Quantifying Selection in Chimpanzee Composite Tool Use at Bossou, Guinea: Novel Material Property Tests of the Archaeological Record

Variables such as rock shape and size affect tool selection decisions of wild chimpanzees during percussive tool use. However, very little is known about how chimpanzees discriminate between different rock types. The lithologies selected for this study were previously analyzed to determine which variables influenced raw material selection by Early Pleistocene hominins. Previous results suggest that hominin selection linked to two mechanical properties: durability and fracture predictability. At the outdoor laboratory, in Bossou, Guinea, we investigated tool selection among a group of wild chimpanzees. We provided them with rock types preferred by hominins to make chipped stone artefacts at the site of Kanjera South, Kenya, which are not available in the modern Bossou habitat. We investigated chimpanzee selectivity by calculating indice of selection based on the use of different rock types. Tests on rock elasticity and micro-hardness allow us to link selection to material properties. Results show that decisions on tool selection by chimpanzees correlate with (invisible) mechanical properties of rocks. Data demonstrate that rock hardness and elasticity drive the selection decisions of both hammers and anvils. This pattern is more evident amongst older individuals with greater efficiency and skill in nut cracking.