The first-ever population study of Western Chimpanzees living on the Guinean-side of Mt. Nimba reveals there are four communities totaling more than 100 adults

Gazing out from the top of the Nimba Mountains, which extend across the borders of Guinea, Cote d’Ivoire and Liberia, can evoke an almost mystical feeling. Conservationists who have stood on the peaks of Mt. Nimba say the vantage point emphasizes how irreplaceable the sprawling massif is.

“It’s really extraordinary to be in and to climb to the top of Nimba, especially towards the end of the rainy season when everything is still green…and you can see out across three countries,” says Maegan Fitzgerald, a primatologist at Re:wild, who has spent a decade studying Western Chimpanzees living in the Nimba Mountains. “You can see out and even recognize how much of Nimba is this mountain island with nothing else like it around.”

Mt. Nimba’s topography has indeed made it a land-locked island of threatened and rare species—some of which are found nowhere else on the planet—and earned it recognition as a UNESCO World Heritage site and a biosphere reserve. The steep mountains–the highest peak is just over 5,500 feet–have also made it difficult to study many of those species. But a study published in March 2023 was able to provide the first-ever estimate of how many Critically Endangered Western Chimpanzees live on the Guinean side of Mt. Nimba.

Chimpanzee populations in West Africa have declined by more than 80% since 1990, so having an estimate of the size of the remaining populations is important for the species, especially since extractive industry projects could lower those numbers. Mt. Nimba has a huge iron-ore deposit underneath it and a key-shaped section has been carved out of the World Heritage site for a mining project. The concession where mining will be permitted overlaps with chimpanzee habitat on the Guinean side of Mt. Nimba.

Counting the number of chimpanzees living on the western side of the Nimba mountains in Guinea was no easy feat. The chimpanzees are not habituated to humans, meaning they avoid researchers, and chimpanzees can much more easily cross and climb the Nimba Mountains’ steep slopes and valleys than their human cousins.

“The Nimba Mountains are an incredible and unique place,” says Katheljine Koops, the lead author on the recent paper estimating the population size of Western Chimpanzees living on the western flank of Nimba in Guinea. “The study site in Nimba is the only mountainous chimpanzee study site in West Africa. When I first went there in 2003, we knew very little about the chimpanzees inhabiting these mountains. I was fascinated to find out more! Chimpanzees are a highly adaptable species, and studying how they live in such an extreme environment has been a journey of discovery.”

Koops and a team of Guinean researchers and partners, who have lived and worked in the Nimba Mountains their entire lives, and international researchers estimated the number of adult chimpanzees on the western flank of the Nimba Mountains between 2003 and 2018 using genetic analysis. It is the first study to use those techniques to estimate the size of a Western Chimpanzee population.

After analyzing DNA in 663 fecal samples, the team determined there are at least 136 Western Chimpanzees living across the western portion of the Nimba Mountains in Guinea.

The genetic analysis also showed that there is gene flow between the communities, meaning that individual chimpanzees have moved between those four communities, mated and successfully had babies. Female chimpanzees usually leave the communities they were born in and immigrate to new or neighboring communities as they enter adolescence, typically between nine and 12 years old.

“The study actually captured some of these migration events,” says Tatyana Humle, primatologist and a senior associate for Re:wild. “It's to say that there is movement. Such dispersal events and gene flow do help to maintain the genetic health of the population–mixing is good and essential for the genetic viability of this unique chimpanzee population. It's genetically viable, and it's mixing, and that's good.”

The key-shaped mining concession that is completely surrounded by the Mt. Nimba World Heritage site overlaps with the habitat of two of the four known chimpanzee communities on the western side of the mountain range in Guinea. Primatologists and conservationists are concerned that the potential activity from mining–building roads for heavy trucks, hauling materials up and down steep mountains, and removing layers of mountain with industrial machinery to get at iron ore deposits–will affect the chimpanzees.

“Of course, we know chimps can cross roads, but there is always the risk [involved] and chimpanzees do perceive that risk and change their behavior in response,” explains Fitzgerald.

The mining concession could become a barrier to chimpanzees migrating between communities. If roads are noisy, wide and have trucks moving along them regularly, it could be enough to deter a chimpanzee from crossing it.

“They can be very wary of crossing in the presence of vehicles, especially large trucks,” says Humle. “It can constrain their use and access to some areas and disrupt the time they allocate to different activities such as foraging, socializing and resting.”

If the four chimpanzee communities become isolated from each other, disrupting the gene flow, it could have consequences for their health and long-term survival. They could become more susceptible to disease or their reproduction rates could drop because of stress caused by mining and associated disturbances.

“You may not see it for years. [I]t may seem like they're doing fine, but they're just aging, and eventually they will age out,” says Fitzgerald. “It can be difficult to see, especially because you can't just follow these communities of chimpanzees. They're not habituated and the terrain is too difficult. They could actually be diminishing as we speak. It's hard to know.”

Since chimpanzees are territorial, primatologists are also concerned about the availability of food, water and nesting sites in areas where they live that are inside the mining concession.

“If they are compelled to move away from the mining project, it will significantly reduce their access to natural food resources and nesting sites,” says Humle. “The Nimba chimpanzees like to nest in the higher altitudes of the mountain range and seasonally seek out fruiting trees that only grow at higher altitudes.”

Any shift in their territories could lead to lethal encounters and heightened competition for resources with neighboring chimpanzee communities or with humans living in the lower elevations.

“The possible risk is that chimpanzees start to range more in the lowlands, and they expose themselves more to hunting, including snares,” says Humle.

In addition to becoming accidental victims of snare-hunting, having closer contact with humans or domesticated animals poses another potential risk for chimpanzees.

“The primary threats to chimpanzees in Nimba are human encroachment and extractive industries,” says Fitzgerald. “Although chimpanzees are rarely the target of hunting, they can get caught in snares set for other wildlife. Zoonoses, diseases transmitted between humans and animals, are also a risk, and become an even greater risk if encroachment and hunting increase.”

The Manon people, who live around the Nimba Mountains, are very tolerant and revere chimpanzees. Their traditional cultural beliefs have helped protect chimpanzees because they believe these primates should not be killed or hunted. Conservationists stress that the best solutions to protect chimpanzees are complex, but will also benefit humans.

“Effective and sustainable conservation of the chimpanzees and overall biodiversity in the Nimba Mountains will require a holistic approach that not only involves legal protections and the scientific study of wildlife, but also addresses the needs of local communities,” says Koops. “Activities such as land-use planning, conservation education, building local research capacities, and improving livelihoods will go a long way towards protecting the Nimba Mountains, its wildlife, and the numerous ecosystem services they provide locally and regionally.”