Guinea’s crab-fishing chimps are in good health, study shows, but threats loom

Guinea’s crab-fishing chimps are in good health, study shows, but threats loom
  • An international team has used genetic censusing — analyzing DNA from fecal samples — to work out the size of a population of critically endangered chimpanzees in Guinea. 
  • At least 136 adult chimpanzees were identified living in four communities on the western flanks of Guinea’s Nimba Mountains, part of a UNESCO World Heritage Site. 
  • Iron ore mining is planned in a northern section of the mountains, in an area the chimpanzees use to disperse between their separate communities. 
  • The scientists warn that human activities that hinder or restrict chimpanzee movements or territories can trigger deadly battles between rival communities of the apes and compromise their genetic diversity.

A team of international scientists has used a new genetic tool to measure the size of a population of crab-fishing chimpanzees living in a pristine mountain range in Guinea where there are plans to mine for iron ore.

The “genetic census” analyzed nearly 1,000 fecal samples collected over 15 years within the Nimba Strict Nature Reserve, a UNESCO World Heritage Site located in the range that straddles the borders of Guinea, Liberia and Côte d’Ivoire. The survey demonstrated that the population of western chimpanzees (Pan troglodytes verus) on the Guinean side, where they’re known for their unique ability to fish for freshwater crabs in shallow streams, is viable and healthy. It numbers more than 136 individuals, with evidence of migration between communities necessary to maintain genetic diversity.

“It’s a healthy and viable population, and [the number of chimpanzees] is actually an underestimate, because when you do genetic analyses based on fecal sampling, infants and juveniles are generally not included,” says Kathelijne Koops, professor of evolutionary anthropology at the University of Zurich, who was the lead author of a recently published study describing the findings.

Through genetic analyses in the laboratory, the research team was able to work out the unique DNA of 136 individual chimpanzees. Armed with that information, they traced the movements of these individuals throughout the western part of the Nimba Mountains, in a breathtakingly beautiful landscape of rainforest interspersed with high-altitude savannas and fast-flowing rivers.

forest and mountain in the Mount Nimba Strict Nature Reserve
The Mount Nimba Strict Nature Reserve, a UNESCO World Heritage site in West Africa, harbors a unique population of western chimpanzees. Image by Kathelijne Koops.

The census also identified four distinct communities of the apes, which are a critically endangered subspecies of chimpanzees, as well as evidence of migration between them.

Female chimpanzees, when they reach sexual maturity, typically migrate away from the community they’re born into. This “dispersal” is essential to maintain the overall population’s genetic diversity.

“We have [evidence of] gene flow between the communities,” Koops tells Mongabay. “Because we sampled for such a long time we even had some instances where we sampled a female first in the range which is clearly in the north, and then a year or so later in the south, allowing us to look at migration of individuals between communities.”

The data gathered by the study will help scientists and conservationists work out where the chimpanzees are in relation to a planned project to mine for iron ore in the Nimba Mountains.

The company behind that venture, Société des Mines de Fer de Guinée, says on its website that it aims to extract 450 million metric tons of ore over a period of 15-25 years. It says it’s tailoring the project to fit in with the landscape and rich biodiversity, and that its impact on the mountain range will be minimal.

But the site earmarked for mining, while technically outside the Nimba nature reserve, stretches across an area that chimpanzees regularly use.

“If [the mining venture] was completely unmitigated, and the chimpanzees couldn’t cross, then the north community would be cut off from the communities in the south,” Koops says.

“It highlights the importance of making sure there are corridors for the chimpanzees to keep this connection between the communities.”

Primatologist Genevieve Campbell, who was not a part of the study, notes that the impacts of mining can spread far beyond the project area. Roads will be built to facilitate access, and the mine will bring an influx of workers, leading to added pressure on natural resources, says Campbell, a senior associate at conservation group Re:wild.

If chimpanzee ranges and movements are constricted or hindered by human activity, whether mining, hunting or agriculture, the resulting overlaps of territory could trigger deadly battles between rival communities, Koops and her co-authors warn in their study.

“The presence of people in areas frequented by chimpanzees also presents a risk of disease transmission, that is, zoonoses, which can result in significant chimpanzee mortality,” they write.

The 2014 Ebola outbreak started close to the Nimba reserve, Campbell says. “It doesn’t seem like a good idea overall to encroach on a high biodiversity site given the known link to pandemic outbreaks. Iron ore is present globally yet we still want to extract it in a World Heritage site and impact a unique site which is also an important watershed for the region.”

a western chimpanzee in a tree
An international research team used genetic censuses to determine the size, composition and distribution of the chimpanzee population on the western flank of the Nimba massif in Guinea. Image by Kathelijne Koops.

Annette Lanjouw, head of the great apes and gibbons program at the Arcus Foundation, an international charity, was not part of the research team but says the study shows it’s now possible to count chimpanzees with greater accuracy and reliability using genetic censusing.

“It still requires [fecal] samples to be collected from each individual, which is not always possible to guarantee. However, it is far more reliable and accurate than alternative techniques,” she says. Those alternative techniques include counting nests, which the chimpanzees in the Guinean study site sometimes build on the forest floor instead of in trees, and linear transects and reconnaissance walks along trails through the forest.

Direct observation of chimpanzees in the Nimba Mountains is challenging. That’s partly due to the uneven terrain: chimpanzees can skim across deep ravines via treetops, leaving their earthbound human observers far behind. But it’s also to do with the difficulty of studying apes that are still very wary of humans. The evidence collected by Koops and her colleagues on crab-fishing behavior, for instance, was documented remotely through the use of motion-triggered cameras.

“The Nimba population of chimpanzees is a critical stronghold of this subspecies,” Lanjouw tells Mongabay, adding that conservation work would need to account for the entire mountain ecosystem, including its hydrological function.

“If the habitat is destroyed and the chimpanzees are lost, they will not be the only species lost,” she says. “The impact on people in the region, as well as further beyond, will be significant.”

Banner image: a western chimpanzee, by Kathelijne Koops.


Koops, K., Humle, T., Frandsen, P., Fitzgerald, M., D’Auvergne, L., Jackson, H. A., … Hvilsom, C. (2023). Genetics as a novel tool in mining impact assessment and biomonitoring of critically endangered western chimpanzees in the Nimba Mountains, Guinea. Conservation Science and Practice. doi:10.1111/csp2.12898

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