- Only found in the rainforests of the Democratic Republic of the Congo, the lesula monkey (Cercopithecus lomamiensis) was first described by scientists in 2012.
- A 2023 Animals study finds that the lesula is mostly terrestrial, unlike the other species of guenon monkeys in the region.
- The study also finds that the lesula is active during the day, has a seasonal reproductive cycle, and lives in family groups of up to 32 individuals, with males dispersing out to form bachelor groups.
- Researchers say the Tshuapa, Lomami and Lualaba Rivers Landscape, where the study was conducted, holds incredible primate diversity.
In 2012, the description of a new monkey species from the Congo Basin with an unforgettable, humanlike countenance made global news. Now, a camera trap study reveals how the lesula (Cercopithecus lomamiensis) has carved out a unique niche on the forest floor.
The lesula is a slender, medium-sized monkey with a long tail, found only in a remote part of the Congo Basin called the Tshuapa, Lomami and Lualaba Rivers Landscape, or TL2, in the Democratic Republic of Congo (DRC). Though long known to local people, the species had remained unknown to science until research teams with the Lukuru Foundation’s TL2 Project, photographed an individual during surveys of the area.
Taxonomic and genetic analysis showed that the lesula was a type of guenon monkey (genus Cercopithecus), the most species-rich group of African primates. Little was known definitively about the lesula’s ecology or behavior when it was first described, but there were already tantalizing hints that the lesula might be quite different from the other mostly arboreal species of guenon monkeys in the landscape.
Now, using camera traps, researchers have found that the lesula spends far more time on the ground than other related monkeys in the area. It’s also active during the day and lives in larger groups than previously thought, according to the 2023 study in the journal Animals. The research also highlights the importance of the TL2 landscape for primates.
“We have six different guenons … living in this landscape and they’re sorting it out … so it’s that diverse and productive that it can support all these different species in the same place at the same time — and the lesula is one of them,” says study author Kate Detwiler, a professor of anthropology at Florida Atlantic University.
The Congo Basin holds the second-largest rainforest on Earth, which covers 178 million hectares (440 million acres) across six countries, and is one of the largest carbon sinks on the planet. These forests contain an astounding array of biodiversity, including iconic mammals like the okapi (Okapia johnstoni), bonobos (Pan paniscus) and forest elephants (Loxodonta cyclotis). The rainforest is also home to more than 1,000 species of birds and 600 types of tree. It also supports countless communities and Indigenous peoples.
Across the region, shifting cultivation and charcoal production are the primary drivers of deforestation, while the commercial bushmeat trade is hitting wildlife hard. The DRC, which contains more than half of all the Congo Basin’s primary rainforest, has the highest rates of deforestation of all the Congo Basin nations.
Yet the DRC’s remote TL2 landscape, surrounded by powerful meandering rivers, remains astoundingly intact, and at 4 million hectares (10 million acres) — the size of Switzerland — is a haven for biodiversity, including the lesula. Since 2016, the core of the area has been protected within the nearly 900,000-hectare (2.2-million-acre) Lomami National Park. The main threat here is poaching.
To figure out how the lesula uses the forest, researchers needed to observe them. But they quickly realized that the classic methods of studying primates — finding them, getting them used to human presence (a process known as habituating) and following the groups — weren’t going to work.
For starters, lesulas are incredibly hard to find, even for seasoned primatologists like Detwiler. She visited the study area for three weeks in 2012, and didn’t manage to see a lesula until the last day, when two unexpectedly crossed the trail in front of her. “And it was for less than 10 seconds!” she recalls. “It was like, wow, they’re really cryptic.”
The researchers were also wary of habituating the lesula because the monkeys are threatened by hunting. Habituation would make lesulas lose their natural fear of humans and put them at greater risk of being killed.
Instead, the researchers decided to use camera traps. Still, that was logistically challenging and expensive, says Junior Amboko, a graduate student at Florida Atlantic University and one of the study authors. The TL2 landscape is very remote; supplies like fuel, batteries and food have to be brought in from bigger towns over a hundred miles away, and travel is slow and difficult.
