Following DeLoache’s [5] definition of symbol (‘‘something that someone intends to represent something other than itself’’, p.66), a token can be considered a symbol since it is arbitrarily related to its referent through the conventions established between the experimenter and the exchanging subject [15,16]. Numerous studies in recent years examined aspects of economic behavior in non-human primates using tokens. For example, tokens were used to test reactions to social inequity [17, but see 18,19], reference-dependent preferences [20], and endowment effects [21]. In all these experiments, monkeys were typically asked to trade with the experimenter valueless objects (the tokens) in exchange for desirable pieces of food. Probing economic preferences using tokens opens a number of important questions. For example, monkeys could psychologically treat tokens as symbols for the food they represent, similarly to how humans treat words or money. Alternatively, exchanging tokens for food could simply result from instrumental conditioning. In this scenario, monkeys exchanging tokens with the experimenters would display a behavior conceptually analogous to the behavior of pigeons operating a lever to obtain food. A closely related question is whether preferences monkeys reveal by the way of tokens are qualitatively and quantitatively similar to those they reveal when they choose between the actual foods. To examine this issue, we adopted a behavioral paradigm that provides a measure of the value capuchins assign to different foods. Subjects choose between two foods, one of which is preferred, offered in variable amounts. When offered the choice between a unit quantity of each food, subjects choose (by definition) the preferred food. However, if the less preferred food is offered in sufficiently large amounts, subjects will choose it. The relative value of the two foods can be inferred from the indifference point—the quantity ratio for which the subject chooses either food equally often [22–24]. Qualitatively, this behavioral paradigm highlights two fundamental traits of economic choice behavior. First, individuals have strict economic preferences: away from the indifference point, their choices are typically very consistent. Second, individuals’ choices satisfy transitivity. In other words, if an individual is indifferent between foods X and Y and if it is indifferent between foods Y and Z, the individual is also indifferent between foods X and Z. Quantitatively, this behavioral paradigm provides an operational measure of the value individuals subjectively assign to different foods. Transitivity is one of the main axioms of standard economic theory and is a fundamental trait of rational decision-making [25,26]. Only a few studies examined preference transitivity in non-human primates. At the behavioral level, both capuchins and rhesus macaques (Macaca mulatta) combine the relative value assigned to three foods (or juices) according to transitivity [23,24]. Furthermore, when rhesus macaques are presented with binary choices between two types of juice in variable amounts, neurons in the orbitofrontal cortex encode the value of the offered and chosen juices in a menu invariant way, suggesting that preference transitivity might be rooted in the activity of these neurons [23]. In this study, we compared preferences revealed in the real (food) and in the symbolic (token) conditions. We trained five capuchins to associate three different foods with three different types of tokens. In separate sessions, we then presented monkeys with pair-wise choices between actual foods (Food condition) or between tokens associated with the same foods (Token condition). Finally, we compared the relative values measured using tokens with those measured with the actual foods. Notably, the cognitive demands of this situation are much more challenging than in previous studies carried out in capuchins [12,13]. Indeed, in order to choose between different quantities of tokens corresponding to qualitatively different foods, Transitivity in Monkeys capuchins should recall the association between each token and the corresponding food, evaluate the amount of each token array, estimate the relative value of the two offers, and finally make up their mind on which option to choose. We envision three possible results. One possibility is that capuchins use the same cognitive mechanism to reason on tokens as they do with food; as a consequence, their performance will not differ in the two contexts. Alternatively, capuchins could find it more difficult to deal with tokens than with food; thus, their choice pattern will be more consistent with transitivity with food rather than with tokens. Finally, tokens may aid capuchins to override the incentive value of the immediately available food, as described for chimpanzees [9– 11], and to achieve psychological distancing (i.e., to separate cognitively from the immediate behavioral environment, thus directing attention away from the salient features of the stimulus, 27–30). If this were the case, capuchins would deal with tokens better than with food, and therefore their choice pattern will be more consistent with transitivity with tokens rather than with food. RESULTS Five captive-born capuchin monkeys were individually tested. Each subject was presented with a ‘‘choice apparatus’’, constituted by a platform with two sliding trays where different quantities of food or tokens (according to condition) were available (Figure 1). The subject could choose one of the two offers (by pulling one of the two sliding trays) on the basis of the amount of food or tokens presented. In the preliminary phase, subjects were offered pair- wise choices between two foods (A:B and B:C) in order to select three foods such as A was preferred to B, and B was preferred to C. Then, in the Food condition, preference transitivity was tested by presenting capuchins with binary choices between different quantities of the three foods, labeled A, B and C in decreasing order of preference (Table 1, Video S1, S2, S3, S4). Subsequently, in the training phase, subjects learned to exchange three valueless tokens for the three types of food used in the Food condition (Table 2). Finally, in the Token condition, preference transitivity was tested by presenting capuchins with binary choices between different quantities of the three tokens, labeled A, B and C in decreasing order of preference (Table 1, Video S5, S6, S7, S8, S9, S10). 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 it was rewarded with one piece of food. Both in the real (food) and in the symbolic (token) conditions, the quantities of the two items offered to the monkey for any given item pair varied from trial to trial (Table 1). We thus obtained in each session three choice patterns corresponding to the three item pairs. In the Food condition, capuchins generally had strict economic preferences (i.e., for offer types away from the indifference point, data points were close to 0% or 100%). Figure 2 shows the behaviour recorded in a representative session in the Food condition. To compute relative values, we fitted each choice pattern with a ‘‘normal sigmoid’’, and we interpreted the underlying Gaussian as a distribution for the relative value. The mean (m) and variance (s2) of the distribution thus represent the estimate for the relative value and the relative error of measure (see Methods for details). In all sessions but one, we could evaluate the relative value of each food pair (see below). As shown in Figure 3, in 96% of these sessions (23 out of 24 sessions), measured relative values satisfied value transitivity (z-test, p.0.05). This held true for all subjects, i.e. Gal (5/5 sessions), Paprica (5/5 sessions), Robot (4/5 sessions, in one session we could not evaluate the relative value of the A:C PLoS ONE | www.plosone.org 2 June 2008 | Volume 3 | Issue 6 | e2414