BLUE MONKEY

Cercopithecus mitis

Geographic Distribution and Habitat

Most commonly known as the diademed or blue monkey, Cercopithecus mitis is an Old World primate widely distributed across Africa in a large variety of environments. This arboreal species dwells high in the canopy of contiguous and fragmented lowland and montane tropical moist forests, riverine and gallery forests, delta and bamboo forests, and sand forests. Blue monkeys tolerate secondary forests, logged forests, and thickets.  

Their territory crosses many countries, including Angola, Burundi, Congo, DRC, Ethiopia, Kenya, Malawi, Mozambique, Rwanda, Somalia, South Sudan, the United Republic of Uganda, Zambia, Zimbabwe, Swaziland, and South Africa.  

There are several sub-species, the reported number of which varies depending on the source. For example, the Sykes monkey is sometimes listed as a species in its own right, and other times is listed as a sub-species of the blue monkey. Sub-species are determined on the basis of DNA analysis, vocalizations, facial patterns, proteins, morphology, and distribution patterns. As of March 2017, seventeen blue monkey subspecies are recognized by the International Union for Conservation of Nature (IUCN).

Blue monkey geographic range map. Map: IUCN, 2019

Size, Weight, and Lifespan

Males are larger than females at 17 pounds (8 kg) on average, versus the female’s average weight of 8 pounds (4 kg). Their body is 19-25 inches (50-65 cm) long and their tail is about the same length as their body.

Maximum lifespan in the wild is 20 years; it is about 30 years in captivity.

Appearance

Contrary to what one would expect, the blue monkey is not actually blue. Their pelage is mostly gray and speckled. Their dark face is pear-shaped, with rounded cheeks and a narrow top of the head. The nose area looks like a droplet that ends with a small mouth and chin, both accentuated with patches of pale, yellowish fur. Their whiskers shoot backward and downward, and they have cheek pouches to store food as they forage. Their almond eyes are very expressive and brown-orange in color. Their eyelids are a light shade of gray, and their forehead is crowned by a whitish diadem. The top of their head is black.

The adult male’s jaw projects out more than that of adult females and they have larger canines.

The fur on their chest is a lighter shade. The long hair on their shoulders and saddle varies from dark gray to greenish or yellowish, and gives the appearance of a mantle.

The blue monkey’s body is rather large. Their forelimbs are short and their hind limbs are slender and well adapted to jumping and arboreal life. Their tail is as long as their body, and is used for balance. Their hands and feet have five long digits, including one opposable digit.

Newborns are brown/black without the grizzled pelage. They sometimes have a faint diadem. Their cheeks do not look round yet, and their head and ears appear disproportionately large compared to the rest of their body.

Photo credit: Charles L Sharp/Creative Commons
Diet

Blue monkeys consume a wide variety of food items, which allow them to survive in fragmented and degraded forests. 

In continuous forests, the blue monkey consumes mostly fruit (40-50%) and young leaves and shoots (30%), and supplements her diet with mature leaves, bark, gum, fungi, flowers, invertebrates, birds, birds’ eggs, reptiles (small snakes and lizards), small primates (galagos), and sometimes small mammals (bats, rodents, squirrels).

In fragmented and degraded forests with a dense herb layer, the blue monkey switches to a more folivorous diet, with shoots and young leaves being the preferred food items, followed by animal prey. They occasionally venture out to raid human crops and gorge on barley or other seeds. In some areas, blue monkeys eat bamboo, which contains high levels of cyanide; it is not clear how they process it. It is not understood how they find adequate supplies of sugar, as their diets require, in such environments.

Where fruit is abundant, the blue monkey prefers fleshy, sweet, ripe fruit over unripe fruit. Figs, which are high in fructose, are a favorite. Interestingly, adult males usually consume more fruit than females and subadults. In other primate species, females eat more fruit, especially around mating time.

The reason the blue monkey is able to have such a diverse diet lies in the morphology of her digestive system. The foregut is used for the digestion of fruit and fermentation of ingested food. The large caecum and colon in the hindgut contain many bacteria that allow fermentation and digestion of fibrous items, such as leaves.