Nevertheless, the researchers set up three camera trap grids between 2013 and 2016, two within the protected area and one in the buffer zone. As the lesula was thought to be mostly terrestrial, they set the cameras along forest trails at ground level — and because they were interested in behavior, they programmed them to video mode.
In all, the cameras recorded 15,000 videos, including nearly 600 clips of the lesula. Analyzing those clips was a monumental task that required a steady stream of research students, Detwiler says. But finally, a picture of the lesula’s distinct behavioral ecology emerged.
Like the other guenon monkeys in the area, the lesula is almost exclusively active during the day, with just a few videos recorded at dawn or dusk, and its reproductive patterns are tied to the rains, like other guenons.
But in other ways, the lesula has found a unique niche. It spends far more time on the forest floor than the other monkeys; there were nearly 600 video clips of lesulas moving around on the ground, compared to only 10 clips of other guenon species.
The videos also revealed that lesulas live in larger groups than previously thought, of up to 32 individuals. Family groups are usually composed of one adult male, numerous females and their young, while males are also found alone or in bachelor groups. However, the authors note that the average group size recorded by the cameras was smaller. This could be because the cameras only recorded animals in front of the lens, and more research is needed. (Unfortunately, the lesulas didn’t pose for a family portrait.)
These findings have practical conservation applications. A previous study using the same camera-trapping data set and combined with data on the frequency of lesula morning calls, concluded that the monkey isn’t currently heavily impacted by hunting. However, the lesulas traveled in smaller groups in the buffer zone where hunters were active, compared to in the national park. Living in smaller, quieter groups might be one way that they evade hunters, Detwiler says. Baseline data on group size could thus help researchers detect when populations are under stress.
John Hart, with the TL2 Project, says the landscape here still holds many mysteries — and new methods like camera traps are helping us peer beneath the trees to reveal the forest’s secrets. Some of the videos showed lesulas in the same frame as blue duikers (Philantomba monticola) and other species. Hart says he hopes further analysis will show if these associations are intentional or if the species are simply “meeting at the same restaurant.”
The study also raises new questions about how lesulas use the forest canopy. Though the species is primarily terrestrial, observations show it also climbs trees — and Detwiler says she wants to know why. She’s already planning a new study, where the team will place camera traps at different heights in the canopy.
The answers could tie into our own evolutionary history. Detwiler says understanding the lesula may shed light on “what Australopithecus was doing when it evolved to come down [from the trees].” Like the lesula, Australopithecus, an early human ancestor that lived some 2 million years ago, walked on the ground but still climbed trees. “So, if those types of species are living in front of us, we can ask, ‘OK, what are you doing up there?’”
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Banner Image: A lesula female and infant in the TL2 Landscape in the central Congo Basin. Lesula live in family groups of up to 32 individuals, usually composed of one adult male, several females, and young. Image by Florida Atlantic University
Citations:
Fournier, C. S., Graefen, M., McPhee, S., Amboko, J., Noonburg, E. G., Ingram, V., … Detwiler, K. M. (2023). Impact of hunting on the lesula monkey (Cercopithecus lomamiensis) in the Lomami River Basin, Democratic Republic of the Congo. International Journal of Primatology, 44(2), 282-306. doi:10.1007/s10764-022-00337-4
Fournier, C. S., McPhee, S., Amboko, J. D., & Detwiler, K. M. (2023). Camera traps uncover the behavioral ecology of an endemic, cryptic monkey species in the Congo Basin. Animals, 13(11), 1819. doi:10.3390/ani13111819
Hart, J. A., Detwiler, K. M., Gilbert, C. C., Burrell, A. S., Fuller, J. L., Emetshu, M., … Tosi, A. J. (2012). Lesula: A new species of Cercopithecus monkey endemic to the Democratic Republic of Congo and implications for conservation of Congo’s Central Basin. PLOS ONE, 7(9), e44271. doi:10.1371/journal.pone.0044271
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