Transitivity in Monkeys Figure 4. Token condition. The three panels show the choice patterns recorded for token pairs A:B, B:C and A:C. Here the subject is offered green chips as tokens A (corresponding to one cheerios each), black plastic tubes as tokens B (corresponding to one piece of dried apricot each), and brass hooks as tokens C (corresponding to one sunflower seed each). The sigmoid fits provide the relative values V(A) = 2.8 V(B) and V(B) = 4.3 V(C); however, when facing the choice between one token A and up to six tokens C, this subject always chose the single token A, thus we could not evaluate the relative value of A vs. C. doi:10.1371/journal.pone.0002414.g004 young children. In humans, the understanding and use of symbolic artifacts develops slowly because simultaneously representing both the concrete object itself and its abstract relation to what it stands for is complex [5]. DeLoache and colleagues have extensively investigated how young children use scale models (i.e., realistic miniature models of a familiar playroom) as a source of information for solving a retrieval problem. In a typical trial, children observed an experimenter hide a miniature toy in the model and were then asked to find the larger toy in the analogous location of the playroom. Children understanding of the model- playroom relation and the ability to successfully find the corresponding larger toy in the playroom developed between 2.5 and 3.0 years of age [3–5]. In chimpanzees tested in a similar version of this task, only a few individuals could inhibit perseverative object-oriented responses and successfully retrieved the hidden item [11]. Young children seem to fail in the scale model task because they are attentive to the real object rather than to what it stands for. When the salience of the model is decreased (for example substituting the scale model with a video clip), or when there is no need for dual representation (children are told that a ‘‘shrinking machine’’ transformed the playroom into the miniature model), performance improves. In contrast, performance declines when the physical salience of the scale model is increased by allowing children to play with the model before performing the task [3,5]. A similar phenomenon might explain our results. In the Token condition capuchins might have focused on the quality of the preferred token disregarding the quantities of the two alternatives, thus choosing this token more often than the corresponding food in the Food condition. Future studies should assess whether preventing the physical interaction with tokens by eliminating the exchange procedure modifies capuchins’ performance. Interestingly, in humans a complete appreciation of the symbolic nature of tokens took long time to be achieved [32]. Around 8500 years B.C. the Sumerians started keeping track of trades by employing a system based on small clay tokens shaped differently depending on the good they stood for. The evolution of this token system reflected Sumerians’ socioeconomic develop- ment. As their trades expanded, Sumerians needed to transport tokens in clay pots and, to readily identify the content of each pot, they engraved on their surfaces the type and number of the tokens contained. Nonetheless, Sumerians did not immediately realized how abstract this symbolic system could be and only after several millennia both number representation and writing evolved from the engraved clay pots. Finally, the increased abstraction of the Token condition did not ameliorate capuchins’ performance, as reported for chimpanzees [9–11] and young children [30,33,34] when symbolic representa- tions substituted for real food. Again, limited mastery of the dual nature of tokens may have prevented capuchins from achieving psychological distancing [27–30]; however, this hypothesis re- quires further studies examining whether tokens enhance perfor- mance in tasks where inhibition is critical for success. In conclusion, capuchin monkeys’ behavior with tokens is not simply the product of instrumental conditioning, though tokens have not gained yet the status of human money. Our findings suggest that capuchins indeed treat tokens as symbols, despite experiencing the cognitive burdens imposed by symbolic repre- sentation. Thus, also non-apes have undertaken the path of symbol use and understanding, though they are far from achieving full symbolic competence. MATERIALS AND METHODS Subjects and apparatus Five captive-born capuchin monkeys (three males, two females, average 15.4 years, range 7–23) were tested. All subjects were already proficient in token exchange and had experience in cognitive and number-related tasks [12,13]. No subject but one (Robot) had already participated in a previous study on transitivity of food preferences [24]. They lived in three groups at the Primate Center of the Institute of Cognitive Sciences and Technologies of CNR, Rome; each group was housed in indoor–outdoor compartments (total area: 65.4–139.5 m3, depending on group size) and tested in one of the two indoor compartments (12.2 m3 each, for all groups). All compartments were furnished with wooden perches, tree trunks and branches. Separation for individual testing was achieved by splitting the group into smaller units by means of sliding doors and then allowing one individual to enter the indoor compartment. This procedure was part of the daily routine. Monkeys were not food deprived for testing. The main meal took place in the afternoon when fresh fruits, vegetables and monkey chow were provided. Water was available ad libitum. This study complied with PLoS ONE | www.plosone.org 5 June 2008 | Volume 3 | Issue 6 | e2414