A new study presents evidence of symbolic reasoning in tufted capuchin monkeys, a South-American species that diverged from humans about 35 million years ago. In the experiment, five capuchins engaged in "economic choice" behavior. Each monkey chose between three different foods (conventionally referred to A, B and C), offered in variable amounts. Choices were made in two different contexts. In the "real" context, monkeys chose between the actual foods. In the "symbolic" context, monkeys chose between "tokens" (intrinsically valueless objects such as poker chips) that represented the actual foods. After choosing one of the two token options, monkeys could exchange their token with the corresponding food.

The researchers examined whether capuchins' preferences in both real and symbolic contexts satisfy transitivity -- a fundamental trait of rational decision-making, according to which if A is preferred to B, and B is preferred to C, then A must be preferred to C.

Capuchins' choices did satisfy transitivity, both in the real context and in the symbolic context. Capuchins systematically preferred item A to B, item B to C, and item A to C both with tokens and with the actual foods. Hence, their preferences were qualitatively similar in both contexts. Quantitatively, however, expressing choices in the symbolic context increased the value distance between the corresponding foods.

For example, when choosing between actual foods, capuchins were indifferent between one Cheerio and two pieces of parmesan cheese, indicating that the value of one Cheerio is equal to two times the value of one piece of parmesan cheese. When choosing between tokens that represented the same foods, the relative value increased -- for example, capuchins were indifferent between one Cheerio-token and four parmesan-tokens.

These results indicate that capuchin monkeys can indeed reason about symbols. However, as they do so, capuchins also experience the cognitive burden of symbolic representation, and in this respect they appear to behave similarly to young children. In sum, though capuchins may not achieve adult-human-like symbolic competence, this study demonstrates that animal species relatively distant from humans have undertaken the path of symbolic use and understanding.

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Journal Reference:

1. Addessi et al. Preference Transitivity and Symbolic Representation in Capuchin Monkeys (Cebus apella). PLoS One, 2008; 3 (6): e2414 DOI: 10.1371/journal.pone.0002414