The Ontogeny of Social Comparisons by Rhesus Macaques (Macaca mulatta)

Copyright: © 2013 Hopper LM, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Introduction For adult humans, the concept of fairness appears to be universal [1], but the ability to recognize or react to an unequal distribution (inequity) is something that develops with age [2-4]. An inequity response occurs if an individual refuses a poor-quality reward for completing the same task as their partner, who received a more desirable reward [5]. In humans, this behavioral response to inequity has been widely studied in both psychology [6, 7] and economics [8], and recently, there has been a surge of interest in how other species respond to inequitable outcomes and what this can tell us about the evolutionary origins of the human response. To date, however, there have been no studies of the ontogeny of the inequity response in any species other than humans. Only certain (adult) nonhuman primate species compare their outcomes to those of others and respond to inequity (Cebus apella [9]; Pan troglodytes [10]; Macaca fascicularis [11]). The primary aim of this study, therefore, was to identify whether an, as yet, unstudied species (rhesus macaques, M. mulatta) responded to inequity and, if so, examine the ontogeny of this response. This would not only provide information specifically about rhesus macaques, but would also give insights into whether the developmental trajectory of this response in nonhuman primates mirrors that seen in human infants and children. We selected rhesus macaques as our study species for a number of key reasons. Although no study of social contrast has been conducted with rhesus macaques, long-tailed macaques (M. fascicularis), which are typified by similar natural histories to rhesus macaques [12-15], have recently been shown to respond negatively to inequity [11]. Additionally, as a response to inequity, requires monitoring rewards gained by social peers, it is notable that Macaca species are able to follow conspecifics' gaze direction, which is essential for monitoring rewards received by social peers (M. mulatta, M. arctoides, M. nemestrina [16]; M. nigra [17]; M. fascicularis [18]). Macaca species are also able to socially learn from conspecifics, leading to the emergence of behavioral traditions (M. fuscata [19,20]) and appear to recognize when they are being imitated (M. nemestrina [21]). Finally, they are known to be able …