ORIGINAL PAPER Evaluation of third-party reciprocity by squirrel monkeys (Saimiri sciureus) and the question of mechanisms James R. Anderson 1 • Benoit Bucher 1 • Hika Kuroshima 1 • Kazuo Fujita 1 Received: 17 December 2015 / Revised: 26 February 2016 / Accepted: 18 March 2016 / Published online: 28 March 2016 Ó Springer-Verlag Berlin Heidelberg 2016 Abstract Social evaluation during third-party interac- tions 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 possi- bility 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 coopera- tive 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. Keywords Non-human primates Á Saimiri sciureus Á Social exchange Á Third-party relationships Á Reciprocity Á Social evaluation Introduction As in a wide range of animal species (Earley 2010), eaves- dropping on social interactions between third parties is common in non-human primates (Cheney 2011). The advantages of eavesdropping for monkeys and apes include acquisition of information about mating opportunities (Crockford et al. 2007), others’ competitive abilities or social dominance (Borgeaud et al. 2013; Cheney et al. 1995), or foraging opportunities (Russell et al. 2008; Subiaul et al. 2008). In the latter studies, for example, chimpanzees eventually learned to beg preferentially from a ‘generous’ human actor who shared food with a third party (beggar), rather than a ‘mean’ actor who refused to share. With the possible exception of studies on inequity aver- sion (for review, see Brosnan and de Waal 2014), studies of non-human primates’ social evaluations of individuals based on third-party interactions are rarely conducted from the perspective of moral judgments or perceptions of fairness. This latter perspective is more typical of studies with humans; moral behavior, including fairness in exchanges of resources, is considered crucial for cooperative societies to function effectively (Alexander 1987; Sun 2013). Signs of moral reasoning emerge early in human infants. For exam- ple, before 2 years of age, infants evaluate the fairness of distributions of items such as food or toys between third parties, expressing negative or surprise reactions when individuals benefit from inequitable sharing (Geraci and Surian 2011; Schmitt and Sommerville 2011; Sloane et al. 2012). Electronic supplementary material The online version of this article (doi:10.1007/s10071-016-0980-7) contains supplementary material, which is available to authorized users. & James R. Anderson j.r.anderson@psy.bun.kyoto-u.ac.jp 1 Department of Psychology, Kyoto University Graduate School of Letters, Yoshida-honmachi, Sakyo, Kyoto 606- 8501, Japan 123 Anim Cogn (2016) 19:813–818 DOI 10.1007/s10071-016-0980-7