Diana Monkey, Cercopithecus diana
DIANA MONKEY
Cercopithecus diana
Geographic Distribution and Habitat
The Diana monkey, also known as the Diana guenon, is found primarily in the forest regions of Upper Guinea in West Africa. Specifically, the species ranges from coastal southeastern Guinea to Sierra Leone, Liberia, and southern Côte d’Ivoire west of the Sassandra River.
The Diana monkey lives in the canopy of primary and older secondary lowland moist forests, and riverine and gallery forests. What exactly are these forest types? Primary forests are those that are undisturbed by humans, while secondary forests are those that have naturally regrown after a period of human-caused disturbance. Riverine forests are those sitting along a river or riverbank, and gallery forests are those formed along riverbanks that flow into otherwise open areas, such as deserts or savannas.
The Diana monkey is found at sea level in Sierra Leone to at least 0.81 miles (1,300 m) above sea level in the Loma Mountains, the highest mountain range in the country.
Two taxa (classification groups) that were once considered subspecies of the Diana monkey have been upgraded to full species status: the Roloway monkey (C. roloway) is found in Côte d’Ivoire and Ghana, and the Dryas monkey (C. dryas) is found in the Democratic Republic of the Congo.
Size, Weight, and Lifespan
Diana monkeys range in size from 16.5-24 inches long (42-61 cm) from head to rump, with a non-prehensile tail (one that is not used for grasping or grabbing) that can be up to 35 inches (89 cm) long. Their weight ranges from 8.8-15.4 pounds (4-7 kg). The species is sexually dimorphic, meaning that there are noticeable physical differences between genders; in the case of the Diana monkey, males are slightly larger than females.
They can live up to 20 years in the wild, so long as they are healthy.
Appearance
Slenderly-built, Diana monkeys have long legs and a long tail to move around and balance among the trees in their habitat. The face and much of the fur is inky black. In contrast, the chest fur, brow-band, throat, and beard (yes, even the ladies have them) are milky white. The inner portion of the thighs and arms is also white, and a white stripe runs down each outer thigh. The back of the thighs and lower back are chestnut brown, and their eyes are light brown.
Infants are light brown to gray in color and develop more striking coloration as they mature to adulthood.
Diet
The Diana monkey is omnivorous, feeding on a wide variety of foods. Their diet consists primarily of fruits, insects and other invertebrates, and flowers. When these items are less available based on the season, leaves become a dietary staple.
Behavior and Lifestyle
The Diana monkey is diurnal (active during the day) and arboreal (tree-dwelling), with the vast majority of their activity taking place in the upper canopy (where they also sleep) and in the middle canopy (where they forage and feed). They navigate their tree-filled habitat walking on all fours, as opposed to leaping, and rarely venture to the ground, due to the risk of encountering predators. Forty to forty-five percent of their active time is spent on foraging and feeding, while 25% of their time is spent traveling throughout their habitat. The Diana monkeys keep things interesting by demonstrating considerable variation in how they budget their time based on the composition of the troop, the season, and the available food in specific locations within a given year.
They add much to the sound backdrop of the forest, as their presence in the forest is a noisy one, given their wide array of calls used for various situations.
Predators, in addition to local humans who hunt them for the bushmeat trade, include the African crowned eagle, leopards, and chimpanzees. To avoid predators, Diana monkeys’ foraging takes place within the middle canopy to avoid being easily spotted by birds of prey. At the same time, this keeps them safely out of reach of ground-based predators. Much of their time is spent in association with other primate species, sharing the important job of listening for predator-avoidance alarm calls, enabling them to retreat when necessary for survival.
Diana monkeys do not construct sleep nests. Rather, they sleep directly upon branches within the upper limits of the trees.
Diana monkeys are named for the crescent-shaped white brow band that resembles the crescent on the brow of the Roman goddess Diana, the protector of wildlife and woodlands.
Diana monkeys are social creatures and live in troops of up to 20 members which consist of one adult male, several females, and their young. Regarding rank within the troop, offspring inherit their mother’s rank in Diana monkey communities. Females remain in the mother’s troop for their entire lives; males, on the other hand, leave the troop shortly before reaching sexual maturity at around three years of age.
The home range of a Diana monkey troop can vary within a range of 72-229 acres (29-93 ha) and depends upon factors including troop size, resource availability, and survival-friendly habitat features. The average distance traveled per day has been reported to range within a distance of 0.62-0.93 miles (1-1.5 km).
Diana monkeys co-mingle with other primate species with whom they share their habitat. These include the western red colobus (Piliocolobus badius), the olive colobus (Procolobus verus), the Campbell’s guenon (Cercopithicus campbelli), the putty-nosed monkey (C. nictitans), and the lesser spot-nosed guenon (C. petaurista). These species all aid one another in sounding the alarm when a predator is spotted. The relationships between the Diana monkey and the colobus species are particularly successful because they are not in competition with one another. While their paths may overlap due to similar foraging goals as both seek leaves and foliage, they have a slightly different niche within the habitat and somewhat different food preferences.
