Why Do These Chimps Like to Throw Rocks at Trees?

A chimpanzee rests on a branch. Abeselom Zerit/Shutterstock

Researchers recently discovered some unusual behavior in chimps living in the forests of West Africa. An adult male in the wild would pick up a rock, throw it at a tree while yelling, and then run away.

Although researchers aren't certain why the chimps fling the rocks, they have a hint: The chimps 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 uncovered this behavior three years ago. Because the actions appeared to be localized, the researchers suggested the purpose was a local tradition and was likely part of some sort of ritual, Phys.org reports. But they weren't sure what purpose the ritual served.

So the group devised more experiments to determine why the chimps were so interested in lobbing rocks at trees. This time around, they went to the same areas but set up microphones to capture the sounds of the stones as they were thrown. While waiting for the chimps to hurl the rocks, the researchers threw the stones at 13 different kinds of trees in various situations.

"It was quite fun, I have to say," coauthor and primatologist Ammie Kalan tells Science.

Researchers analyzed all the recordings and found that the chimps tended to prefer throwing rocks at trees that made lower, longer-lasting sounds. Often these were trees that had exposed roots.

In their findings, published in Biology Letters, the researchers write that "low-frequency sounds travel further in the environment and are better suited for long-distance communication." In addition, more resonant sounds will last longer in the environment.

But if chimps 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.

Because the researchers saw chimps stick to the same trees and never choose new ones, 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 where to find them.

So, there are still questions to be answered. In the meantime, you can watch the chimps toss rocks at trees in the video below.