protocols approved by the Italian Health Ministry and all procedures were performed in full accordance with the European law on humane care and use of laboratory animals. Subjects were tested individually in the indoor compartment; the apparatus was a black plastic table (65 cm664 cm613.5 cm) with two sliding aluminum trays (6.5640 cm; 2.5 cm high), positioned at 32 cm distance from one another. Each tray had two holes (1.4 cm in diameter), one at each end; one served to allow the subject’s pulling, whereas the other hole allowed the experimenter to block the tray by inserting a pin into it. All subjects were already familiar with the apparatus [12,13]. The experiment proceeded in phases. In the preliminary phase, subjects were tested for their binary food preferences. Subse- quently, preference transitivity was tested in choices between foods (Food condition). Then, in the training phase, subjects learned to exchange tokens for food. Finally, preference transitivity was tested in choices between tokens (Token condition). Preliminary phase Capuchins’ preference for two pairs of foods (A:B and B:C, see below) was assessed. First, we carried out the preference test for the pair A:B, and then for the pair B:C. In each session, capuchins faced binary choices between different quantities of a pair (A:B or B:C). Before the beginning of each session, eight familiarization trials were carried out by presenting pair-wise comparisons between the two foods (one piece each). According to subjects’ preferences, foods were referred to as food A (high-preferred) and food B (low-preferred). Similarly, when testing the other food pair, foods were referred to as food B (high-preferred) and food C (low- preferred). Individual preferences varied so that labels A, B, and C referred to different foods for different subjects (Table 2). For the A:B pair, in each trial capuchins faced a binary choice between one or two pieces of food A and one to five pieces of food B. Therefore, the following comparisons were presented: 2A:1B, 1A:1B, 1A:2B, 1A:3B, 1A:4B, and 1A:5B; each combination was presented eight times for a total of 48 trials in a pseudo-random order. The left/right arrangement of the offers was counterbal- anced within each session. Each subject received one session a day for a total of five sessions. The same procedure was employed for the B:C pair. Testing was carried out by two experimenters: experimenter 1 sat in front of the subject’s indoor compartment, with the apparatus placed on the floor between the experimenter and the capuchins’ compartment. Placed between the experimenters were two opaque containers, each containing pieces of one type of food. Experimenter 2 sat next to experimenter 1 and during baiting she covered the apparatus with an opaque screen to prevent the subject from seeing the process. Then, experimenter 2 lifted the opaque screen and experimenter 1 pushed the apparatus towards the wire mesh, so that the monkey could pull one of the two trays. Both experimenters refrained from looking at the apparatus so as not to provide cues to the subject. The inter-trial interval was about 10 s. Food condition In each session, capuchins faced choices between different quantities of three foods, labeled A, B and C in decreasing order of preference. Choices were binary and trials with the three pairs of foods (A:B, B:C and C-A) were interleaved pseudo-randomly. Before the beginning of each session, nine familiarization trials were carried out by presenting for three times all the possible pair- wise comparisons between the three foods (one piece of food of each type). Foods were referred to as food A (high-preferred), food B (medium-preferred), and food C (low-preferred). Transitivity in Monkeys In each trial capuchins could face a binary choice between: (1) one or two pieces of food A and one to four pieces of food B, (2) one or two pieces of food B and one to four pieces of food C, and (3) one or two pieces of food A and one to six pieces of food C. In a session, each comparison was presented four times. For each food pair, according to each individual’s indifference point, the type and number of trials presented varied across sessions, from a minimum of 60 to a maximum of 64 trials per session; no subject received all the possible 72 comparisons within a session (Table 1). The left/right arrangement of the offers was counterbalanced within each session. Each subject received one session a day for a total of five sessions. All the other features of the procedure were the same as in the preliminary phase. Token condition The same five capuchins were presented with the Token condition after completing all the five sessions in the Food condition. We used the same subjects and apparatus as in the Food condition. (a) Tokens. Tokens were objects of similar dimensions, differing in shape, material and color; in particular, we used green chips, black plastic tubes, and brass hooks. These objects were familiar to the subjects but never used in previous studies. The three tokens were pseudo-randomly assigned to the three types of food across subjects (Table 2). (b) Training. Subjects learned to associate each type of token to one of the three foods used in the Food condition (Table 2). Therefore, token A was associated with the high-preferred food, token B with the medium-preferred food, and token C with the low-preferred food. The training procedure consisted of placing 12 tokens of the same type (i.e., associated to the same type of food) into the indoor compartment, and repeatedly saying ‘give me’ to the monkey while requesting a token, with left hand outstretched and palm up. The reward was given upon the placement of one token into the experimenter’s left hand. There was a 10-s interval between one trial and the next one. Incorrect exchanges, in which tokens were thrown or incorrectly placed into the experimenter’s hand, were not rewarded. Moreover, when the subject did not exchange a token within 30 s, the trial was considered incorrect and a new trial started after 10 s. Subjects received a training session per day. Each session consisted of two blocks of 12 trials each, for a total of 24 trials. Criterion was set at 90% correct responses within two consecutive sessions. Each subject was trained to exchange one type of token (A, B, or C) at a time, and the order in which they learned to exchange the three tokens was randomly determined. When criterion was reached for all types of token, subjects received nine sessions of consolidation with tokens A, B and C alternated across days. Capuchins completed training (including the nine sessions of consolidation) in an average of 18.261.7 sessions. In particular, they reached criterion in an average of 2.860.4 sessions for token A (range: 2–4), 2.860.8 sessions for token B (range: 2–6), and 3.461.4 sessions for token C (range: 2–9). The rate of training of the present study is similar to that reported for capuchins learning to associate different type of tokens with different quantities of food [12,13]. (c) Procedure. In each session, capuchins faced binary choices between different quantities of three tokens (A:B, B:C, and A:C). Before the beginning of each session, nine familiarization trials were carried out by presenting for three times all the possible pair-wise comparisons between the three tokens (one token of each type). According to subjects’ preferences, tokens were referred to as token A (associated to the high-preferred PLoS ONE | www.plosone.org 6 June 2008 | Volume 3 | Issue 6 | e2414