In Guinea-Bissau, Chimpanzees Have Been Throwing Stones at a Tree for 10 Years—and Scientists Don’t Know Why

 Walking through the savanna woodland of Boe National Park in Guinea-Bissau, you might stumble upon a strange sight: a tree with deep gashes scarred into its bark, surrounded by hundreds of rocks piled high around its roots.

​There are no people nearby, no human construction, and no obvious explanation. It is not an ancient ruin—it is direct evidence of a chimpanzee habit that has left primatologists worldwide completely baffled.

​Scientists call this accumulative stone throwing—repeatedly hurling rocks at the exact same tree or hollow trunk until a mound builds up. The most startling part is that this ritual has continued unbroken for years, and in some areas, for over a decade.

Who First Discovered This?

In 2016, a team led by primatologist Ammie Kalan first documented this behavior.

​Her team set up hidden camera traps throughout the forest—motion-activated cameras that record without people present, allowing researchers to capture natural wildlife activity without disturbance. What the footage revealed was striking: adult male chimpanzees would approach the same tree one after another, pick up a rock from the ground, slam it with full force against the trunk, and calmly walk away.

​This was not a one-time incident. The same individuals returned on different days, repeating the exact same sequence over and over.

​Kalan's team identified this ritual across four separate chimpanzee populations in West Africa, and researchers believe more sites will turn up with deeper exploration.

Why This Is Different from Normal Tool Use

Chimpanzees using rocks is nothing new. For decades, scientists have known that certain chimp groups use stones like hammers and anvils to crack open tough nuts for food, making it a classic example of tool use by animals.

​Accumulative stone throwing is entirely different. It has no clear connection to feeding or survival. The rocks are not aimed at nuts or fruit—they are thrown straight into tree trunks and hollow bases, over and over, across many years, with no visible reward. That lack of practical purpose is what makes it such an intriguing puzzle.

​While hurling these rocks, the chimps frequently make loud vocalizations known as pant-hoots—a booming, long-distance call chimps use to signal their presence and position to the group. They also bang heavily on the tree's flared buttress roots using their hands and feet.

​Both pant-hooting and drumming are well-known parts of male display behavior used to project dominance and territory. Because of this overlap, researchers suspect that throwing rocks might simply be an amplified, modified version of that same routine.

What Is This Behavior Really? A Signal, a Border, or Something Deeper?

Current research makes one thing clear: not all chimp populations do this, even in areas where trees and rocks are everywhere. This shows it is not an automatic instinct. Instead, it looks like learned behavior—an example of animal culture passed down across generations within specific groups, much like human customs.

​Scientists have proposed several leading explanations:

• ​Long-Distance Signaling: Smashing rocks against hollow trunks makes a loud thud that travels far, acting as a form of animal communication to broadcast presence or intention.
• ​Boundary Marking: It may serve as a way of claiming territory, marking travel routes or safe resting spots.
• ​Symbolic or Ritual Sites: A more intriguing theory suggests these trees have social or symbolic value, resembling the way early human communities built stone mounds or cairns to mark important spots.

​None of these theories have been proven yet. The reality is that researchers still do not know what these trees mean to the chimps, or why they keep returning to throw rocks at them year after year.

Returning to the Wild: What New Research Reveals

In 2026, a research team returned to remote corners of Boe National Park to dive deeper into this mystery.

​Working through the "Pan African Programme" at the Max Planck Institute for Evolutionary Anthropology, scientists are no longer just logging stone-throwing events. Instead, they want to pinpoint which specific individuals take part, what triggers the action, and how nearby chimps respond. Alongside motion cameras, researchers are now deploying bioacoustics gear to identify chimps by voice and analyze their social behavior.

​Reaching these sites is grueling work involving miles of trekking on foot through dense brush and blistering heat. Still, researchers consider the effort essential. Chimpanzees are our closest living relatives, and decoding this puzzling habit could reveal how early communication and stone-use traditions first took root in human evolution. This is not just a study of chimpanzees—it could be a window into our own deep past.

A Secret That Must Be Solved Before It Disappears

Along with this research comes an urgent concern. This region of Boe faces constant pressure from industrial mining operations. Expanding deforestation and land excavation cause severe habitat destruction, putting these rare chimpanzee groups at risk. This loss could erase this remarkable behavior forever—before scientists can fully decipher it. That is why field researchers are working against the clock; this is not merely an academic puzzle, but a window into the past that could slam shut at any moment.

What These Trees Really Tell Us

We often assume that traditions, symbolic thinking, and sacred rituals belong exclusively to human beings. Yet the mounds of stones gathering in the forests of Boe remind us that the line between us and other animals might not be as sharp as we believe.

​Until science solves this mystery, every rock that cracks against wood carries an unanswered question—one that might end up teaching us just as much about our own evolutionary roots as it does about chimps.