- A new study from Côte d’Ivoire highlights the urgent need to integrate chimpanzee cultural preservation with conservation.
- The study documents the loss of a socially learned behavior — a mating signal — among a group of chimpanzees following the poaching of all of the group’s male members.
- Once lost, behaviors that could be crucial to chimpanzee survival take years to reemerge.
- Researchers say it’s essential to preserve entire chimpanzee communities and their cultural knowledge, as well as simply protecting individuals.
Male chimpanzees in Côte d’Ivoire’s Taï National Park use distinct “auditory gestures” to attract females. However, researchers have found that when the males die, these behaviors can disappear with them.
The solicitation gestures used by these male western chimpanzees (Pan troglodytes verus) include tapping their heels or knuckles against fallen branches or tree trunks, shaking branches or saplings, or noisily tearing leaves off stems.
In one of the four chimpanzee communities monitored for the past 45 years by the Taï Chimpanzee Project, a research and conservation group, the “knuckle-knock” was the main solicitation gesture used by males since 1991.
But severe hunting pressure drastically reduced the chimpanzee population in Taï, from around 3,000 in the early 1980s to just 300 by 2010. The part of the Taï forest occupied by the northern community and their knuckle-knocking males was particularly vulnerable due to its proximity to villages, providing easy access to hunters. In 2008, the group’s last adult male, known to researchers as Nino, was killed for his meat.
In the aftermath, a female took over as leader, and while the group continued functioning as a unit, the absence of multiple adult males meant there were no individuals left to perform and reinforce the knuckle-knock gesture. Today, even though young males have since grown up and assumed leadership, the behavior has not reemerged. Instead, it has been replaced by another gesture: the “heel-kick.”
“We’ve shown that a behavior that is totally learned can be lost,” says Mathieu Malherbe, a primatologist at Germany’s Max Planck Institute for Evolutionary Anthropology and lead author of the study.
While the loss of an auditory gesture may not seem critical, other socially learned skills used in Taï — such as using rocks to crack open nuts or stick tools to break open termite mounds — are essential for chimpanzee survival, particularly during times of food scarcity.
“If you go and disrupt, basically, the possibility for individuals to pass on any knowledge, then you have a huge issue,” Malherbe says.
Part of the challenge is that these skills take years to acquire. A study published last year by Malherbe and colleagues analyzed more than seven years of video footage from 70 wild chimpanzees using tools to extract food in the Taï research area, home to around 200 of the park’s now 1,000-strong chimpanzee population. The study found that while young chimpanzees acquire basic tool-use skills between the ages of 5 and 6, they continue refining them over time and may not master them until adulthood, around age 15.
Male eastern chimpanzees (Pan troglodytes schweinfurthii) in Uganda’s Budongo Central Forest Reserve, on the other side of the continent, share some auditory gestures with those used in Taï, as well as others that are unique. Some, like the knuckle-knock, have different meanings. In Taï, it’s used to attract mates, while in Budongo it’s not. This all suggests that chimpanzees have a pool of genetically inherited gestures to choose from. But while one might expect such genetically hardwired gestures to reemerge over time, the studies in Taï show how long it takes: Twenty years after the last males in the northern group used the knuckle-knock, the behavior remains absent. That’s a full generation in chimpanzee life terms.
“Long-term studies like the Taï Chimpanzee Project are incredibly important because they are uniquely positioned to explore questions regarding the stability of chimpanzee cultures,” says David Morgan, senior scientist at Chicago’s Lincoln Park Zoo. Morgan, who was not involved in Malherbe’s study, has studied chimpanzees in Central Africa, notably in the Republic of Congo’s Nouabalé-Ndoki National Park.
“We have yet to record losses of socially learned behavior in the chimpanzees we study in Central Africa,” he tells Mongabay. However, he points out that with habitat degradation occurring in both East and West African forests, it’s clear that losses are occurring.
“The loss of chimpanzee populations means that there are fewer opportunities for site-specific new traditions to arise in the face of changing environments.”
The fragility of socially learned behaviors was brought home to Malherbe in 2018, at the start of his Ph.D. studies. While he was at the research camp, colleagues returned from the field with reports of a young female chimpanzee behaving unusually. A recent arrival to the northern group, she had built a nest on the ground and rolled on it — an action unfamiliar to the other chimps. Malherbe had just read a paper about this behavior, known as “defending the castle,” which had been observed in a separate chimpanzee group in the park’s south.
“She basically created the ground nest, rolled on it, and no one came to play with her,” Malherbe says. “This [was] only about [trying to transmit] play behavior, but it could be about behaviors that are very important for their survival.”
For a behavior to become established within a group, it must be transmitted, understood and maintained over time, Malherbe says. But, just like in human communities, the loss of knowledgeable individuals can negatively impact the survival of an entire population.
“We need to preserve communities and [the] population [in order] to not lose the cultural knowledge,” he says, “part of which is essential for chimpanzee survival.”
Citations:
Boesch, C., Gotanegre, A., Hillers, A., Kouassi, J., Boesch, H., Kizila, P., & Normand, E. (2020). Lessons learned while protecting wild chimpanzees in West Africa. American Journal of Primatology. doi:10.1002/ajp.23209
Malherbe, M., Kpazahi, H. N., Kone, I., Samuni, L., Crockford, C., & Wittig, R. M. (2025). Signal traditions and cultural loss in chimpanzees. Current Biology, 35(3), R87-R88. doi:10.1016/j.cub.2024.12.008
Malherbe, M., Samuni, L., Ebel, S. J., Kopp, K. S., Crockford, C., & Wittig, R. M. (2024). Protracted development of stick tool use skills extends into adulthood in wild western chimpanzees. PLOS Biology, 22(5), e3002609. doi:10.1371/journal.pbio.3002609
FEEDBACK: Use this form to send a message to the author of this post. If you want to post a public comment, you can do that at the bottom of the page.
Credits
