Bone-related behaviours of captive chimpanzees (Pan troglodytes) during an excavating experiment

Abstract

After stone tools, bone tools are the most abundant tool type in the hominin archaeological record. However, due to their poor preservation, they are relatively scarce, limiting our understanding of the behavioural processes that led to their production and use. Extant primates constitute a unique source of behavioural data to build and shape hypotheses about the potential uses that our hominin ancestors might have given to bone tools. In this study, we investigated the behavioural responses of two groups of captive chimpanzees (Ntotal=42) to detached and cleaned bones while participating in a foraging task aimed at eliciting excavating behavior. Each chimpanzee group was provided with bones of different characteristics and the two groups differed in their respective levels of experience with tool excavation (one group being mostly inexperienced while the other had ample experience in stick tool excavation). We found that several individuals from the inexperienced group used the provided bones as tools during the task. No individuals from the experienced group used bones as excavating tools. Instead, these chimpanzees performed other bone-related behaviours (not observed in the inexperienced group), such as hammering and tool-assisted marrow consumption. Given that previous experience in tool excavation did not predict the use of bones as tools in the excavating task, we hypothesize that the differences in behavior observed between the two chimpanzee groups were due to the characteristics of the bones they were provided with. Furthermore, our results suggest that object characteristics rather than material determine whether chimpanzees perceive an object as a suitable excavating tool.