The Sounds of Trees When Hit by Rocks Thrown by Chimps

What are the sounds that come from trees when those trees are hit by rocks thrown by chimpanzees? A new study addresses that question:

Chimpanzees use tree species with a resonant timbre for accumulative stone throwing,” Ammie K. Kalan, Eleonora Carmignani, Richard Kronland-Martinet, Sølvi Ystad, Jacques Chatron and Mitsuko Aramaki, Biology Letters, 18 December 2019, Article ID:20190747.

The authors, at the Max Planck Institute for Evolutionary Anthropology, Germany, and Aix Marseille University, France, explain:

We conducted field experiments to test whether impact sounds produced by throwing rocks at trees varied according to the tree’s properties. Specifically, we compared impact sounds of AST [accumulative stone throwing] and non-AST tree species. We measured three acoustic descriptors related to intrinsic timbre quality, and found that AST tree species produced impact sounds that were less damped, with spectral energy concentrated at lower frequencies compared to non-AST tree species. Buttress roots in particular produced timbres with low-frequency energy (low spectral centroid) and slower signal onset (longer attack time). In summary, chimpanzees use tree species capable of producing more resonant sounds for AST compared to other tree species available.

They touch on the question of whether this be music:

Studies on how variation in the sound-production properties of different tree species might affect animal behaviour are lacking despite observations of chimpanzees and palm cockatoos drumming on trees. In comparison, humans fashion a variety of wooden musical instruments whereby the quality of sound for each instrument is dependent upon the intrinsic sound properties of the tree species used, otherwise referred to as ‘timbre’. In particular, it has been shown that mechanical properties of wood species such as internal friction, density and the longitudinal modulus of elasticity are important aspects that instrument makers take into account when selecting tree species.