Chimpanzees are a musical lot, and percussion is an important part of their repertoire. But one particular behavior continues to fascinate zoologists: every so often, chimpanzees (Pan troglodytes) are caught on camera, yelling and throwing rocks at a good, solid tree trunk, in what some have even described as a kind of ritual.
Footage collected throughout a nature reserve in Guinea-Bissau across five years shows a series of fascinating variations on this phenomenon, captured by researchers from Wageningen University and the German Primate Research Centre.
Their records reveal, on multiple occasions, lone adult male chimpanzees hanging out by a sturdy tree trunk before rousing into a frenzy of hoots and hollers that are punctuated by the satisfying 'thunk' of solid stone against wood, like the sting of a joke.
It's similar to another kind of 'drumming' chimps do with their hands or feet on the hollow buttress roots of trees, but this technique follows a different structure, with silence preceding the drumming. 'Stone-assisted drumming', as the researchers call it, tends to go the other way round.
Of course, someone always has to bend the genres: this male seems to have combined drumming and rock-throwing into a more experimental cacophony.
In a few instances, these outbursts and their accompanying instrumentation appear to elicit a response from other chimpanzees nearby, which suggests it could be a form of communication used when individuals are further apart.
"It may be that these loud, low-frequency sounds are meant to carry further than typical within-group communication," behavioral biologist Sem van Loon suggests. "The acoustic properties of a stone striking a tree make that feasible in densely forested areas."
These strange videos may add to our understanding of this percussive behavior, but for now, its true purpose remains a mystery to us humans.
The research was published in Biology Letters.