By Eva Frederick

In forests across West Africa, scientists have caught some grown male chimpanzees engaging in a strange behavior. As seen in the video above, they pick up a rock, hoot, throw the stone at a tree, and run away. At popular trees, small piles of rocks build up.

The reason is still a mystery, but a new study reveals a clue: The chimps seem to prefer to throw rocks at trees that create a richer, longer lasting sound when struck. This suggests the chimps are chucking the stones either as a method of communication—or simply because they like the sound.

Ammie Kalan, a primatologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and co-authors were the first to report the rock-throwing behavior in 2016. For the past few years, Kalan has been trying to figure out why the chimps do it.

She knew that some of the animals “drum” on the buttresses of trees to communicate their whereabouts to members of their group in other parts of the forest. If the rock-throwing had a similar purpose, the chimps might choose trees that sounded the loudest when hit.

Kalan reached out to two groups of researchers in France who study sound perception and acoustics, and together they designed an experiment to test the differences in the timbre of different timber. The test involved Kalan herself chucking rocks at 13 different kinds of trees and recording the resulting thuds. “It was quite fun, I have to say,” she says.

Once they had the recordings, the researchers analyzed how percussive the noise was, how bright or muted it sounded, and how long it took to fade away. It turned out that chimps’ favorite trees to throw rocks at were those with a lower, longer-lasting sound, such as the Treculia tree, the researchers report today in Biology Letters. It didn’t matter much what the trees looked like, although those with tall, raised “buttress” roots tended to be the most popular.

So far, researchers have only observed the rock-throwing behavior in four groups of chimps in Guinea-Bissau, Guinea, Liberia, and Ivory Coast. The fact that not all chimps do it—even when plenty of nice-sounding trees are available—is “intriguing,” says Andrew Whiten, a zoologist at the University of St. Andrews in the United Kingdom who was not involved in the study. “This suggests the behavior is a local cultural tradition.”

During the course of the experiment, Kalan never caught a chimp in the act of choosing a new tree to throw stones at—instead they kept banging away at those that already had piles of rocks at their base. In the future, she hopes to find out how the chimps single out their throwing trees in the first place. Maybe, she says, the locations of the trees have something to do with nearby resources like food and water, and the sound is a signal to others of where to find them.

Kalan says the behavior could also be a display of male dominance, because the main rock throwers are usually adult males. There’s also the chance the chimps just do it for fun, although Kalan thinks this in unlikely. “Play behavior is a little less structured, a little more impromptu,” she says. (One of chimps’ favorite ways to play is grabbing each others’ feet while walking so they fall over, she notes.) Still, Kalan can’t rule out this hypothesis completely—nor the idea that the stone-throwing behavior is just the chimps’ very own version of rock music.