In forests all over West Africa, scientists spotted some adult male chimpanzees acting strangely. The animals lifted stones, whistled and threw them at the tree, then ran away. Small heaps of stones accumulate near such trees.
The reason is still a mystery, but this study provides a clue: chimpanzees seem to prefer to throw stones at trees, which, when struck, create a richer, longer sound. This suggests that chimpanzees throw stones either as a way to communicate or to listen to sounds.
Ammi Kalan, a primatologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, was the first to report the strange behavior of chimpanzees. For the past several years, she has been trying to figure out why chimpanzees do this.
The sea otter knew that some animals drummed on the tree supports to communicate their whereabouts to members of their group in other parts of the forest. If throwing stones had a similar purpose, chimpanzees could choose the trees that sound loudest, the expert concluded.
Later, a group of researchers in France who study sound perception and acoustics developed an experiment to test the differences in timbre of different trees. During the test, Kalan herself threw stones at 13 different types of trees and recorded the hits she received.
After they had the tapes, the researchers analyzed the noises from stones hitting different trees. It turns out that the chimpanzee's favorite trees to throw rocks at are trees with a lower, longer sound. The researchers noted that tall trees with erect roots were the most popular.
Researchers observed the behavior of stone throwers in only four groups of chimpanzees in Guinea-Bissau, Guinea, Liberia and Cote d'Ivoire. Scientists noted that not all chimpanzees throw stones, which is very intriguing for researchers. According to Andrew Whiten, a zoologist at the University of St Andrews in the UK, this suggests that this behavior is a local cultural tradition.
Kalan noticed that chimpanzees do not choose new trees to throw, but use those at the foot of which there are already stones. In the future, she hopes to figure out how chimpanzees distinguish their throwing trees in the first place. According to her, the location of the trees has something to do with nearby resources, such as food and water, and the sound is a signal for other monkeys to notify of this place.
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Kalan says this behavior can also be a manifestation of male dominance, because the main stone throwers are usually adult males. There is also a chance that chimpanzees are just doing it for fun, and it is possible that stone throwing is just the chimpanzee's own version of "rock music".