Behavior and Lifestyle

Blue monkeys spend most of their time in the canopy, rarely descending to the ground. Groups divide their day into chunks of activities: sunbathing in the early hours of the morning or after a rainstorm when temperatures are low, foraging for food, resting at midday, and more foraging, before retiring for the night.

When foraging, individuals usually do not stay next to each other—especially when the crown of the fruit tree is not large enough. Small trees with a 6.5 feet (2 m) crown usually do not accommodate more than two monkeys; medium trees with a crown up to 25 feet (8 m) can accommodate 3-8 monkeys; larger trees can host more monkeys. The usual distance between individuals of the same group while foraging is 300 feet (100 m) or more when the groups are large.

At night, the group gets back together to sleep. Blue monkeys do not build nests. They huddle on branches in subgroups of 2-4 individuals.

Fun Facts

Blue monkeys have been observed taking bites of meat and eating leaves at the same time.

Daily Life and Group Dynamics 

Blue monkey groups have set territories and well-defined boundaries, which they expect other groups to respect. Territorial boundaries are not necessarily linked to a prized food item, although most antagonistic behavior between groups can be triggered by such a food item.

They live in groups that typically include 10-25 individuals. The groups are composed of one male, several adult females, plus juveniles and subadults of both genders.

Females are philopatric (i.e., remain in their natal group) and males disperse when they reach maturity. Males can be solitary during their transition from their natal group to a group of their own. They can also associate with other males and live the “bachelor” life for a while. These all-male groups are fluid—members can come and leave as they please—and usually include 2-12 individuals.

During mating season, non-resident males may follow or join a group for a few hours or a few months. The resident male may be challenged by a non-resident male and lose to him. The new resident male may remain in place for a few months, or up to six years.

When a group gets too big, it splits in two and divides the territory accordingly. It only takes about a month for the two groups to develop antagonistic relations with each other.

Females seem to instigate most of the aggression that occurs within the group or with an outside group. Within the group, these events tend to occur while feeding, but do not result in fights. A high-ranking female may interrupt a low-ranking female by approaching. The low-ranking female retreats and the conflict is avoided. If conflict is unavoidable, it usually ends quickly and injuries are not life-threatening. Rank is important in blue monkey society, but it does not seem to be related to the age of a female and does not seem to impact reproduction success. Rank dominance is also more apparent in times of food shortage.

When antagonistic behavior occurs towards a neighboring group, the attacking females (adults and juveniles) chase and bite their opponents. Males and youngsters of both sexes are rarely involved. In fact, juvenile males have no second thoughts when they feel like hanging out and playing with young males of a neighboring group. Combats between female opponents may last several minutes and cause severe or even fatal injuries. After the fight, females of the same group cluster with 2 or 3 partners and engage in intense allogrooming sessions (mutual social grooming), sometimes switching partners. This is strikingly different from their regular behavior, since blue monkeys prefer to keep at a safe distance from one another. This species is not particularly social; females spend less than 5% of their time engaged in social activities, such as grooming, and grooming between mother and daughter is not consistent, especially when they are close in age.

In most primate societies, strong female coalitions are advantageous. This is not the case with the blue monkey. In fact, studies have shown that blue monkey females who maintain weak bonds with the same or different partners have a lower mortality rate than those who maintain stronger bonds. Female coalitions are few, even if females have been occasionally observed protecting one another against adult male assaults and against potential predator attacks.

Blue monkeys share overlapping territory with different primate species, and their relationships can vary greatly based on who their neighbor is. They avoid baboons and chimpanzees, for instance, but occasionally associate with red colobus monkeys or red-tailed monkeys. Conflicts, when they occur, usually involve only a few individuals. Random cohabitation with another primate species can be beneficial and protect the blue monkey against some predators. Sometimes cohabitation is more than peaceful and a few cases of hybridization between red-tailed monkeys (another guenon species) and blue monkeys have been observed in East Africa.

