Lesula, Cercopithecus lomamiensis
LESULA
Cercopithecus lomamiensis
Geographic Distribution and Habitat
The lesula is an African monkey, residing in the Lomami Basin in the Democratic Republic of the Congo. Their range is between the Lomami and Tshuapa rivers, in central Congo. Their habitat consists of mature, lowland rain forests. These forests are humid and consist of tall evergreen trees, Gilbertiodendron dewevrei. They spend time high in the canopy, but also make frequent trips to the ground. Their range is roughly 6,564 square miles (17,000 sq km), and they live at an altitude of 1,312–2,346 feet (400–715 meters) above sea level.
The lesula was made known to the international scientific community in 2007, though it had been known to local populations for as long as they can remember. It was confirmed in 2012 that it was, indeed, a separate species from its close relative, the owl-faced monkey (Cercopithecus hamlyni).
The two species are separated by the Lomami River and it is thought that, while they may have been a single species at one point, isolation due to the river led to its unique speciation. They are both, however, guenons—which is the common name for members of the genus Cercopithecus. This makes the lesula only the second African monkey species to be described by science since 1984. Its discovery came when a researcher found a lesula tied to a post in a village, presumably being kept as a pet. Local populations have always referred to this monkey as a “lesula,” therefore scientists believe it’s appropriate to keep it the same.
Size, Weight, and Lifespan
The lesula is a sexually dimorphic species meaning, in this case, that their size is dependent on their sex. Males are larger, weighing 8.5–15 pounds (4–7.1 kg), and females weigh 7–8.5 pounds (3.5–4 kg). Males are 18–25 inches (47–65 cm) in length, and females are 15–16.5 inches (40–42 cm). Since the lesula is a relatively newly discovered species, there is no reliable data on their lifespan in the wild or in captivity. Other guenons live to be roughly 25 years, so the lesula may have a similar trajectory.
Appearance
The lesula is easily recognized for its unique features. They have a blonde and gray coat, with mostly blonde being present on their chest. Their legs are a dark gray color, as well as their tails, which may appear amber in some light. Infant and juvenile lesulas have a primarily blonde coat, which allows for parents to distinguish them easily in the trees. Males, after reaching sexual maturity, have bare bottoms, and their genital area is blue.
The lesula has a distinguished face, with a long and thin nose and narrow set eyes. From their brows to just under their lower lip, the skin is bare and ranges from a light pink to tan color. These monkeys have often been regarded as having human-looking eyes, this is because they share a white sclera with us, which is the area of the eyeball that surrounds the iris and pupil. Many primate species have a dark tan or brown sclera, so the lesula is particularly striking to our human eye because of this.
Diet
The lesula subsists on a diet primarily of fruit, making it a frugivore. They do eat insects, leaves, flowers, and root-plants, but they are best suited (and greatly prefer) to eat ripe fruit. They find their food in the canopy, and occasionally descend to the forest floor to forage among roots and even discarded scraps from other primates.
Behavior and Lifestyle
Lesulas are diurnal, meaning they are awake during the day and sleep in the night. They are semi-arboreal, so they spend some of their time in the canopy and some on the forest floor. Lesulas spending time on the ground is not usual for many African monkey species, as it can be dangerous and exposing.
They begin each day by loudly vocalizing to establish their territory and make their presence known to other groups. They can be heard between 5:45 AM to around 6:40 AM. After this morning bout of vocalizations, they remain largely quiet for the rest of the day.
Like many other monkeys, their day is spent foraging, grooming, resting, and playing.
For the lesulas’ cousin, the owl-faced monkey (C. hamlyni), smell is vital in their life. Using a scent gland on their chest, they are able to identify individuals who have left their scent. While it is not confirmed that the lesula communicates this way, similar chest rubbing and scent marking has been observed in captive lesulas.
Some researchers use the common noun “lesula” to refer to both the singular and plural name of the species, much like the word “deer” refers to one or several deer.
