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Chimps Throw Stones to Trees, Communicate with the Impact Sounds

Researchers observed West African chimpanzees regularly throwing rocks at trees to produce sounds.

For several years a team of researchers has observed male chimpanzees in West Africa throwing rocks at trees and then running away.

It's still not clear why the adult chimpanzees engage in this behavior but new research aims to shed some light on it. 

Scientists including Ammie Kalan, a primatologist at the Max Planck Institute for Evolutionary Anthropology who first observed the behavior known as accumulative stone-throwing or AST in 2016, have concluded they may be making music. 

RELATED: CHIMPANZEES' COMMUNICATIVE GESTURES SHARE SIMILARITIES TO SPOKEN HUMAN LANGUAGE 

Researchers recorded impact sounds 

To test that theory the researchers conducted field experiments to record the impact sounds produced when the chimpanzee threw rocks at trees. They compared impact sounds produced by throwing rocks at trees that had been used for the act and ones that hadn't. The research was published in journal Biology Letters.

"We predicted that chimpanzee AST tree species produce sounds that have energy concentrated at lower frequencies and a greater resonance since these impact sounds would be optimal for long-distance communication," wrote the researchers in a release highlighting the work. "Accordingly, we predicted that chimpanzees use AST tree species that possess the following physical features because they may aid the production of low frequency, high resonating sounds: trees with a large diameter, buttress roots, and hollow cavities, formed either by roots merged together or a hollowed-out tree trunk."

The fieldwork took place in Boé, Guinea-Bissau, from February to June 2017 encompassing a 50 km area. The researchers found a total of 39 AST sites, which had visible wound marks from repeatedly being hit with rocks and accumulation of rocks at the trunk. Of the 39 sites, 21 had fresh impact signs. The trees used for AST were only one of seven species, implying the chimpanzee chose those trees for a particular reason. 

Chimps are rocking out thanks to the certain trees

The researchers recorded multiple simulated chimpanzees throws on 27 trees which were then sent to the lab for acoustic analyses removing any information about the tree species. Only 125 of the 172 impact sounds recorded were able to be analyzed. The analysis found patterns that show acoustic timbre differences between sounds generated from impacting one material to another.

"Overall, this study suggests that at least one function of AST behavior is sound production. Low-frequency sounds travel further in the environment and are better suited for long-distance communication," wrote the researchers.  "Only 39 individual trees had any signs of use by chimpanzees out of the potentially hundreds of AST trees available. Future research should focus on testing the factors influencing individual tree and tool selection, including testing more tree species, and more trees per species. Additional studies investigating putative cultural aspects of AST would also be important for their potential to assist chimpanzee conservation efforts in the wild." 

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