Abstract
Monkeys can learn the symbolic meaning of tokens, and exchange them to get a reward. Monkeys can also learn the symbolic value of a token by observing conspecifics but it is not clear if they can learn passively by observing other actors, e.g., humans. To answer this question, we tested two monkeys in a token exchange paradigm in three experiments. Monkeys learned token values through observation of human models exchanging them. We used, after a phase of object familiarization, different sets of tokens. One token of each set was rewarded with a bit of apple. Other tokens had zero value (neutral tokens). Each token was presented only in one set. During the observation phase, monkeys watched the human model exchange tokens and watched them consume rewards (vicarious rewards). In the test phase, the monkeys were asked to exchange one of the tokens for food reward. Sets of three tokens were used in the first experiment and sets of two tokens were used in the second and third experiments. The valuable token was presented with different probabilities in the observation phase during the first and second experiments in which the monkeys exchanged the valuable token more frequently than any of the neutral tokens. The third experiments examined the effect of unequal probabilities. Our results support the view that monkeys can learn from non-conspecific actors through vicarious reward, even a symbolic task like the token-exchange task.
Highlights
Since the first work on chimpanzees [1,2], a number of studies have examined the ability of non-human primates to exchange tokens for foods, in capuchin [3,4] and chimpanzees [5], or to obtain either food or tools, in capuchins [6]
Some individual chimpanzee and capuchin monkeys were able to represent the value corresponding to the sum of different tokens [8]
Monkey 1 was more restless in the cage, extending arms toward the human model as if trying to obtain the food and showing a greater difficulty with paying attention to the task
Summary
Since the first work on chimpanzees [1,2], a number of studies have examined the ability of non-human primates to exchange tokens for foods, in capuchin [3,4] and chimpanzees [5], or to obtain either food or tools, in capuchins [6]. A more recent study showed that some capuchins understood the association between tokens and quantities of food [7] This later result represents evidence that the symbolic meaning of a token can go beyond its simple association with the reward. Some individual chimpanzee and capuchin monkeys were able to represent the value corresponding to the sum of different tokens [8]. Limitations in both the understanding and use of tokens have been reported in several experimental paradigms, either when tokens were not used by monkeys to obtain a reward [9] or when monkeys failed to plan the token’s use ahead to maximize future rewards [10]
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