For years, researchers have been perplexed by a strange phenomenon happening deep in the forests of West Africa: chimpanzees throwing rocks at trees. A new study, published this month in the journal Biology Letters, sheds some light on the situation. When a chimp wants to huck a stone, it chooses a tree that sounds like a bass drum.

“Accumulative stone throwing,” as it’s known, was first described by Jane Goodall more than 50 years ago. The behavior tends to occur when male chimps get particularly excited: shaking side to side, giving excited “hoot-pant” vocalizations, and, on occasion, hurling a rock at a tree.

For a long time, primatologists weren’t quite sure what to make of this behavior. Sure, it was a unique example of an animal using an object to make a sound—a rare form of tool use only documented in great apes and cockatoos. But what exactly did it mean? Four years ago, a group of researchers set up trail cameras all over West Africa to find out.

What they learned surprised them. Accumulative stone throwing was not a spontaneous burst of excitement that manifested in smashing a stone against the nearest solid object. Instead, the chimpanzees seemed to be deliberately targeting the same trees over and over again. Over time, these hapless trees have ended up with a collection of stones strewn around their trunks.

So what’s so special about these trees? To sort this out, a group of scientists led by primatologist Ammie Kalan flew back to West Africa with a straightforward research goal: to throw their own rocks at these trees and see what happens.

Kalan first went to the accumulative stone throwing sites discovered in the team’s first study and recorded which tree species were being targeted. It turns out that chimpanzees, faced with a literal forest of options, choose only six tree species to throw rocks at. In a remarkably simple experiment, Kalan grabbed some rocks of her own and threw them at more than 80 of these trees, recording the sound made by the rocks using a microphone.

Next, Kalan made a list of seven other tree species that were common in the area, but which chimpanzees never targeted for accumulative stone throwing. She found 40 of these trees and again recorded the impact sound made when she threw a rock against their trunks and roots.

By comparing the acoustic characteristics of rocks hitting these two groups of trees, Kalan discovered that the trees used by chimps produced a much lower-pitched noise than the trees chimps never threw rocks at. Importantly, lower-pitched noises tend to travel further in a landscape, which is why, for example, a loud bass rhythm from a car stereo can wake you up at night. By deliberately hitting trees capable of making a sound that can be heard deep into the forest, the chimpanzees seemed to be using stone throwing as a long-distance communication signal.

However, the jury is still out as to what that sound is actually communicating. One possibility is that it’s a threat meant to let other nearby chimpanzees know the extent of the rock hurler’s territory. But the researchers point out that this does not explain why a chimp would specifically choose to throw stones. While the thud of a rock against a tree trunk is loud, it doesn’t carry nearly as far as other, more common signals in the chimp-noise repertoire, like hand-drumming against roots or hoot-panting.

An intriguing alternative possibility is that rock throwing may be a cultural tradition, since the behavior is only practiced by certain troops of chimps. Rather than communicating some specific bit of information, perhaps throwing a stone is simply a ritual that chimpanzees do whenever they go to a specific place in the forest– not unlike certain human cultural groups that pile stones beside sacred trees. In this case, certain trees may be targeted by the apes simply because they produce the “right” sound for the tradition.

In the future, the research team hopes to excavate these stone throwing sites to uncover just how long chimpanzees have been hucking stones in the same spots in the forest. By better understanding how our closest relatives may be practicing cultural rituals over time, Kalan and the team hope to uncover the origins and significance of human behavior and traditions—and perhaps even our affinity for a meaty bass rhythm.