food), token B (associated to the medium-preferred food), and token C (associated to the low-preferred food). In each trial capuchins could face a binary choice between: (1) one or two tokens A and one to six tokens B, (2) one to three token(s) B and one to six token(s) C, and (3) one or two token(s) A and one to six token(s) C. In each trial, after choosing an offer type capuchins were required to exchange the token(s) selected (one at a time) before the experimenter administered the next trial; each token exchange took a few seconds and was rewarded with one piece of food (Table 2). Typically, capuchins exchanged correctly, and in the very few cases in which they did not do so, the experimenter gave the token back to the subject so that s/he could exchange it again. In a session, each comparison was presented four times. For each token 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 76 trials per session in a pseudo-random order; no subject received all the possible 88 comparisons within a session (Table 1). The left/right arrange- ment 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 Food condition. The study took place between February and May 2007. Analysis of choice patterns We analyzed choice patterns using the method employed by Padoa-Schioppa & Assad [22,23]. We refer to ‘‘relative’’ values because behavioral analyses allow measuring quantities of different goods on a common value scale up to a scaling factor. Our measure of relative value rests on the assumption of linear indifference curves: if a monkey is repeatedly offered the choice between quantities qX and qY of items X and Y (offer qYY : qXX), the rate of ‘‘Y’’ choices only depends on the ratio qY / qX [23]. Choice patterns recorded for each pair of items X and Y are expressed as a function of log(qY / qX), where qX and qY are, respectively, the quantities of items X and Y offered to the monkey. For each item pair, we then fit the percentage of ‘‘Y’’ choices with a normal sigmoid, which is a normal cumulative distribution function of the form S(x) = #x2‘ = N (t, m, s) dt. We interpret the underlying Gaussian (which has mean m and variance s2) as a probability distribution for the log relative value, and we compute the estimated relative value of the two items n = exp(m). The relative value corresponds to the indifference point, i.e. the ratio of quantities for which the monkey would choose either item equally often. We indicate with V(X) the value of X, and with nX:Y the relative value of items X and Y, such that V(X) = nX:Y V(Y). For each session, we thus obtain the three probability distributions for the log relative values u = log(nA:B), v = log(nB:C) and w = log(nA:C). Under the assumption of linear indifference curves, indifference transitivity is satisfied if the following relationship holds statistically true: nA:B * nB:C = nA:C. We refer to this condition as ‘‘value transitivity.’’ Testing whether values satisfy transitivity reduces to testing whether the identity u+v = w holds statistically true. Because u, v and w are all normally distributed variables, transitivity violations can be identified with a z-test [23]. Supporting Information Video S1 Food condition. Carlotta, a female capuchin, has a choice between one cheerios (food A, on the left) and three pieces of parmesan cheese (food B, on the right). She selects the three pieces of parmesan cheese by pulling the corresponding tray. Found at: doi:10.1371/journal.pone.0002414.s001 (4.79 MB MPG) Transitivity in Monkeys Video S2 Food condition. Carlotta has a choice between one cheerios (food A, on the left) and three sunflower seeds (food C, on the right). She selects the single cheerios by pulling the corresponding tray. Found at: doi:10.1371/journal.pone.0002414.s002 (4.78 MB MPG) Video S3 Food condition. Carlotta has a choice between three sunflower seeds (food C, on the left) and one piece of parmesan cheese (food B, on the right). She selects the three sunflower seeds by pulling the corresponding tray. Found at: doi:10.1371/journal.pone.0002414.s003 (4.56 MB MPG) Video S4 Food condition. Carlotta has a choice between five sunflower seeds (food C, on the left) and one cheerios (food A, on the right). She selects the five sunflower seeds by pulling the corresponding tray. Found at: doi:10.1371/journal.pone.0002414.s004 (5.82 MB MPG) Video S5 Token condition. Carlotta has a choice between one green poker chip (token A, corresponding to one cheerios, on the left) and three brass hooks (token C, corresponding to one sunflower seed each, on the right). She selects the token A by pulling the corresponding tray. Found at: doi:10.1371/journal.pone.0002414.s005 (5.28 MB MPG) Video S6 Token condition. Carlotta has a choice between three black plastic tubes (token B, corresponding to one piece of parmesan cheese each, on the left) and one green poker chip (token A, corresponding to one cheerios, on the right). She selects the three tokens B by pulling the corresponding tray. Found at: doi:10.1371/journal.pone.0002414.s006 (9.35 MB MPG) Video S7 Token condition. Carlotta has a choice between one black plastic tube (token B, corresponding to one piece of parmesan cheese, on the left) and three brass hooks (token C, corresponding to one sunflower seed each, on the right). She selects the single token B by pulling the corresponding tray. Found at: doi:10.1371/journal.pone.0002414.s007 (6.34 MB MPG) Video S8 Token condition. Carlotta has a choice between one black plastic tube (token B, corresponding to one piece of parmesan cheese, on the left) and four brass hooks (token C, corresponding to one sunflower seed each, on the right). She selects the four tokens C by pulling the corresponding tray. Found at: doi:10.1371/journal.pone.0002414.s008 (10.39 MB MPG) Video S9 Token condition. Carlotta has a choice between one green poker chip (token A, corresponding to one cheerios, on the left) and five brass hooks (token C, corresponding to one sunflower seed each, on the right). She selects the single token A by pulling the corresponding tray. Found at: doi:10.1371/journal.pone.0002414.s009 (5.53 MB MPG) Video S10 Token condition. Carlotta has a choice between six brass hooks (token C, corresponding to one sunflower seed each, on the left) and one green poker chip (token A, corresponding to one cheerios, on the right). She selects the six tokens C by pulling the corresponding tray. Found at: doi:10.1371/journal.pone.0002414.s010 (12.60 MB MPG) PLoS ONE | www.plosone.org 7 June 2008 | Volume 3 | Issue 6 | e2414