Wild bearded capuchin (Sapajus libidinosus) select hammer tools on the basis of both stone mass and distance from the anvil
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Contemporary optimization models suggest that animals optimize benefits of foraging and minimize its costs. For wild bearded capuchins (Sapajus libidinosus), nut-cracking entails cost related to lifting the heavy stone and striking the nut and additional cost to transport the stone if it is not already on the anvil. To assess the role of stone mass and transport distance in capuchins’ tool selection, we carried out three field experiments. In Experiment 1, we investigated whether transport distance affected choice of a tool by positioning two stones of the same mass close and far from the anvil. Capuchins consistently selected the closer stone, effectively reducing transport costs. In Experiment 2, we examined the trade-off between the cost of transport and the effectiveness in cracking by positioning two stones of different mass close and far from the anvil. Most subjects significantly preferred the closer stone, regardless of mass, whereas others preferred the heavier stone regardless of transport distance. In Experiment 3, we changed transport distance of both stones while maintaining the same distance ratios as in Experiment 2. Capuchins maintained the preferences expressed in Experiment 2, with the exception of one subject. Overall, our findings indicate that (1) individuals vary in their sensitivity to distance of transport, (2) a few meters are perceived as a substantive cost by some monkeys, and (3) monkeys’ body mass affects their decisions. We also developed a non-dimensional Preference index (P) defined as a function of the stone mass and the transport distance to describe monkey’s choice.
KeywordsNut cracking Tool transport Stone mass Cost-benefits Optimization Cebus
Thanks to the Familia M for permission to work at Fazenda Boa Vista, Noemi Spagnoletti, Giulio Casali, and Alessandro Albani for their assistance in data collection. Thanks to Elsa Addessi for revising the manuscript and to the anonymous referees for their constructive comments. This study was supported by the Leakey Foundation, Sapienza University of Rome, the University of Georgia LACSI Tinker Graduate Research Award, Consiglio Nazionale delle Ricerche Short Term Mobility Program. Permission to work in Brazil was granted by IBAMA and CNPq.
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