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de Waal, F. B. M., Dindo, M., Freeman, C. A., & Hall, M. J. (2005). The monkey in the mirror: hardly a stranger. Proc. Natl. Acad. Sci. U.S.A., 102(32), 11140–11147.
Abstract: It is widely assumed that monkeys see a stranger in the mirror, whereas apes and humans recognize themselves. In this study, we question the former assumption by using a detailed comparison of how monkeys respond to mirrors versus live individuals. Eight adult female and six adult male brown capuchin monkeys (Cebus apella) were exposed twice to three conditions: (i) a familiar same-sex partner, (ii) an unfamiliar same-sex partner, and (iii) a mirror. Females showed more eye contact and friendly behavior and fewer signs of anxiety in front of a mirror than they did when exposed to an unfamiliar partner. Males showed greater ambiguity, but they too reacted differently to mirrors and strangers. Discrimination between conditions was immediate, and blind coders were able to tell the difference between monkeys under the three conditions. Capuchins thus seem to recognize their reflection in the mirror as special, and they may not confuse it with an actual conspecific. Possibly, they reach a level of self-other distinction intermediate between seeing their mirror image as other and recognizing it as self.
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Dindo, M., & De Waal, F. B. M. (2007). Partner effects on food consumption in brown capuchin monkeys. Am. J. Primatol., 69(4), 448–456.
Abstract: It has been claimed that capuchin monkeys (Cebus apella) show inequity aversion in relation to food rewards for a simple exchange task. However, other factors may affect the willingness of a monkey to consume foods of high or low value in the presence of a conspecific. In this study, pairs of monkeys were presented with unequally valued foods, but without any task-performance: they simply received the food under four experimental conditions. By looking at the rate of collection and consumption of low-valued cucumber slices we expected to see variation dependent on whether the partner either had 1) cucumber (equity), 2) grape (inequity), 3) inaccessible cucumber or 4) inaccessible grape. Testing 12 adult capuchin monkeys, our findings differed from those of other authors in that the monkeys failed to show negative reactions to inequity, but rather responded with scramble competition (i.e., fast food collection) in the presence of a conspecific without access to food. They also showed facilitated consumption in the presence of a conspecific consuming high-valued food. Possibly, (in)equity plays a different role if food serves as a reward for a task rather than if it is simply made available for consumption. Am. J. Primatol. 69:1-9, 2007. (c) 2006 Wiley-Liss, Inc.
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