Transitivity in Monkeys Figure 2. Food condition. The three panels show the choice patterns recorded for food pairs A:B, B:C and A:C, respectively. In the first panel, the x axis represents the offer type, and different offer types are ordered by the ratio of qB / qA, where qA and qB are the quantities of foods A and B offered to the subject. The y-axis represents the percentage of trials in which the subject chose item B. Analogously, in the second and third panel, the y-axis represents the percentage of trials the subject chooses food C. In this session, the subject is offered cheerios as food A, dried pineapple as food B, and rice krispies as food C. The sigmoid fits provide the relative values V(A) = 0.9 V(B), V(B) = 2.4 V(C), and V(A) = 2.2 V(C); therefore, subject’s choices satisfy value transitivity since 0.9 * 2.4 , 2.2. doi:10.1371/journal.pone.0002414.g002 p = 0.04, g2p = 0.80), whereas the variability in capuchins’ performance did not significantly differ across conditions (F1,3 = 3.51, p = 0.16, g2p = 0.54). DISCUSSION Our results can be summarized as follows. Qualitatively, prefer- ences revealed by the way of tokens were similar to those measured with the actual foods. Specifically, when choosing between tokens, capuchins displayed strict economic preferences and choices satisfied transitivity since the first session for all subjects but one. These results confirm and extend previous findings obtained in non- human primates faced with choices between real foods or juices Figure 3. Food condition. The x-axis represents the product nA:B* nB:C, the y-axis represents nA:C, and each data point represents one of the 24 sessions consistent with value transitivity. doi:10.1371/journal.pone.0002414.g003 [23,24] and with relative numerousness judgments between food or tokens [13]. Quantitatively, however, values measured by the way of tokens differed systematically from those measured with the actual foods. In particular, for any pair of foods, the relative value of the preferred food tended to increase when monkeys chose between the corresponding tokens. As a consequence, while in the Food condition it was generally possible to assess the relative value of the items, in about half of the sessions carried out in the Token condition this was not the case. The fact that choice patterns were otherwise qualitatively similar in the two conditions suggests that by increasing the number of the less preferred tokens (e.g., presenting more than 6 tokens C vs. 1 token A) we might induce capuchins to choose the less preferred but more numerous type of token. Overall, these results suggest that capuchins use similar cognitive mechanisms when evaluating options in both real and symbolic contexts. Indeed, capuchins’ preferences satisfied tran- sitivity in both contexts. At the same time, tokens were not dealt with exactly as the food they stand for, since relative values were higher in the Token condition than in the Food condition. Several factors could account for this result. First, the high memory load due to recalling the association between each token and the corresponding food. However, high memory load should have led to a more ‘‘noisy’’ pattern of choice in the Token condition, but this did not seem to be the case, since the variability in capuchins’ performance (as measured by the variance s2) did not significantly differ between the Food and Token conditions. Alternatively, monkeys’ behavior could be explained by a decreased motivation due to the delayed feedback inherent in token exchange. This hypothesis seems, however, unlikely because our subjects never refused to participate and completed all the token trials. Moreover, the same two individuals for which tokens increased most the relative values between item pairs (Sandokan and Carlotta) were the best performers in a previous study on the estimation and combination of token quantities, where temporal discounting was at stake [12]. Nonetheless, since each token exchange for the corresponding food takes a few seconds, we cannot rule out that capuchins may have discounted the offers involving a higher number of tokens [31]. Finally, the different results obtained in the Token condition compared to the Food condition could reflect a difficulty to achieve a dual representation [3]. Specifically, capuchins may find it difficult to grasp the dual nature of tokens as it is the case for PLoS ONE | www.plosone.org 4 June 2008 | Volume 3 | Issue 6 | e2414