Fig 1.
Location and view of the Kumamoto Sanctuary (A); close-ups of the nut cracking experiments (B and C). View of the instructor (SH) and chimpanzee interacting during the experiments (D). See S1 Video for a short video clip of the experiments.
Table 1.
Summary of the stone samples used in the experimental sessions.
The number of strikes per nut was obtained by dividing the total number of nuts cracked by the total number of strikes.
Table 2.
Basic measurements (in mm and g) of percussive artefacts.
After the experiments, artefacts showed no visible modifications in size and only two lost mass owing to the detachment of fragments.
Fig 2.
Percussive positives and refit from active element O11.
See description of groups in Table 3 caption. All positives from Group 5 were detached during four separate nut cracking sessions. First, the positive from Group 4 (1), located on the left lateral side of the plane, was detached. This was followed by the two positives from Group 2.3 that conjoined on a dorsal-ventral refit located on the proximal plane (2). The last positive of the sequence is the fragment from Group 2.3 located on the right side of the horizontal plane (3), which was detached in a different experimental session.
Table 3.
General measurements (in mm and g) of positives detached during the experiments.
Group categories follow the classification established by de la Mora and de la Torre [13] and expanded by Arroyo [61]. Group 2.3: positives that mimic characteristics of knapping flakes having platforms, bulb of percussion, and impact point; Group 4: irregular fragments without any butt, bulb, or platform; Group 5: positives smaller than 20 mm on which features such as percussive marks, platforms, or impact points are absent.
Fig 3.
A) Piece O15 with crushing marks on the left lateral edge. 1. Residues identified before cleaning (80×, scale 500 μm); 2. Detail of crushing (20×, scale 1 mm); 3. Impact point (20×, scale 500 μm); 4. Pit associated with a crushed area (40×, scale 500 μm). B) Piece O48. 1. Detail of residues (16×, scale 3 mm); 2 and 3. Crushing (25× and 20×, both scales 1 mm).
Table 4.
Summary of use-wear traces identified on the pounding tools used in the experimental programme.
Fig 4.
A) Passive element O74. 1. Detail of residues (50×, scale 500 μm); 2 and 3. Impact points (50× and 80×, both scales 500 μm). B) Active element O72. 1. Residues on the working surface (80×, scale 500 μm); 2. Impact point (40×, scale 800 μm).
Fig 5.
A) Piece O35. 1. Detail of crushing associated with detachment of small crystal fragments (80×, scale 500 μm). 2. Residues (80×, scale 500 μm). B) Piece O11. 1 and 2. Crushing with associated microfractures (10× and 20×, scales 5 mm and 2 mm). 2. Impact point (50×, scale 700 μm).
Fig 6.
Spatial distribution analysis of pounding tools used in the experimental programme (see S1 Table for details).
Analysis was not conducted on lava tools O74 and O128, as they did not develop macroscopic traces.
Fig 7.
Mechanisms that explain use-wear patterns identified on pounding tools used by captive chimpanzees: A) Compression; B) Compression and peripheral contact; C) Direct impact.
Fig 8.
Comparison of different passive elements showing similarities in the wear pattern.
A) Tool use by a captive chimpanzee; B) Anvil from SHK, Olduvai Gorge (Leakey’s collection); C) Anvil used to crack/open nuts by modern humans; D) Anvil used to tenderize meat (C and D from de la Torre et al., Figs 6B and 8C [14]).