Stones a big hit: how jungle chimpanzees make rock music

In the forests of west Africa some chimps have an unusual custom: they hurl stones at trees. Zoologists are working out why.

Chimpanzees are thought to be rock music fans. Picture: AAP

In the forests of west Africa, some chimpanzees have adopted an unusual custom: they hurl stones at trees. Now zoologists have begun to unravel the mystery of why.

A study suggests that the apes seek out trees that produce a pleasing, resonant chime when struck.

They are, it seems, creating their own form of rock music.

When Western researchers were first told by locals that chimps were responsible for the collections of stones found around certain trees in Guinea-Bissau, they were sceptical.

In some cases, hollow trunks were full of rocks. In others, the stones formed cairn-like towers.

Camera traps confirmed that the chimps, not people, were the culprits, but the purpose was still unclear.

Was it a way for males to show off? A means of laying claim to territory? A form of long-distance communication?

Ammie Kalan, of the Max Planck Institute for Evolutionary Anthropology in Leipzig, decided to test whether the acoustic properties of the trees might be significant.

Although the jungle has at least 40 arboreal species, the chimps throw rocks at only seven of them. Dr Kalan’s team found 39 trees that the chimps had targeted. They lobbed their own rocks and analysed the sounds they made.

The apes, it appears, have an ear for timbre. Their stone-throwing trees were found to have “low damping coefficients”, which means that they produced a sustained, sonorous sound when struck.

According to a study published in the Royal Society journal ­Biology Letters on Wed­nesday, their favoured trees also produced low-frequency tones, of a kind that would travel far in the jungle.

By contrast, the trees shunned by the chimps produced boring dull thuds.

“The trees they use are the ones that resonate, like when you hit a xylophone and the note echoes in the room. It’s a particularly nice sound,” Dr Kalan said.

She suspects that the stone throwing emerged from behaviour seen in all wild chimpanzees, known as “buttress drumming”, which involves feet and hands being used to bang on tree trunks.

Buttress drumming is far noisier than stone throwing, however, which would suggest that the latter has a purpose other than long-range communication.

“The stone-throwing produces just one beat,” Dr Kalan said.

“From the perspective of long-distance communication, buttress drumming is better.

“So the stone throwing still seems somewhat bizarre. There must be an additional purpose to it.”

The Times