Transitivity in Monkeys Figure 1. Experimental apparatus. The apparatus is positioned in front of the indoor compartment and the experimenter is nearby the apparatus, facing the subject. The subject can reach each tray through the corresponding opening in the wire mesh (8.5 cm63.8 cm each). This photo depicts Robot, a male capuchin, who has just selected three black plastic tokens (each corresponding to one piece of dried apricot, food C) preferring them to one brass hook (corresponding to one cheerios, food A). Robot is exchanging the first plastic token for one piece of dried apricot. doi:10.1371/journal.pone.0002414.g001 FOOD 2A: 1B…1A: 4B 2B: 1C…1B: 4C 2A: 1C…1A: 6C 2A: 1B…1A: 4B 2B: 1C…1B: 4C 2A: 1C…1A: 4C 2A: 1B…1A: 4B 2B: 1C…1B: 4C 2A: 1C…1A: 6C 2A: 1B…1A: 4B 2B: 1C…1B: 4C 2A: 1C…1A: 6C 2A: 1B…1A: 4B 2B: 1C…1B: 4C 2A: 1C…1A: 6C TOKEN 1A: 1B…1A: 6B 2B: 1C…1B: 6C 3A: 1C…1A: 6C 2A: 1B…1A: 5B 3B: 1C…1B: 4C 2A: 1C…1A: 6C 2A: 1B…1A: 5B 3B: 1C…1B: 5C 2A: 1C…1A: 6C 2A: 1B…1A: 5B 2B: 1C…1B: 6C 1A: 1C…1A: 6C 2A: 1B…1A: 4B 3B: 1C…1B: 6C 1A: 1C…1A: 6C Carlotta Paprica Robot Sandokan Gal A B C A B C A B C A B C A B C food pair), Sandokan (5/5 sessions), and Carlotta (4/5 sessions, in one session her behavior was not consistent with transitivity; p = 0.03). For all subjects but Robot, the first session of the Food condition was always consistent with transitivity. In the Token condition, in 13 out of 25 sessions we could not evaluate the relative value of at least one of the three pairs because the subject consistently chose one of the two types of token. Figure 4 shows the behavior recorded in one of the sessions in which we could not evaluate the relative value of the A:C token pair. In the remaining 12 sessions, we could evaluate the relative values of all the three token pairs and capuchins generally had strict economic preferences. As shown in Figure 4, in all these 12 sessions measured relative values satisfied transitivity (z-test, p.0.05). In particular, this held true for four out of five subjects, Table 1. Type of trials presented in the Food condition and in the Token condition for each subject and for each item pair. SUBJECT ITEM PAIR i.e. Gal (5/5 sessions), Paprica (4/5 sessions), Robot (2/5 sessions), and Sandokan (1/5 sessions); for all the above subjects, the first session of the Token condition was always consistent with transitivity. To compare the results obtained in the Food condition and in the Token condition, we performed a repeated measures MANOVA (including sessions where it was possible to evaluate the relative value of each item pair, N = 24 in the Food condition and N = 12 in the Token condition) . The results showed that there was no significant relationship between the estimate of the relative value and its variability, corresponding respectively to the mean and variance of the Gaussian distribution (F2,2 = 11.65, p = 0.08, g2p = 0.92). The relative value was significantly higher in the Token condition than in the Food condition (F1,3 = 12.35, Table 2. Food and token triads for each subject. SUBJECT ITEM FOOD cheerios parmesan sunflower seed pistachio dried pineapple sunflower seed cheerios black olives dried apricot cheerios dried apricot sunflower seed cheerios dried pineapple rice krispies TOKEN green chip black plastic tube brass hook black plastic tube brass hook green chip brass hook green chip black plastic tube green chip black plastic tube brass hook brass hook green chip black plastic tube Carlotta Paprica Robot Sandokan Gal A vs. B B vs. C A vs. C A vs. B B vs. C A vs. C A vs. B B vs. C A vs. C A vs. B B vs. C A vs. C A vs. B B vs. C A vs. C doi:10.1371/journal.pone.0002414.t001 A items are preferred to B items, and B items are preferred to C items. doi:10.1371/journal.pone.0002414.t002 PLoS ONE | www.plosone.org 3 June 2008 | Volume 3 | Issue 6 | e2414