Guenons typically live in groups of 10 to 40 members. When it comes to the lesula, however, this may vary. Some field observations have observed no more than 5 individuals in a group, and these groups were largely composed of females. Having such a small group, when compared to their guenon relatives, may have been an ecological adaptation in order to maximize their resources. In some cases, however, lesula groups of up to 38 individuals have been observed. This leads many primatologists to believe that we have only scratched the surface of the lesulas unique behavior and social system.
Lesulas are wary of humans and flee quickly when spooked. Humans are among their primary predators, so while fleeing is advantageous for the lesula, it makes learning more about them rather difficult for researchers. When they are spotted, though, it is often with other primate species in the area.
For small lesula groups, they are primarily female. Males will roam and appear to come in and out of groups, if they are welcome to do so. Females are the philopatric sex, meaning they stay in the group for generations. Males, conversely, will disperse when they reach sexual maturity.
As mentioned previously, lesulas are quiet during the day, rarely making calls. They are known to vocalize loudly in the early hours of the morning primarily to establish their home boundaries. Their calls are described as loud, resonating “booms” the sound of which can be heard from far away. Calls made by males can travel a considerable distance due to specialized air sacs located in the larynx.
Lesulas may use scent marking like their cousins, the owl-faced monkey (C. hamlyni), as they have similar anatomy that would allow for it. This has yet to be confirmed, though. The owl-faced monkey (C. hamlyni) has a similar boom-like call, which adds to their communication similarities.
Due to the lesulas’ shy and cryptic nature, researchers have not directly observed their reproductive process. We do know a few things, though, and can make inferences based on their relatives. They are polygynous, meaning that one male mates with multiple females. Females reach sexual maturity from 3 to 6 years old, upon which they will approach a mature male to copulate. Males reach maturity at 4.5–7 years old, and maturation is marked by their genitals and rump turning a vibrant blue.
They most likely have a mating season that fluctuates around yearly rainfall. This indicates that there is a birth season too. When a female becomes pregnant she will gestate for 5–6 months and often becomes reclusive and especially cryptic during this time. She will give birth to a single infant. Twin are rare. The infant is wholly reliant on its mother for the first 2 years or so. They are weaned at 6 months old, but they rely on their mothers to teach them how to survive for a while after that. A female will generally become pregnant again after a 2-year period of childrearing.
Like all frugivores, the lesula is a seed disperser in its environment. By eating the fruit of nearby trees, the lesula will either discard seeds to the ground in the process of eating, or will digest the seed and drop them in another area of the forest, complete with a surrounding of fertilizer! The lesula plays an especially important role in the forest as a secondary seed disperser. When a different monkey species has discarded a fruit to the ground, the lesula will forage for those leftovers on the ground. This is an important niche, and helps the seeds to germinate farther than they may have otherwise.
The International Union for Conservation of Nature assesses the lesula’s threat level as Vulnerable (IUCN, 2019) appearing on the IUCN Red List of Threatened Species. Its population is decreasing. Lesulas are targeted for bushmeat, and also fall prey to traps set for other animals, such as duikers and bush pigs. They are also prey for non-human predators, such as the crowned eagles and leopards.
The lesulas’s environment is subject to human development and deforestation. This habitat loss is not a result of subsistence farming, which is sustainable and low impact, but rather due to commercial farming rapidly entering the area. Rice plantations, natural resource use, and further developing agriculture are all ongoing threats to the lesula’s habitat.
Hunting is controlled inside of Lomami National Park; however, outside of it there is little that is done when a lesula is hunted.
The lesula is not listed by name in the Convention on International Trade in Endangered Species (CITES), although because it is a member of the genus Cercopithecus, it is included under Appendix II. The eastern-central Tshopo Province has protected the lesula at the provincial level, which is a step in the right direction. Protection at the national level is still needed, though. Lomami National Park, which is in the Tshopo Province, is patrolled by rangers regularly; however, only about 20% of the park is covered during these rounds.
Protection actions such as education and community awareness at the local and national level is suggested, as it is important to educate people about how vulnerable the species is. More research is needed so that we may know more about the species. Its relatively recent discovery has left us lacking information regarding unique behavior and group dynamics. It is also imperative so that we may continue to estimate their population numbers accurately.
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Written by Robyn Scott, January 2024