Blue Monkey

The blue monkey (Cercopithecus mitis) is a widespread and highly variable forest guenon of eastern, central and southeastern Africa. Regional names include diademed monkey, Sykes’ monkey and samango monkey, while several distinctive geographic forms also have well-known names of their own, such as the golden monkey (Cercopithecus mitis kandti).

Colour varies greatly across the range, but the coat is often olive-grey with darker arms and legs, dark exposed skin and a pale or yellowish brow band. Head-and-body length is about 50–65 cm, with a tail of similar length. Males may weigh up to about 9 kg, while females generally weigh 4–5 kg.

Taxonomy within the Cercopithecus mitis complex remains unusually fluid. Some primate classifications split Sykes’ monkeys, silver monkeys, golden monkeys and several other regional forms into separate species, while Mammal Diversity Database v2.5 retains a broad C. mitis and includes forms that have also been treated as C. albogularis, C. doggetti and C. kandti.

The form photographed on this page at Cape Vidal is Stairs’ white-collared monkey (Cercopithecus mitis erythrarchus), one of the forms commonly called samango monkey in southern Africa. Under the broad MDD treatment, this and other regional forms remain within one highly variable species, although their taxonomic rank is still debated.

I have also seen the geographically distinctive population around Lake Manyara and Ngorongoro in northern Tanzania that was described in 2020 as the Manyara monkey (C. m. manyaraensis), although I have never photographed it. Mammal Diversity Database v2.5 currently treats manyaraensis as a synonym within the broader C. mitis, while the name is still used for this local population in parts of the literature.

Blue monkeys are primarily forest animals, occupying evergreen, riverine, coastal and montane forest with good canopy cover and reliable access to water. They are strongly associated with continuous or well-connected tree cover, although they can use forest edges and some disturbed forest where enough canopy remains.

Fruit, flowers and leaves form most of the diet, supplemented by insects and other invertebrates and occasional small vertebrates. At Cape Vidal in South Africa, samango monkeys maintained a fruit-rich but flexible diet through the year despite strong seasonal changes in food availability, showing how the species can shift among plant foods without abandoning its overall preference for fruit.

Blue monkeys are diurnal and mainly tree-dwelling, spending much of the day moving and feeding in the canopy. They are often shy in less disturbed forest, but animals accustomed to people may spend considerably more time on the ground.

Blue monkeys typically live in groups of roughly 10–40 animals centred on related females and their young. The social system is usually polygynous—one resident adult male lives with several adult females—but it is flexible: males from outside the group can enter during the breeding season, and some groups may temporarily contain more than one adult male.

Females normally remain in the group in which they were born and form long-term social relationships and a clear dominance hierarchy. Competition over fruit can make rank more important, and higher-ranking females may gain better access to preferred foods during difficult periods. Grooming, vocalisations, facial expressions and body posture help maintain these relationships and coordinate the group.

Males leave as they mature. In South African populations, young males may remain with their birth group until around six years of age, after which they can live alone or associate temporarily with one or two other bachelors before attempting to join a female group. Long-term data from Kenya found that resident males held this role for an average of 2.8 years, but with wide variation from 1 to 8 years. Genetic studies also show that males from outside the group can father young, so the resident male is not the only male that reproduces within a group.

Young monkeys also become part of a wider social network early in life. The species shows alloparental care, in which individuals other than the mother help handle, carry or care for the young. Large juvenile females in particular may act as caretakers before producing young of their own, gaining experience with infants in the process.

Predation has shaped some of the blue monkey's most distinctive communication. In the Ugandan form Cercopithecus mitis stuhlmanni, adult males give different alarm calls to crowned eagles (Stephanoaetus coronatus) and leopards (Panthera pardus): eagle threats mainly trigger “hacks”, whereas leopard growls trigger “pyows”. Experiments using recorded calls showed that listeners respond in ways that match the type of predator being signalled.

Risk also changes how the monkeys use different heights in the forest. A South African experiment found that samango monkeys accustomed to people moved closer to the ground when human observers were present, apparently because people reduced the perceived danger from predators on the ground. The monkeys therefore adjust where they spend time according to where the immediate risk seems greatest.

Because fruit is so important in the diet, blue monkeys can be significant seed dispersers. In a study in Rwanda, 94% of examined blue-monkey dung samples contained intact seeds. By carrying fruit through the canopy and later depositing seeds away from the parent plant, the monkeys can contribute to forest regeneration.

The effect is not identical for every plant species: some seeds are dropped close to feeding trees or damaged during handling. Even so, their fruit-rich diet and regular movement through forest mean that the monkeys spread many seeds while feeding.

Reproduction is often seasonal, with births tending to coincide with periods of higher food availability. In southern African populations, births are concentrated mainly in the warmer wet season. Females play an active role in partner choice rather than reproducing only with the resident male.

Gestation lasts about five months and normally results in one young. Females become sexually mature at around three years, although they often do not produce their first young until several years later; in some South African populations the first birth occurs at about seven years of age. There are commonly around two years between births, and individuals can live for several decades, with maximum ages around 30 years reported.

Local beliefs around blue monkeys have been documented particularly well around Kakamega Forest in western Kenya, where the local form is Cercopithecus mitis stuhlmanni. Some hunting families have traditionally regarded monkey meat as having medicinal value, and blue monkeys in particular have been associated with remedies for back problems in women.

These beliefs are local rather than representative of the species across its whole range, and researchers report that they are becoming less widespread as livelihoods and access to modern healthcare change. They nevertheless show that blue monkeys can occupy a distinct cultural as well as ecological place in communities living beside the forest.

Blue monkeys can become strongly accustomed to people where forests meet camps, roads and tourist facilities. At places such as Sodwana Bay and Cape Vidal in South Africa, samango monkeys may approach people and take unattended food. Easy access to human food can reinforce ground use and close contact with people.

Elsewhere in the range, forest clearing, hunting and conflict around cultivated foods create a much less tolerant relationship. The species therefore shows both sides of coexistence with people: it can adjust its behaviour to human presence, while that same proximity may expose local populations to persecution or habitat loss.

The blue monkey is listed as Least Concern on the IUCN Red List. The broad species remains widespread and locally common in many protected forests, although forest loss, fragmentation, hunting and conflict with people cause declines in parts of the range.

The broad species-level category also hides very different conservation situations among regional forms. The golden monkey (Cercopithecus mitis kandti) is assessed as Endangered, while the Somalia white-collared monkey (Cercopithecus mitis zammaronoi) is Critically Endangered. By contrast, Stairs’ white-collared monkey (Cercopithecus mitis erythrarchus)—the form photographed on this page—is globally assessed as Least Concern, although forest fragmentation remains an important regional concern.