Investigating reactions of squirrel monkeys ( Saimiri sciureus ) towards unequal food distributions in a tray-pulling pa
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ORIGINAL ARTICLE
Investigating reactions of squirrel monkeys (Saimiri sciureus) towards unequal food distributions in a tray‑pulling paradigm Benoit Bucher1,2 · Maxime Bourgeois1 · James R. Anderson1 · Hika Kuroshima1 · Kazuo Fujita1 Received: 19 June 2019 / Accepted: 13 April 2020 © Japan Monkey Centre and Springer Japan KK, part of Springer Nature 2020
Abstract The fact that squirrel monkeys do not routinely cooperate in the wild has been proposed to explain their failure to show disadvantageous inequity aversion (i.e., negative reactions when receiving less than a partner) in an experimental exchange. Here we assessed whether the use of a tray-pulling paradigm, allowing for a larger variety of unequal testing situations, would bring additional insights into inequity aversion in this species. Squirrel monkeys were tested in pairs in which only the donor could pull a tray baited with food to within reach of itself and a recipient. Using pairs with different social relationships, we examined donors’ frequencies of pulling both in the presence and absence of a recipient, as well as across three different food distributions: equal, qualitative inequity (higher-value reward for the recipient), and quantitative inequity (no food reward for the donor). Results showed that female donors pulled the tray less often in the quantitative inequity condition with an out-group female recipient than when alone. However, such discrimination was not observed when females were with female in-group and male out-group recipients. By contrast, male donors did not adjust their pull frequencies according to the recipient’s presence or identity (female and male out-group recipients). These results point towards possible disadvantageous inequity aversion in female squirrel monkeys. However, alternative hypotheses such as increased arousal caused by out-group female recipients cannot be ruled out. We discuss the data in line with major theories of inequity aversion and cooperation in primates. Keywords Inequity aversion · Cooperation · New world monkey · Squirrel monkey
Introduction Cooperation, defined as joint and costly actions in order to get mutual benefits (Clutton-Brock 2009; Dugatkin 1997), is widespread in the animal kingdom, even between nonkin individuals (Clutton-Brock 2002, 2009; Kappeler and van Schaik 2006). A sense of fairness, and particularly inequity aversion, has been proposed as one of the psychological mechanisms supporting the evolution of cooperation in humans (Brosnan and de Waal 2014; Fehr and Schmidt 1999; Tomasello 2016). Inequity aversion can be divided into two types: an advantageous type in which an individual reacts negatively after receiving a better outcome than a * Benoit Bucher [email protected] 1
Department of Psychology, Graduate School of Letters, Kyoto University, Kyoto 606‑8501, Japan
Japan Society for the Promotion of Science, Chiyoda, Tokyo 102‑0083, Japan
2
partner, and a disadvantageous type in which an individual reacts against earning less than a partner (Fehr and Schmidt 1999)
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