Blue Monkey, Cercopithecus mitis
BLUE MONKEY
Cercopithecus mitis
Geographic Distribution and Habitat
A monkey of many names, the blue monkey (aka: Sykes’ monkey, diademed monkey, gentle monkey, and samango) also inhabits many different types of forests across Africa. They are primarily found in central and eastern Africa in the countries of Ethiopia, Kenya, Malawi, Mozambique, Rwanda, The Democratic Republic of the Congo, Burundi, Eswatini, Somalia, Tanzania, Uganda, South Sudan, Zambia, Zimbabwe and South Africa. There is also an isolated population found on the west coast of the continent in Angola.
Blue monkeys can be found in tropical mountain and lowland forests, mangroves, bamboo forests, and delta forests. They live in forests along rivers (known as riverine forests), and gallery forests that grow between wet and dry landscapes. They are found in the sand forests of South Africa, which are characterized by ancient sand dunes and unique plant species that have adapted to the dry nutrient-poor soils. Blue monkeys are also found in secondary forests that have regrown after experiencing deforestation and logged forests. While adapted to so many forest types, they are not found in open woodlands or grasslands, which seem to act as barriers for the species. They require large forest ranges and seem to have a difficult time in small forest fragments that have been separated by deforestation.
With the many common names used for them, it may be of no surprise that blue monkeys have undergone a bit of an identity crisis. There are many recognized subspecies, and over the years scientists have debated whether some of these subspecies should be considered their own species or not. Primarily, these distinctions have been made on differences in appearance and range. Currently, there are 16 recognized blue monkey subspecies across their range.
Size, Weight, and Lifespan
From head to hindquarters, the blue monkey is typically 19.7–25.6 inches (50-65 cm) in length, plus their tail which is nearly equal in length. They are sexually dimorphic, with males weighing 13–19.6 pounds (5.9-8.9 kg) and females weighing around 8.44 pounds (3.83 kg).
Blue monkeys typically live up to 35 years of age.
Appearance
Despite their name, blue monkeys are not really blue in color. Their fur is mostly black, although it can sometimes have a slight blueish shine in the light. The fur on their back is gray or olive (depending on subspecies) and has the appearance of a cape draped over them. The fur on their cheeks is bushy making their heads appear rounded, and speckled with white. Around their mouth is short white fur that spreads onto the throat and matches their white brow ridge (also called a diadem due to the crown-like appearance). The fur on top of their head is a stark black and looks almost like a cap.
Diet
In addition to their many names and forests they inhabit, blue monkeys eat a wide variety of food and are omnivorous. They primarily rely on fruit and, like other guenons, have several adaptations for their primarily frugivorous diet. These adaptations include rounded molars, simple stomachs (unlike the complex stomachs of leaf-eating monkeys that are similar to ruminants), and cheek pouches that let them take their food to-go. Scientists think that the cheek pouches in blue monkeys and many other African fruit-eating monkeys help avoid predation. They’re able to stuff as much food in their cheek pouches as they can while in a riskier area and then enjoy the food later when they can get to a safer spot. Although their favorite food is fruit, they’re not very picky and will try most foods they can get their hands on. This includes leaves (mostly young leaves that are easier on the stomach), flowers, vertebrates (like lizards, birds, and small mammals including other primates like bushbabies), insects, and occasionally tree gums.
Because blue monkeys inhabit many different forest types over a wide range, there is a lot of variability in their diets based on what they have access to, meaning not all blue monkeys will necessarily eat everything listed here. For example, some groups have a diet that consists of 25% fruit, and others with a diet made up of 91% fruit.
Behavior and Lifestyle
As a forest dependent species, it’s no surprise that blue monkeys spend most of their time high up in the trees, meaning they are arboreal. They move quadrupedally (on all fours) through the tree branches, using their long tails to help them balance. Blue monkeys are diurnal and are active during the day and sleep during the night in designated sleeping trees.
Their home range size ranges from 0.15–1.37 square miles (38 ha – 335 ha), and they will typically travel around 0.8 miles (1300-1400 m) a day. During their daily commute, blue monkeys do not cross their path, meaning they won’t visit the same place twice in a day. On average, they’ll cover about 1/3 of their home range in a day, moving in a circular pattern.
Blue monkeys are predated on by crowned eagles, chimpanzee, and humans.
Blue monkeys live in groups of around 4-65 individuals, with 1 adult male and multiple females. Outside males may join the group temporarily during the mating season, and sometimes will even stick around for a few years. The presence of outside males is most common in larger groups. With more females, it is difficult for the resident male to guard them all from competing males. When males mature, they leave their natal group (the group they were born in) and become solitary or join a bachelor group until they are able to find a group with females to join.
Female blue monkeys do not socialize often and typically prefer to rest and forage alone. When they do decide to socialize, it’s typically by grooming each other. Because of the lack of socialization among females, scientists have a hard time defining their social hierarchy.
