Evaluation of third-party reciprocity by squirrel monkeys (Saimiri sciureus) and the question of mechanisms
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Social evaluation during third-party interactions emerges early in human ontogeny, and it has been shown in adult capuchin monkeys who witness violations of reciprocity in object exchanges: Monkeys were less inclined to accept food from humans who refused to reciprocate with another human. A recent study reporting similar evidence in marmoset monkeys raised the possibility that such evaluations might be based on species’ inherent cooperativeness. We tested a species not renowned for cooperativeness—squirrel monkeys—using the procedure used with marmosets and found a similar result. This finding rules out any crucial role for cooperative tendencies in monkeys’ responses to unfair exchanges. We then tested squirrel monkeys using procedures more similar to those used in the original study with capuchins. Squirrel monkeys again accepted food less frequently from non-reciprocators, but unlike capuchins, they also strongly preferred reciprocators. We conclude that neither squirrel monkeys nor marmoset monkeys engaged in emotional bookkeeping of the type that probably underlies social evaluation in capuchin monkeys; instead, they employed one or more simple behavioral rules. Further comparative studies are required to clarify the mechanisms underlying social evaluation processes across species.
KeywordsNon-human primates Saimiri sciureus Social exchange Third-party relationships Reciprocity Social evaluation
This research was supported by JSPS Grant-in-Aid for Research Activity Start-up No. 2688503 to JRA, and by JSPS Grant-in-Aid for Scientific Research (A) No. 25240020 and that for Priority Areas No. 25118002 to KF. Housing and care of the monkeys adhered to Kyoto University Primate Research Institute’s Guide for the Care and Use of Laboratory Primates (2nd ed.). The study was approved by the Animal Experiment Committee of the Graduate School of Letters, Kyoto University.
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