Delay Discounting for Primary and Secondary Rewards in Capuchin Monkeys

Self-control is considered one of the features distinguishing humans from other animals. When facing the choice between a small immediate reward and a large delayed reward (intertemporal choice task) people tolerate longer delays than non-human animals. However, animal studies have always used food rewards whereas human studies mostly used monetary rewards, and people are more impulsive over food than over money. Here, we presented 10 capuchins with choices between: (i) 2 vs. 6 food items (Food delay condition), (ii) one low-value token, exchangeable for 2 pieces of food, vs. one high-value token, exchangeable for 6 pieces of food (Token delay condition). The small option was available immediately, and the large option was available after 80 seconds. Moreover, capuchins received a Food control condition and a Token control condition, i.e., choices between 2 vs. 6 pieces of food or between one low-value token and one high-value token, both available immediately. If capuchins shared with humans an enhanced tolerance for delay with secondary rewards, one would expect them to be more impulsive with food than with tokens. In contrast, our subjects chose the larger delayed option more in the Food delay condition than in the Token delay condition. Probably, in the intertemporal choice task the preference for the larger delayed option is partly due to an impulsive preference for the quantity rather than to a high delay tolerance. Tokens decreased the perceptual salience of the larger option, thus reducing the choices for the larger delayed option. This interpretation, if confirmed by further studies, would have important implications for studies on temporal preferences in non human animals. Funded by ISTC-CNR and American Society of Primatologists.