Investigating reactions of squirrel monkeys (Saimiri sciureus) towards unequal food distributions in a tray-pulling paradigm

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.