The blue monkey or diademed monkey is a species of Old World monkey native to Central and East Africa, ranging from the upper Congo River basin east to the East African Rift and south to northern Angola and Zambia. It sometimes includes the Sykes’, silver, and golden monkey as subspecies.
Despite its name, the blue monkey is not noticeably blue: it has little hair on its face, and this does sometimes give a blue appearance, but it never has the vivid blue appearance of a mandrill, for example. It is mainly olive or grey apart from the face (which is dark with a pale or yellowish patch on the forehead – the “diadem” from which the species derives its common name), the blackish cap, feet and front legs, and the mantle, which is brown, olive or grey depending on the subspecies. Typical sizes are from 50 to 65 cm in length (not including the tail, which is almost as long as the rest of the animal), with females weighing a little over 4 kg and males up to 8 kg.
The blue monkey is found in evergreen forests and montane bamboo forests, and lives largely in the forest canopy, coming to the ground infrequently. It is very dependent on humid, shady areas with plenty of water. It eats mainly fruit and leaves, but will take some slower-moving invertebrates. It prefers to live in tall trees which provide both food and shelter, and is therefore, like almost all guenons, suffering from the loss of its natural habitat. Where pine plantations replace natural forest, the monkey may be treated as a threat by foresters, since it sometimes strips the bark from exotic trees in a search for food or moisture. It is also hunted for bush meat.
The blue monkeys live in female-philopatric social systems where females stay in their natal groups while males disperse once they reach adulthood. As a result, blue monkey groups usually consist of one male with several females and infants, giving rise to matrilinear societies. Occasionally, solitary males are observed which are probably transient, having left their natal group in search of a new group (Rudran 1978).
In these female-bonded societies, only 5–15% of monkeys’ activity budget is occupied by social interactions and the most common social interactions within a group are grooming and play.Relationships between group members vary: infants interact most frequently with their peers and adult or juvenile females and are rarely seen near adult males (Rudran 1978).
Alloparenting is common among blue monkeys. The most common infant handlers are juvenile females and usually one infant is carried by a number of alloparents. One hypothesis is that this allows the infant to learn to socialise at an early stage in life.
Interesting female-female relationships exist among blue monkeys. This relationship is believed to be shaped by their feeding ecology, which, in turn, is shaped by between-group and within-group competition. Blue monkey females exhibit strong, aggressive competition between groups and between other species because of their territorial character but milder though more frequent competition within groups. Even though earlier beliefs were that blue monkeys are not territorial, more current extended research shows that earlier researchers misinterpreted the results because social interactions overall are infrequent. Moreover, overall agonism rates in blue monkeys are very low. Within group conflicts are mild and infrequent because females tend to distance themselves from one another and feed at different sites, thus avoiding competition. Though it was believed that blue monkeys are egalitarian, current extended research confirms that there actually is linear dominance hierarchy in female blue monkeys, which becomes more apparent when food resources are scarce.
The mating system is polygynous, and there is a corresponding sexual dimorphism in size, with the males substantially the larger sex. Females normally give birth every two years, during the onset of the warm, rainy season; gestation is around five months, and the infants are born with fur and with their eyes open. Group sizes range from 10 to 40, containing only a single adult male. It is often found in groups with other species of monkeys such as the red-tailed monkey and various red colobus monkeys.