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Across Africa, chimpanzee communities exhibit large behavioural diversity not exclusively explained by ecological or genetic variation. However, excluding subtle environmental factors is difficult when comparing far-apart populations. Taï National Park offers a unique opportunity to explore the impact of social learning on behavioural diversity. The Taï South and Djouroutou communities live only 60 km apart in a continuous ecosystem. However, these groups apply semi-different techniques when nut-cracking. Djouroutou chimpanzees crack five species of nuts, exclusively using stone tools. Taï South only cracks four of those species, using stone and wooden tools, and does not exploit abundant Sacoglottis gabonensis nuts. Moreover, this group uses stone and wooden tools. Here, we compared tool material availability and tool choice. Our results showed that both groups did not differ in available nut trees and access to different materials. The communities responded to these ecological conditions differently than predicted if availability was the only factor responsible for shaping behaviour. This highlights the potential role of cultural behaviour in wild chimpanzee feeding ecology.
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 often relied on implicit assumptions or theoretical arguments about how tool features would influence foraging efficiency, and comprehensive measures of actual efficiency are still missing. We used field observations of nut-cracking efficiency and previously published estimates of energetic costs to investigate the rate of net energy intake as a function of hammer weight and hammer material. While stones allowed for a generally more efficient performance, nut-cracking efficiency depended on an interaction of hammer weight and material. Relative performance of stones and wood varied according to the ripeness of the nuts. Chimpanzees’ tool selection tends to optimize nut-cracking in many respects. Nonetheless, we also observed a few mismatches between efficiency and selection, some of which may be explained on cognitive, motivational or cultural grounds.
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