This study aims at studying cognitive and behavioral mechanisms displayed by capuchin monkeys in tool choice for cracking nuts. We forecast that the hammer's shape has a role in the tool choice. As perception / action theory predicts, there should be a broad range of variability between subjects and this should be directly related to the subject's experience of cracking nuts. Moreover, as the optimal foraging theory predicts, the subject's choice should be positively correlated with the nut cracking efficiency provided by each tool type (or shape). Tool choice in nut cracking by capuchin monkeys is a critical and necessary component to succeed: the animal may choose such as the most appropriate tool or modify to achieve goal. This choice is possible through the process of detecting and relating affordances, as a consequence of individual learning by trial and error, and observational learning of group member's behavior. These suitable choices have effect not only in obtaining food resource success, but also on their behavior efficiency, increasing benefits and reducing risks. Considering the tools, it is possible to choose the tool to be used to hit the nut (hammer), the tool (substrate or object) on which the nut will be placed to be broken (anvil), and even the nut to be cracked. We propose an experimental study in which we aim to evaluate the role of affordances detection related to objects that may be used as percussive tools and their relationships. We predict that monkeys chose hammers with optimal weigh according to their own weigh, and with a shape that allows for optimal prehension.
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