Based on one study regarding cooperation and competition between Diana monkeys and putty-nosed monkeys, it was concluded that, despite overlapping territories and occasional food competition (which is solved to an extent by the Diana monkeys’ willingness to feed on invertebrates instead of fruit during times of limited resources), the two species cooperatively use predator-protection strategies. These include the use of sentries (lookouts) and shared responsibility for the recognition of alarm calls. This provides benefits to both species and minimizes potential negative consequences that could arise from territory sharing.
Despite their relatively tolerant relations with other primates, Diana monkeys have been known to be territorial and intolerant towards competing troops within their own species. (This might be compared to a primate version of a “family feud”.) Although such encounters are uncommon, when two troops encounter one another, females, subadults, and juveniles have been observed fighting, while males engage in loud calls, jumping displays, and head-bobs to intimidate one another.
The Diana monkey uses a wide range of visual social signals for communication, including head-bobs and facial expressions.
As is common among primates, tactile (touch) communication is important for the Diana monkey. Not only do activities such as grooming help to remove debris and parasites from their skin and fur, but such activities also help to reinforce social bonds among troop members.
The most noteworthy form of communication among Diana monkeys, however, is sound. Diana monkeys are well-known for being alert to alarm calls from other primate species in their shared habitats. They have the additional bonus of being able to use specific alarm calls for different predators. This allows them to take appropriate defensive strategies depending on which predator is nearby. Leopards and birds of prey, such as the African crowned eagle, are stealth predators, relying on the element of surprise to sneak up on and take down their prey. Stealth predators are more likely to abandon their hunt if an alarm call has blown their cover. Meanwhile, pursuit predators, such as chimpanzees, are attentive to the alarm calls of their prey in order to locate them. Therefore, Diana monkeys will remain as silent as possible if a chimp is spotted or heard so as not to alert this predator to their presence within a given location. To maintain territorial boundaries, Diana monkeys also utilize a long-distance call to warn other troops to stay away.
Diana monkeys are polygynous, meaning one male mates with all of the adult females in the troop. They breed seasonally, and females become sexually receptive about once a month. The reproductive patterns of Diana monkeys have not been extensively studied, but it is known that females do not display genital swelling (which in other species typically indicates readiness to mate). Gestation (pregnancy) lasts for around five months, after which time the female gives birth to a single offspring. Young infants have been observed in Sierra Leone’s Tiwai Island Wildlife Sanctuary most frequently in the dry months of January and February, suggesting that births peak from December through February.
While infants are born in a well-developed condition, with eyes open and with the ability to grasp onto their mothers, the mothers rarely allow their infants to leave their protection. Mothers carry their offspring on their backs and nurse them for the first six months of life. As the offspring grow and wean off the mother’s milk, they become very playful. These playful behaviors may include pulling on and biting the mother’s tail, as well as bounding around her, assuring that the mother is never given any peace.
Juveniles reach sexual maturity at around three years of age. Shortly before reaching this critical point, male offspring will leave their birth troop to hopefully establish a troop of their own. Females, on the other hand, remain with their natal (birth) troop for the rest of their lives.
With a large part of their diet consisting of fruit, the Diana monkey aids in the regeneration of the forest by dispersing seeds through their feces as they move around the habitat. As a prey species, they also play a role in feeding local predators within that habitat.
The International Union for Conservation of Nature lists the Diana monkey as Endangered (IUCN, 2019), appearing on the IUCN Red List of Threatened Species.
The main threats facing the Diana monkey arise from hunting as well as from habitat degradation and loss.
The Diana monkey’s relatively large body size, along with the value of their meat and skin make them a preferred game species for local people throughout their range of Guinea, Liberia, and Sierra Leone. Subsistence hunting (done in order to feed one’s family) is widespread, and laws against commercial hunting and other illegal activities (such as habitat encroachment and mining) are poorly enforced even within protected areas. In areas with high hunting pressure, Diana monkeys have become more rare when compared to areas with relatively lower hunting pressure.
Large-scale deforestation in the region caused by charcoal production, logging, mining, and conversion to agricultural land, continues to reduce the amount of available land for the Diana monkey. Between 2001 and 2017, Guinea, Liberia, and Sierra Leone each lost between 14%-21% of their total forest cover, and the dominant reasons for forest loss continue to this day. As forests become more fragmented, Diana monkeys are less able to establish home ranges, forage widely, and safely move through the habitat.
As a result of these combined factors, it is suspected that the total population of Diana monkeys has decreased by more than 50% during the last three generations!
The Diana monkey is listed in Appendix I 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. They are also listed in Class A of the African Convention on the Conservation of Nature and Natural Resources.
The Diana monkey is found in a number of protected and semi-protected areas throughout their range, including: Tai National Park, Cavally Classified Forest, Goin-Debe Classified Forest, Tiwai Island Wildlife Sanctuary, Gola Forest National Park, Loma Mountains National Park, and Sapo National Park.
Conservation actions needed to save the Diana monkey include improved protection enforcement and management in the areas in which the species is found, as well as increased communication and public awareness regarding the plight of the species.
Further research is needed on the Diana monkey’s taxonomy (classification), population size, distribution and trends, life history and ecology, and survival threats. Additionally, monitoring of population trends is also is necessary to better understand the species and its conservation needs.
Ultimately, the combination of these conservation actions and further research may potentially save this endangered species from extinction.
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Written by Sienna Weinstein, March 2024