Abstract # 79:

Scheduled for Thursday, August 18, 2005 07:00 PM-09:00 PM: (Cambridge/Oxford Room) Poster Presentation

Individual reward history, social comparison, and rejection of differentially valued goods in Capuchin monkeys (Cebus apella)

A. M. Ruggiero1, P. G. Roma2, A. Silberberg2 and S. J. Suomi1
1NIH/NICHD, Poolesville, Maryland, USA, 2Department of Psychology, American University, Washington, DC
     Recent literature proposes that monkeys reject “unequal pay,” i.e., they reject a lower-valued food after witnessing a neighboring conspecific receive a higher-valued food, because of a socially rooted “inequity aversion.” We hypothesized that individual reward history would be a more salient point of comparison for monkeys offered differentially valued goods. To test this, 8 adult female capuchin monkeys (Cebus apella) were individually offered 25 cucumber slices (low-value food). They were subsequently tested in rank-matched pairs, where in Phase I one monkey was offered a grape (high-value food, n = 4) followed by their partner being offered a cucumber (n = 4) for 25 trials/animal/day for two days; in Phase II, both groups were offered cucumbers in an identical procedure. This sequence of experimental phases was repeated over two cycles. After each cycle, individuals were simultaneously offered grape and cucumber for 25 trials to determine preference. In Phase I, the monkeys offered cucumbers did not reject more than their grape-receiving counterparts (independent t(6) = -1.724, P = 0.136), that is, they accepted “unequal pay.” In Phase II, monkeys with a grape history rejected significantly more cucumbers than their partners with a cucumber history (t(6) = 3.679, P = 0.010), that is, they rejected “equal pay” by contrasting their rewards with previous rewards rather than with rewards offered to their neighbors. Preferences for grapes did not change throughout the experiment. Although recent work suggests that monkeys have an inequity aversion, these data suggest that previous experience is a better predictor of rejection.