I spent many months in the field, along with many other researchers, trying to figure out what these chimps are up to. So far we have two main theories. The behaviour could be part of a male display, where the loud bang made when a rock hits a hollow tree adds to the impressive nature of a display. This could be especially likely in areas where there are not many trees with large roots that chimps would normally drum on with their powerful hands and feet. If some trees produce an impressive bang, this could accompany or replace feet drumming in a display and trees with particularly good acoustics could become popular spots for revisits.
On the other hand, it could be more symbolic than that – and more reminiscent of our own past. Marking pathways and territories with signposts such as piles of rocks is an important step in human history. Figuring out where chimps' territories are in relation to rock throwing sites could give us insights into whether this is the case here.
Even more intriguing than this, maybe we found the first evidence of chimpanzees creating a kind of shrine that could indicate sacred trees. Indigenous West African people have stone collections at “sacred” trees and such man-made stone collections are commonly observed across the world and look eerily similar to what we have discovered here.
Stone throwing - in action and on site. Top line: Adult male tossing, hurling and banging a stone. Bottom line: Stones accumulated in a hollow tree; typical stone throwing site; and stones in between large roots. Kühl et al (2016), Author provided
A vanishing world
To unravel the mysteries of our closest living relatives, we must make space for them in the wild. In the Ivory Coast alone, chimpanzee populations have decreased by more than 90% in the past 17 years.
A devastating combination of increasing human numbers, habitat destruction, poaching and infectious disease severely endangers chimpanzees. Leading scientists warn us that, if nothing changes, chimps and other great apes will have only 30 years left in the wild. In the unprotected forests of Guinea, where we first discovered this enigmatic behaviour, rapid deforestation is rendering the area close to uninhabitable for the chimps that once lived and thrived there. Allowing chimpanzees in the wild to continue spiralling towards extinction will not only be a critical loss to biodiversity, but a tragic loss to our own heritage, too.
You can support chimps with your time, by instantly becoming a citizen scientist and spying on them at www.chimpandsee.org, and with your wallet by donating to the Wild Chimpanzee Foundation. Who knows what we might find next that could forever change our understanding of our closest relatives.