Blue monkeys aggressively maintain the boundaries of their range from other groups. They will act as if there is an invisible line separating one group’s territory from their neighbors. As long as everyone is on their side of the line, everything’s peaceful. However, sometimes when a neighboring group isn’t near the boundary, blue monkeys will sneak in to access more enticing food sources. If the trespassing group is spotted, the resident group will alert each other and chase the other group back over the boundary. Defending the group’s territory is almost entirely managed by the females, although the resident male may occasionally decide to show his support. The level of aggression between groups depends largely on the population density of blue monkeys in the area (that is, how many monkeys are around). A forest is only able to provide enough food, space, and other resources for so many animals, so if those resources are close to being fully spent (which scientists refer to as carrying capacity), competition for these resources will be higher between the blue monkeys as well as other species using the same resources.
While adults prefer to keep to themselves most of the time, younger blue monkeys are much more social and spend lots of time playing together. They’ll even play with the young monkeys from other groups while their parents fight over territory!
Blue monkeys have been known to engage in polyspecific associations, which means they’ll hang out with other monkey species in a way that changes their behavior or the other species’ behavior rather than simply tolerating each other. Where their ranges overlap, they have been known to associate with red colobus, black-and-white colobus, gray-cheeked mangabeys, and Angolan colobus. They have also been known to associate with red-tailed monkeys, and have even produced fertile hybrids with them.
Blue monkeys have a range of vocalizations they use to communicate with each other. Many of their vocalizations are used to let other group members know about nearby dangers, such as the ka or karain calls (named for how they sound) which alert the presence of predators from the sky. Pyows are used to signal predators from the ground and are also used by males during aggressive encounters with other males. This is the second most common call for blue monkeys and is louder and longer in duration than most other calls. The most common call is the boom, a low-tonal call that males will emit after encounters with rival blue monkey groups or predators. Males will also give a boom call when approached by females. Unlike the other two instances when booms are used, these male-female encounters typically result in grooming sessions between the two.
Despite what the name implies, boom calls are very difficult for human observers to hear. However, scientists have figured out that other blue monkeys can hear a boom call from up to 0.62 miles (1000 m) away.
During more intense fights between males, the “loser” will sometimes emit a nasal scream. Most often, this will be the non-resident male in the group.
Females will use a loud chirping sound to alert other females in the group of a threat, such as a trespassing blue monkey group. When approaching males to initiate grooming, females will also emit low grunts.
Blue monkeys will also communicate using body language. This can include anything from facial expressions to head-bobbing to indicate a threat, or more overt signals such as lunging, slapping, or chasing other individuals.
Blue monkeys have a loose reproductive season, with births occurring year-round but primarily towards the end of the dry season and into the start of the wet season when resources begin to become more available. Gestation is on average 176 days but can range from 162–190 days. Females will typically give birth for the first time by 7 years of age.
While both sexes may initiate sexual advances, females tend to be more flirtatious. Male blue monkeys actually tend to be more selective of which females they mate with and will sometimes reject a female’s advances. Researchers have connected the male’s selectiveness to the female’s cycle, with ovulating females who are most likely to conceive being more attractive to males. Because female blue monkeys do not have any visual ovulatory cues, such as the sex swellings seen in chimpanzees and many other primates, there may be another cue such as olfactory (smell) signals that clue the male in on the female’s likelihood to conceive.
Blue monkeys play a role in maintaining the forests they live in by acting as seed dispersers. Since the monkeys often also consume the seeds from the fruits they eat, the seeds are able to be moved via a monkey’s stomach throughout the forest and dropped in a packet of fertilizer (or rather monkey poop). While perhaps a more passive contribution, seed dispersers help maintain the biodiversity of the forest and play an important part in the ecosystem.
The blue monkey is classified as Least Concern by the International Union for Conservation of Nature (IUCN, 2017), appearing on the IUCN Red List of Threatened Species. While the species as a whole is listed as least concern, the status of subspecies ranges from Least Concern to Critically Endangered, and the overall population is predicted to decrease. Blue monkeys throughout their range are threatened by habitat destruction for development of farmland, logging, and human settlements. Blue monkeys are also hunted for bushmeat and use in traditional medicine. Because the species is known to raid farms, blue monkeys are targeted by farmers in retaliation for destroying their crops and are viewed as nuisances.
Although the overall population is not considered highly fragmented, some sub-populations have been more impacted than others by habitat loss, particularly those located outside of protected areas. These populations have had trouble adapting to their homes being divided. Because the species is forest-dependent they do not travel over large open areas of ground and are subject to local extinction should their forest range become too small.
Blue 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.
Attempts have been made to relocate blue monkeys that live too close to humanized areas, however, this is a new conservation method for arboreal primates and requires more trial and error to establish an effective protocol. This method is very tricky to apply effectively and often has a low success rate.
Another method for maintaining blue monkey populations suggested by scientists is the use of ecological corridors. Also known as wildlife corridors, these are either natural or man-made structures that connect forest fragments in a way that animals can safely cross. With ecological corridors implemented, blue monkeys and other species have an easier time accessing food sources and mating opportunities.
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- https://www.iucnredlist.org/species/4221/196007901
- https://cites.org/eng/app/appendices.php
Written by Lina Rademacher, August 2024