Evolutionary origins of money categorization and exchange: an experimental investigation in tufted capuchin monkeys (Sapajus spp.).

Money is a cultural artefact with a central role in human society. Here, we investigated whether some features of money may be traced back to the exchange habits of nonhuman animals, capitalizing on their ability to flexibly use tokens in different domains. In Experiment 1, we evaluated whether capuchins can recognize token validity. Six subjects were required to exchange with the experimenter valid/familiar tokens, valid/unfamiliar tokens, invalid tokens, and no-value items. They first exchanged a similar number of valid/familiar and valid/unfamiliar tokens, followed by exchanges of invalid tokens and no-value items. Thus, as humans, capuchins readily recognized token validity, regardless of familiarity. In Experiment 2, we further evaluated the flexibility of the token-food association by assessing whether capuchins could engage in reverse food-token exchanges. Subjects spontaneously performed chains of exchanges, in which a food item was exchanged for a token, and then the token was exchanged for another food. However, performance was better as the advantage gained from the exchange increased. Overall, capuchins recognized token validity and successfully engaged in chains of reverse and direct exchanges. This suggests that-although nonhuman animals are far from having fully-fledged monetary systems-for capuchins tokens share at least some features with human money.