Chimps Throw Rocks at Trees Because of the Sound It Makes, Not Because It's a Sacred Ritual

Chimps that throw stones at trees and let the rocks accumulate at the base do so because they like the sound it makes—not because the trees are sacred and hitting them with stones is part of an unexplained ritual, as had been previously suggested.

In 2016, a team of scientists announced they had discovered this perplexing behavior in four chimpanzee communities in West Africa. The research, published in Scientific Reports, put forward different hypotheses about the stone-throwing, with one gaining a significant amount of attention—that this may be "ritualized behavior [that] may have implications for the inferences that can be drawn from archaeological stone assemblages and the origins of ritual sites."

However, a team of scientists led by Ammie Kalan, from the Max Planck Institute for Evolutionary Anthropology, Germany, has now carried out experiments suggesting the chimps select which trees to throw rocks at based on their acoustic properties. The team believes the behavior relates to communication, rather than any ritualistic practice.

"To this day, the behavior has not been observed anywhere else including the multiple long-term research sites where chimpanzees have been studied for decades,"Kalan told Newsweek. "The behavior immediately caught our attention since the trees where the stones accumulate at the base or in the hollow grooves are sites that the chimpanzees repeatedly revisit to throw rocks at the trees, leaving behind a conspicuous visual signal in these chimpanzee territories.

"Moreover, stone tool-use is generally rare in primates, and even more so for non-foraging related behaviors. The non-foraging tool-use, site fidelity and potential communicative and social aspects to this behavior are what caught my attention."

In their study, published in Biology Letters, the team simulated the chimp's behavior by throwing rocks at trees and comparing the sound different tree species produced. They then compared their recordings to those of the trees that the chimps threw stones at.

Findings showed the chimps were selecting trees based on the sound they made when the stones hit them—the sounds produced had energies concentrated at lower frequencies, meaning the sound would linger for longer in the environment. The team also found that surrounding trees were similar in terms of their bark and size, but did not produce the same sound. These trees were not selected as stone-throwing targets, giving further evidence to suggest the chimps were choosing trees based on their acoustic properties.

Most of the chimps recorded throwing stones were adult males. Of the 19 seen engaging in this behavior, one was a juvenile and one was an adult female. Just before throwing the stones, in almost all cases the chimp would emit a long-distance vocalization. After the rock hits the tree, the chimps appear to listen for a response.

Kalan said she thinks the stone-throwing is related to communication, but what it all means is still unclear. "This is what I will try my best to figure out next," she said. "What I find interesting though is why are the sites so rare and why is the behavior so rare in chimpanzees in general."

It is thought that the stone-throwing may be a cultural tradition passed on through social learning, but more research will be needed to confirm this. Kalan also said she is keen to investigate the significance of the locations of the trees selected. "Are they close to particular resources or at the edge or center of the territory? Also, as mentioned above, what might these sites potentially convey to other chimpanzees passing by? What meaning might they have? These are the questions I'm interested in studying next," she said.