West African chimpanzees like to throw rocks at trees, and a new study has found it's probably because they appreciate the deep resonant thump made on impact. Although it might have begun as a way to communicate locations, it's possible this behavior, which appears to be restricted to a small number of groups, could evolve into something significant.
In 2016, Dr Ammie Kalan of the Max Plank Institute for Evolutionary Anthropology reported not only the fondness some chimpanzees have for close-range hurling of rocks at trees but that only certain trees got this treatment. The pattern was so clear, piles of rocks can be seen at the base of the chosen trees. Why they do this, however, remained a mystery – one that was compounded by pre-throwing hooting and a pattern of running away after hitting the tree.
Might it be a ritual? Did the rock-throwers have a grudge against these specific trees? Could they be trying to knock out fruit or scare potential prey? Kalan noticed the trees chosen for this treatment (known as accumulative stone throwing or AST) always came from certain species. After investigating the common AST tree features, Kalan thinks she has a partial answer.
In Biology Letters, Kalan reports she and her co-authors “found that AST tree species produced impact sounds that were less damped, with spectral energy concentrated at lower frequencies.”
Lower frequency sounds travel further, particularly in an environment like a rainforest. The deep pitch produced by the rock throwing seems to be a way to communicate at a distance. If so, it is an exceptionally rare example of tool use by a non-human animal for communication purposes, rather than to access food, although palm cockatoos turn seedpods into drumsticks.
Chimpanzees have been known for many years to drum on the sides of trees or their roots, apparently to signal their location, but using rocks takes the whole thing to a new level. The behavior has only been seen in West African chimpanzees, not in the better studied East African populations or even in the geographically closer central African chimps. However, the fact chimpanzees can throw stones, either as crude weapons or simply as displays of hostility, was reported by Jane Goodall decades ago.
Rather than throwing the rock once, some chimpanzees prefer to beat it repeatedly against the tree, creating a deep drumming. Speculatively, this could represent the first music, an early Things of Stone and Wood. Most, but not all rock throwing, was done by adult males.
So what kind of tree does a chimpanzee like to rock with? Tall “buttress” roots help produce the best sound, frequently from the Treculia genus, whose large “breadfruits” the chimps also sometimes cut up with sharp stones.