Tool choice on the basis of rigidity in capuchin monkeys
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Wild capuchin monkeys select stone tools to crack open nuts on the basis of their weight and friability, two non-visual functional properties. Here, we investigated whether they would select new stick-like tools on the basis of their rigidity. In Experiment 1, subjects faced an out-of-reach reward and a choice of three unfamiliar tools differing in color, diameter, material, and rigidity. In order to retrieve the reward, capuchins needed to select the rigid tool exemplar. Capuchins gathered information regarding tools’ pliability either by (1) manipulating the tools themselves (manipulation condition), (2) observing a human demonstrator repeatedly bending the tools (observation condition), or (3) seeing the tools placed on a platform without any manipulation taking place (visual static condition). Subjects selected the rigid tool above chance levels in both the manipulation and observation conditions, but not in the visual static condition. In Experiment 2, subjects needed to select and use a flexible tool to access a liquid reward (as opposed to the rigid tool, as in previous experiment). Again, capuchins selected above chance levels the appropriate tool (i.e., flexible), thus demonstrating a good appreciation of the relation between the tool properties and the task requirements.
KeywordsTool-use Object properties Problem solving Primates Monkeys
We thank Maria Elena Miletto for her help during the experiments, Noemi Spagnoletti for the photograph in Fig. 1, and Anna Bordonali for her help in calculating inter-observer reliability. We are also grateful to the Bioparco Foundation for hosting the laboratory where the experiment was carried out, G. Fidanza and our keepers M. Bianchi and S. Catarinacci. We acknowledge financial support by the grant IM-CleVeR (Intrinsically Motivated Cumulative Learning Versatile Robots), financed by the FP7 of the European Commission (Grant agreement n. 231722). Héctor M. Manrique was supported by an Exchange Grant within the framework of the European Science Foundation Research Networking Program on “Evolution of Social Cognition: Comparisons and integration across a wide range of human and non-human species”.
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