Anything for a cheerio: Brown capuchins (Sapajus [Cebus] apella) consistently coordinate in an Assurance Game for unequal payoffs.

Unequal outcomes disrupt cooperation in some situations, but this has not been tested in the context of coordination in economic games. To explore this, we tested brown capuchins (Sapajus [Cebus] apella) on a manual version of the Stag Hunt (or Assurance) Game, in which individuals sequentially chose between two options, Stag or Hare, and were rewarded according to their choices and that of their partner. Typically, coordination on Stag results in an equal highest payout, whereas coordinating on Hare results in a guaranteed equal but lower payoff and uncoordinated play results in the lowest payoff when playing Stag. We varied this structure such that one capuchin received double the rewards for the coordinated Stag outcome; thus, it was still both animals' best option, but no longer equally rewarding. Despite the inequality, capuchins coordinated on Stag in 78% of trials, and neither payoff structure nor their partner's choice impacted their decision. Additionally, there was no relationship between self-scratching, a measure of stress in capuchins, and choices. After completing the study, we discovered our reward, cheerios, was sufficiently valuable that in another study, capuchins never refused it, so post hoc we repeated the study using a lower value reward, banana flavored pellets. Capuchins completed only 26% of the pellet trials (compared to 98% with cheerios), constraining our ability to interpret the results, but nonetheless the monkeys showed a decrease in preference for Stag, particularly when they received fewer rewards for the coordinated Stag outcome. These results reinforce capuchins' ability to find coordinated outcomes in the Stag Hunt game, but more work is needed to determine whether the monkeys did not mind the inequality or were unwilling to sacrifice a highly preferred food to rectify it. In either case, researchers should carefully consider the impact of their chosen rewards on subjects' choices.

Capuchin monkeys (Sapajus [Cebus] apella) play Nash equilibria in dynamic games, but their decisions are likely not influenced by oxytocin.

Comparative approaches to experimental economics have shed light on the evolution of social decision-making across a range of primate species, including humans. Here we replicate our previous work looking at six pairs of capuchin monkeys' (Sapajus [Cebus] apella) responses to scenarios requiring both coordination (Assurance Game) and anti-coordination (Hawk-Dove Game). This then provides a foundation for assessing their responses to two additional games, one with a scenario of beneficial cooperation with a temptation to defect (Prisoner's Dilemma) and one with an environment requiring changing strategies within short temporal proximity (Alternating Economic Game). We additionally explored the effects of exogenous oxytocin on decision-making. Oxytocin did not affect decisions in any of our games. Results from the first two games largely replicated our previous findings. Responses to the Prisoner's Dilemma were more varied than was seen in previous games, with pairs respectively cooperating, defecting, and failing to establish stable strategies. Such variability indicates that this game may be a good assay for individual differences in social decision-making. Finally, capuchins were able to flexibly switch between their previously established strategies within each of the different games, even when the games were presented within the same session, requiring strategy adjustments within short temporal proximity. These results build on earlier findings showing that capuchins can alter decision-making strategies as the context demands, which is likely essential for decision-making in naturally occurring contexts.

The influence of reward quality and quantity and spatial proximity on the responses to inequity and contrast in capuchin monkeys (Cebus [Sapajus] apella).

Recent evidence within the field of comparative psychology has demonstrated that small differences in procedure may lead to significant differences in outcome. Therefore, failing to fully explore the impact of different contexts on a behavior limits our ability to fully understand that behavior. A behavior that has exhibited substantial variation, both within and across studies, is animals' responses to violations of their expectations, either when expectations were based on another's outcome (inequity) or one's previous outcome (contrast). We explored this further in capuchin monkeys, focusing on the following 2 factors that often vary in such tests but have not yet been rigorously explored: the relative values of the food rewards and the degree of separation of the subjects. Concerning the first, we examined responses to violation of expectations when the difference between what was expected (or what the partner got) and what was received differed in either quality or quantity. Concerning the second, we compared responses when the 2 individuals were separated by a clear partition (barrier condition) versus sharing the same enclosure without the partition (no-barrier condition). Our results suggest that responses to inequity are most likely to emerge when the food received is low-value food, regardless of the difference between the actual and the expected outcome. However, capuchins did not respond differently to different quantities of rewards, nor did the degree of separation between subjects significantly affect results. We consider the implications of this work for both studies of violation of expectation and other cognitive and behavioral tasks. (PsycINFO Database Record