This male chimpanzee in a nature reserve in Guinea-Bissau has just thrown a stone, captured on a camera trap. Photo: Stichting Chimbo
West African chimpanzees use stones as well as their hands and feet for communication. Subtle variations in context point to possible differences in meaning.
Throwing stones and striking trees with them: remarkable behaviours that contribute to the complex communication system of West African chimpanzees (w), according to scientists from Wageningen University and the German Primate Center in Göttingen (Biology Letters, 7 May).
Using camera traps and the assistance of local field guides, the research team collected video footage over several years from five sites in a nature reserve in Guinea-Bissau. Substantial piles of stones at the base of tree trunks indicate which trees are worth studying. The researchers analysed recently observed stone use by chimpanzees, as well as the already recognised hand and foot drumming on hollow tree roots – a behaviour audible over long distances that serves as a means of communication for group coordination and expressions of dominance. ‘When a chimpanzee uses a stone as a communication tool, we call this stone-assisted drumming‘, says behavioural biologist and first author Sem van Loon. ‘It mainly involves throwing, but sometimes the animal keeps its hand on the stone.’ Because stone throwing closely resembles regular drumming in terms of behavioural dynamics, the team wondered whether the chimpanzees use stones merely as a variation, or whether this phenomenon has its own function within the social repertoire of these animals.
Although unknown at the outset of the study, the accumulation of stones – as territorial marking, for example – does not appear to be the sole purpose of stone throwing. In 78 per cent of cases, chimpanzees use stones already present at the base of the tree, rather than bringing new ones.
Call
Just like hand and foot drumming, stone-assisted drumming appears to occur predominantly in adult males, in situations of excitement or social tension. The two behaviours occur both independently and in combination. Subtle behavioural differences in context nonetheless suggest a difference in content, according to the authors. For instance, chimpanzees more often produce a characteristic loud call before beginning to drum when they are throwing or striking with stones than when they use their limbs. After a drumming session, however, they vocalise less following stone use than after hand or foot drumming. Van Loon: ‘Stone-assisted drumming appears to be an alternative form of drumming behaviour, presumably a signal with a slightly different meaning, just as different drumming frequencies or combinations with, for example, screaming are expected to convey different messages.’ The precise function of this behaviour remains a matter of speculation.
‘Stone-assisted drumming is observed in only four locations in West Africa, one of which we selected for this study’, says Van Loon. ‘There are also indications of individual and local geographical variation, and adolescent chimpanzees appear to copy stone use from adults – both of which are indications of cultural evolution.’ The use of tools for communicative purposes is exceptional in the animal kingdom, according to behavioural biologist and senior author Marc Naguib. He emphasises: ‘It is important to realise that culture is part of nature, and that it can therefore also be lost if protection is insufficient.’