Communication

Like all primate species, blue monkeys communicate through vocalizations, body posture, and facial expressions. Interestingly, unlike other primates, the blue monkey does not seem to use any special signal or call to recruit coalition partners.

Males regularly use six identified calls: ant, boom, ka, katrain, nasal screams, and pyow, which are loud and at a low frequency. Females and juveniles of both sexes can be heard chirping, grunting, growling, or geckering. Infant calls are much quieter.

Some calls occur in specific contexts, while others occur in many contexts. Pyows, for instance, draw attention to the caller and serve multiple purposes. They can be spontaneous, for no specific reason, or they can be used to alert the group to the presence of a predator. They occur during intergroup fights—especially male-male aggression—but are also used to attract a mate during mating season. They are loud tonal calls that can be heard half a mile (1 km) away. Humans can hear them at a distance of 0.2 miles (400 m). They are usually uttered in clusters of 3 or 4 at ten-second intervals, and can be combined with other calls.

Booms are very low frequency calls. Not audible to humans, they can be perceived by blue monkeys up to half a mile away (1 km). Males use this call to interact with females in their group or with individuals outside their group. Booms can be followed by pyows. When female aggression occurs, it is not unusual for males to utter series of booms and pyows.

Ants are short calls that are linked to the presence of terrestrial predators, like leopards, dogs, or snakes. They are not predator-specific.

Kas are short, loud calls that seem related to aerial predators.

Katrains are a rapid sequences of kas that are separated by an “urrr” sound. They are used in association with kas and pyows and are related to aerial predators.

Nasal screams are used by the male recipient of an intense aggression and can be combined with other vocalizations. These are only heard by individuals in close proximity to the caller.

Geckers, eerns, and grunts are quiet vocalizations. Geckers are short raspy sound sequences. Eerns are produced with a partially or fully closed mouth. Grunts are short and used by females and juveniles of either sex.

During incursions, members on the periphery of the impacted group chirp loudly when they notice intruders and start chasing them out.

Other ways of communicating include physical displays. Aggression is manifested through chases, slaps, bouncing, glaring, head-bobbing, bites, and growling.

Cowering, presenting hindquarters, geckering, and trilling are used by submissive individuals to avoid conflict.

Reproduction and Family

Female blue monkeys become adults and give birth to their first offspring at six or seven years of age. Adult males leave their natal group between the ages of six and eight.

Females go into estrus every 28 days in the wild and every 32 days in captivity. Both males and females can make their interest known to the object of their love by following him or her and shaking their heads. However, some males resent too much attention and can become aggressive when a female is too persistent. In blue monkey society, males mate with multiple females, whereas females only mate with one male.

Breeding can occur throughout the year, but births seem to be more frequent in the dry season, after a gestation period of 176 days. Babies weigh less than a pound at birth (400 g), they have fur, and their eyes are open. They are carried by their mothers for the first two weeks of their life. Juvenile and subadult females can play the role of babysitter and are allowed to carry infants as young as 2 or 3 days old. Mothers and infants stay in close physical contact for about six weeks, at which time infants start eating solid food. From 6 to 10 weeks of age, infants interact much less with their mothers and start interacting with other infants and juvenile females. By the time they are 22 weeks old, they only spend 10% of their time with their mothers. However, they can continue suckling until they are 2 years old.

Infant mortality is high and is 18-25% higher when a new resident male takes over a group. Infanticide forces females into estrus, although it does not increase their reproduction chances. Indeed, females are most likely to conceive if they do not have dependent infants and did not give birth in the last two years.

Photo credit: Charles L Sharp/Creative Commons
​Ecological Role

Blue monkeys play an important role is seed dispersals for fruit and other plants.

Conservation Status and Threats

The International Union for Conservation of Nature Red List (IUCN, 2017) categorizes Cercopithecus mitis as Least Concern, but some sub-species are categorized as either Vulnerable (such as Cercopithecus mitis boutourlini), Endangered (Cercopithecus mitis kandti), or Critically Endangered (Cercopithecus mitis zammaronoi). This is because the species is found in many African countries and different environments. All blue monkey populations are effected by habitat loss and fragmentation, but to varying degrees depending on their location.

