IUCN redlist: Djam-djam Monkey
- IUCN redlist: Bale Monkey
- gbif.org: Bale Monkey
- Parks.it: Ethiopia
- Darwin Initiative: Biodiversity Monitoring in Forest Ecosystems in Bale Mountains National Park, Ethiopia
Written by Nami Kaneko, Oct 2025