Are capuchin monkeys (Cebus apella) inequity averse?

Abstract

It has been reported that capuchin monkeys reject a less preferred food (LPF) when they see a partner capuchin receive a more preferred food (PF) for performing the same task. This behaviour was taken as evidence of 'inequity aversion', but an alternative hypothesis is that capuchins reject the LPF because of the mere presence of the PF. We tested this hypothesis in a paradigm, which consisted of presenting two different foods (one PF and one LPF) on a tray and allowing the capuchin to take only the LPF. Refusals to initiate the trial and refusals to take and eat the LPF were higher when the PF was hidden (hiding condition) and when the PF was accumulated in sight but out of reach of the subject (accumulation condition) compared to when two pieces of LPF were placed on the tray (control condition). Interestingly, the subject behaved as in the control condition when its partner was given and ate the PF (partner condition). We argue that capuchins' refusals were due to the frustration of seeing and not obtaining the PF, and that seeing the partner eating increases the LPF acceptance.

Figure 1

(a) Correlation between the average latency to initiate a trial (LatI) and the order of trials during a session (the first and the second sessions of a condition are analysed together). LatI increased significantly across the session in the hiding, accumulation and partner conditions. No correlation between LatI and the order of trials was observed in the control condition. (b) Correlation between the average latency to take and eat the less preferred food (LatT) and the order of trials during a session (the first and the second sessions of a condition are analysed together). No correlation between LatT and the order of trials was observed in any condition.

Figure 2

(a) Average latency to initiate a trial (LatI) in the control, hiding, accumulation and partner conditions. LatI was significantly longer in the hiding and accumulation conditions than in the control condition, whereas LatI in the partner condition was not significantly different from LatI in the control condition. LatI in the accumulation condition was significantly longer than LatI in the hiding condition. (b) Average latency to take and eat the less preferred food (LatT) in the control, hiding, accumulation and partner conditions. LatT was significantly longer in the hiding and accumulation conditions than in the control condition, whereas LatT in the partner condition was not significantly different from LatT in the control condition. The longest average latency was observed in the hiding condition. ^*p<0.013.

Figure 3

Percentage of total refusals: total refusals to initiate a trial (RI, black with white dots) and to take and eat the less preferred food (RT, white with black dashes) in each condition. The percentage of total refusals in the hiding and accumulation conditions differed significantly from that of the control condition. ^*p<0.001.