An energetic model of foraging optimization: wild chimpanzees hammer selection for nut cracking

The chimpanzees of the Taï forest, Côte d’Ivoire, crack highly nutritious Coula edulis nuts using anvils and hammers. While using tools to access encased food items provides obvious benefits, the energetic gain of tool-assisted foraging can be further increased by optimal selection of tools. Previous studies of animal tool selection typically relied on implicit assumptions or theoretical arguments about how tool features would influence foraging efficiency, and comprehensive measures of actual efficiency are still missing. In this study, we used field observations of nut-cracking efficiency in the Taï North community and previously published estimates of energetic costs involved in this behaviour to investigate the rate of net energy intake as a function of hammer weight and hammer material (stone or wood). We found that stones allow for a generally more efficient performance (up to 90% higher net rate of energy intake compared to wooden hammers of similar weight), as a result of increased speed in opening and consuming the nuts, despite a similar number of strikes per nut. However, we also showed that nut-cracking efficiency depends on an interaction of hammer weight and material, and that the relative performance of stones and wood varies according to the ripeness of the nuts. We combined the outcome of our analyses with previously described patterns of tool selection, finding that chimpanzees’ tool selection tends to optimise nut cracking in many respects. Nonetheless, we also observed a few mismatches between efficiency and selection, some of which may be explained on cognitive and motivational grounds. Our results highlight the importance of validating assumptions about tool use efficiency with field data, even more importantly as cultural aspects are also known to influence tool selection in Taï chimpanzees.