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Researchers recently discover unusual behavior of chimpanzees living in the forests of West Africa. An adult chimpanzee in the wild would pick up a rock, throw it at a tree while shouting, and then run away. Researchers aren't certain why the chimpanzees flingthe rocks, but they have discovered that they seem to prefer trees that create longer-lasting, more resonant* sounds when struck.

A team of researchers from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, first discovered this behavior three years ago. Because the actions appeared to be localised, the researchers suggested the purpose was a local tradition and was likely to be part of some sort of ritual*. But they weren't sure what purpose the ritual served. So the group designed more experiments to find out why the chimpanzees were so interested in throwing rocks at trees. This time around, they went to the same areas but set up microphones to get the sounds of stones as they were thrown. While waiting for the chimps to fling the rocks, the researchers threw stones at 13 different kinds of trees in various situations.

"It was quite fun, I have to say," expert Ammie Kalan tells Science. Researchers analysed all the recordings and found that the chimpanzees seemed to prefer throwing rocks at trees that made lower, longer-lasting sound. Often these were trees that had bared roots*.

In their findings, published in Biology Letters, the researchers write that "low-frequency sounds travel further in the environment and are better suited to long-distance communication." In addition, more resonant sounds last longer in the environment. But if chimpanzees were looking to communicate, it would be more effective for them to drum on the trees or choose the ones that made the loudest noises when struck.

The researchers found that chimpanzees stick to the same trees and never choose new ones and that location could be a factor. Kalan tells Science that maybe the locations of the trees have something to do with nearby resources like food and water, and the sound is a signal to others about where to find them.