Selection and transport of tools to crack open nuts by semifree-ranging capuchin monkeys (Cebus sp): experimental manipulation of costs, quality of "hammers" and the distance and positioning between nuts, "hammers" and "anvils"
The spontaneous use of tools by capuchin monkeys has been studied by the Laboratory of Cognitive Ethology in wild and semifree-ranging groups. In the Tietê Ecological Park, the use of stones to crack open jerivá's nuts has been the target of naturalistic studies for over a decade. Recently, we start to study this behavior also through "experimental interventions" to deepen our understanding of various aspects of the selection and transport of the tools employed by the animals. The present study extends these experiments, evaluating tool selectivity of the monkeys as a function of "hammers" properties (shape and weight) and the effects of the distance between available "anvils" and "hammers" on this selectivity. We will analyze the effect of conspecifics' presence on the simultaneous transport of coconuts and "hammers", a peculiarity previously observed in the behavior of our subjects (and seen by some authors as a result of cognitive limitations), which, we hypothesize, may result from an "audience effect" associated with the risk of theft of these resources. The second experiment aims to examine the underlying planning in the use of tools in nut cracking by the animals, analysing the sequential organization of their visits to a nut cracking site where nuts, "hammers", and "anvils" are set evenly apart from each other. (AU)
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