The blue monkey groups at Kibale National Park (Uganda) have decreased by approximately 30% in logged areas over the last fifteen years compared to unlogged areas; by contrast, the blue monkey population in logged areas of the Budongo Forest Reserve (Uganda) is 3.7% higher than in unlogged areas. Kakamega Forest National Reserve (Kenya) has been under attack by logging companies for many years. Old indigenous trees have been and continue to be harvested legally and illegally for rare timber. Although only mature trees (30-35 years old) should be harvested and replanting should occur, it does not always go as it should. Habitat fragmentation has been accentuated by excisions reserved for settlement, schools, and tea plantations. Logging and the lack of rain have also contributed to the shortage of fresh water as rivers are drying up. This not only affects the blue monkey species, but also the local human population and many other species.

The situation is much worse in Ethiopia, where Cercopithecus mitis boutourlini is confined to natural and riverine forests. As large trees are decimated for profit, blue monkeys—which prefer these for roosting, feeding, and predator avoidance—are losing out, and could very well become extinct in a few years.

Conservation Efforts

Although the blue monkey species has proven able to adapt to fragmented habitats, more studies are needed, and regional-specific plans to prevent its decimation should be developed.

SPECIAL THANKS

Special thanks to Dr. Marina Cords, Professor at Columbia University, Department of Ecology, Evolution and Environmental Biology and Department of Anthropology, for providing facts and documentation for this Primate Species Profile.

References:
  • IUCN redlist 2017-3
  • Old World Monkeys – edited by Paul F. Whitehead, Clifford J. Jolly – Chapter on:  “Agonistic and affiliative relationships in a blue monkey group” – Marina Cords
  • Primates of Western Uganda – edited by Nicholas E. Newton-Fisher, Hugh Notman, James Durward Paterson, Vernon Reynolds – Multiple functions and signal concordance of the pyow loud call of blue monkeys – Marina Cords and James L. Fuller
  • The Kakamega Forest – Unesco – World Heritage Centre website
  • Kenya Daily Nation news – ‘Concerns raised as KFS permits harvesting of forest trees’ by Nicholas Komu and ‘Forces of greed, activism to decide fate of Kakamega Forest’ by Benson Amadala
  • The Vocal Repertoire of Adult Male Blue Monkeys (Cercopithecus mitis stulmanni): A Quantitative Analysis of Acoustic Structure – James Lewis Fuller
  • Friendship among adult female blue monkeys (Cercopithecus mitis) – Marina Cords 
  • Mammals of Africa – Volume II – Primates – edited by Thomas M. Butynski, Jonathan Kindgon and Jan Kalina –  “Cercopithecu mitis Gentle Monkey (Diademed Monkey, Blue Monkey, Sykes Monkey).
  • Group size but not dominant rank predicts the probability of conception in a frugivorous primate – Su-Jen Roberts and Marina Cords
  • Ecological Flexibility in Boutourlini’s Blue Monkeys (Cercopithecus mitis boutourlinii) in Jibat Forest, Ethiopia: A Comparison of Habitat Use, Ranging Behavior, and Diet in Intact and
  • Fragmented Forest – Dereje Tesfaye – Peter J.Fashing – Afework Bekele – Addisu Mekonnen – Anagaw Atickem
  • Stronger social bonds do not always predict greater longevity in a gregarious primate – Nicole A. Thompson – Marina Cords
  • Blue Monkey (Cercopithecus mitis stuhlmanni) Predation on Galagos – Thomas M. Butynski
  • Population size, habitat association and dietary composition of Boutourlini’s blue monkeys (Cercopithecus mitis boutourlinii) in Komto Protected Forest, Western Ethiopia – Mosissa Geleta and Afework Bekele.

Written by Sylvie Abrams, June 2018