Geographic Distribution and Habitat
The Bale monkey (Chlorocebus djamdjamensis), also known as the Bale Mountains grivet, Bale Mountains vervet, and Djam-djam, is endemic to the highlands of Ethiopia and found east of the Rift Valley in the Bale Mountains and Sidamo Highlands. It is one of six species in the genus Chlorocebus, but it is unique among them in having a very restricted distribution and specialized habitat.
Unlike other members of Chlorocebus (such as vervets or grivets) that are habitat generalists and found in a wide variety of habitats, the Bale monkey exclusively inhabits either bamboo forests or areas that used to contain bamboo forest. Ecologically speaking, it therefore has more in common with species such as golden monkeys and bamboo lemurs than it does with other members of Chlorocebus. Black-and-white colobus monkeys are sympatric with Bale monkeys in many locations and researchers suspect that there is some hybridization between grivet monkeys and Bale monkeys at certain sites.
Size, Weight, and Lifespan
There is some sexual dimorphism in this species; males weigh up to 14.1 lb (6.4 kg), while females weigh up to 10.8 lb (4.9 kg). Combined head and body length measures about 16.5–23.6 in (42–60 cm) for males and 11.8–19.7 in (30–50 cm) for females. The Bale monkey’s tail is distinctly shorter than the closely related grivet or vervet monkey at around 18–29.2 in (46–76 cm) in males, and 16–26 in (41–66 cm) in females.
Since they are less studied than other primates, the average lifespan of this species in not known, although similar species can live to be around 25 years old.
The Bale monkey has grayish-brown coat, with some yellow and red tones, especially on the top of the head. This pelage turns white on the underside of their torso, arms, and legs. Their dark faces are framed on either side by fluffy white whiskers, giving them the impression of having a white beard and moustache. Their hands, feet, and ears are all a blackish-brown. They look quite similar to their close relatives, the vervet monkey and grivet, but can be distinguished by their shorter tail, shorter facial whiskers, and barely visible white brow.
As well as living in a specialized habitat, Bale monkeys also have a highly specialized diet. While grivets and vervets have a wide-ranging diet and are considered opportunist omnivores, the majority of the Bale monkey’s diet comes from just one plant: bamboo. The young leaves of bamboo account for over 75% of the food consumed by Bale monkeys. The rest of their diet generally consists of plant matter from just a handful of other species, although they do sometimes eat small animals.
Unfortunately, at several human-dominated sites, Bale monkeys have lost much of their natural diet of bamboo through deforestation. To supplement their diet, they engage in crop-feeding; Bale monkeys have been known to feed on barley, other cereals, vegetables, and fruit that are grown by local farmers. The resulting human-wildlife conflict has led to the hunting and trapping of these monkeys by farmers.
Behavior and Lifestyle
Bale monkeys spend the majority of their time feeding and moving, and only a relatively small amount of time resting or engaging in social activities. In the wet season, they spend more time feeding than during the dry season. They are a diurnal species, sleeping during the day, and traveling approximately 0.62 miles (1 km) each day.
Bale monkeys living in bamboo forests are primarily arboreal, spending most of their time in the trees and only rarely descending to the ground. However, populations living in forest fragments, where much of the bamboo has been eradicated, spend more time traveling and foraging on the ground.
Unlike their closest relatives, Bale monkeys inhabit a highly specialized niche, living primarily in bamboo forests and feeding on the young leaves of bamboo.
They are one of the least-studied African primates and endemic to a small area in Ethiopia.
Depending on the site, group sizes can range from 9 to 60 individuals, although average sizes tend to be around 20 individuals. Each group has a home range of around 0.05–0.07 square miles (12–18 ha).
Many aspects of their social lives remain a mystery, although similar species live in multi-male, multi-female groups, where males will migrate to a new group upon reaching sexual maturity.
Being much less studied than other African primates, little is known about the communication of the Bale monkey. However, similar species communicate via a range of modalities, including the use of vocalizations, gestures, facial expressions, and scent marking. It is likely that Bale monkeys use a range of calls and gestures to communicate with one another, although these have yet to be described by researchers.
Very little is known about reproduction in the Bale monkey, although other members of the Chlorocebus genus give birth once or twice a year to a single infant. Infants are generally weaned by about 6 months of age and usually reach sexual maturity at around 4–5 years of age.
Bale monkeys likely have a lesser impact on the environment than frugivorous (fruit-eating) primates, who aid in seed dispersal, since they primarily feed on young leaves. Nonetheless, their preference for the young leaves of bamboo likely has a limiting effect on the growth of this fast-growing plant.
The Bale monkey is currently listed as Vulnerable by the International Union for Conservation of Nature, appearing on their Red List of Endangered Species (IUCN, 2019). As with so many other primates, the greatest threat to Bale monkeys is habitat destruction. The (currently legal) destruction of the bamboo forests that they inhabit, for local consumption and commercial trade, is almost certainly leading to population decline.
For populations in human-dominated landscapes, hunting or trapping by humans is also a major threat, which arises when these monkeys feed on human cultivated crops. This again relates to habitat loss; as their primary food source, bamboo, is removed, Bale monkeys need to find other food sources and often turn to crops.
The Bale monkey is listed on Appendix II of the Convention on International Trade in Endangered Species (CITES) and on Class B of the African Convention on the Conservation of Nature and Natural Resources.
Scientists studying Bale monkeys recommend that bamboo harvesting be outlawed in key areas of Bale monkey habitat in order to prevent their extinction. They also recommend planting bamboo and other trees within and between forest fragments to allow for migration between populations. Further research into this elusive species is also required to make more informed assessments and recommendations for their conservation.
- Mekonnen, A. (2008). Distribution of the Bale monkey (Chlorocebus djamdjamensis) in the Bale Mountains and its ecology in the Odobullu Forest, Ethiopia—A study of habitat preference, population size, feeding behaviour, activity and ranging patterns. Addis Ababa University, Addis Ababa (Ph. D. thesis, MSc thesis).
- Mekonnen, A., Bekele, A., Hemson, G., Teshome, E., & Atickem, A. (2010). Population size and habitat preference of the Vulnerable Bale monkey Chlorocebus djamdjamensis in Odobullu Forest and its distribution across the Bale Mountains, Ethiopia. Oryx, 44(4), 558-563.
- Mekonnen, A., Bekele, A., Fashing, P. J., Hemson, G., & Atickem, A. (2010). Diet, activity patterns, and ranging ecology of the Bale monkey (Chlorocebus djamdjamensis) in Odobullu Forest, Ethiopia. International Journal of Primatology, 31(3), 339-362.
- Mekonnen, A., Bekele, A., Fashing, P. J., LERNOULD, J. M., Atickem, A., & Stenseth, N. C. (2012). Newly discovered Bale monkey populations in forest fragments in southern Ethiopia: evidence of crop raiding, hybridization with grivets, and other conservation threats. American Journal of Primatology, 74(5), 423-432.
- Mekonnen, A., Fashing, P. J., Bekele, A., Hernandez‐Aguilar, R. A., Rueness, E. K., Nguyen, N., & Stenseth, N. C. (2017). Impacts of habitat loss and fragmentation on the activity budget, ranging ecology and habitat use of Bale monkeys (Chlorocebus djamdjamensis) in the southern Ethiopian Highlands. American journal of primatology, 79(7), e22644.
- Mekonnen, A., Fashing, P. J., Sargis, E. J., Venkataraman, V. V., Bekele, A., Hernandez‐Aguilar, R. A., … & Stenseth, N. C. (2018). Flexibility in positional behavior, strata use, and substrate utilization among Bale monkeys (Chlorocebus djamdjamensis) in response to habitat fragmentation and degradation. American journal of primatology, 80(5), e22760.
Written by Jennifer Botting, PhD, August 2020