When is it worth waiting for? Food quantity, but not food quality, affects delay tolerance in tufted capuchin monkeys

When faced with choices between smaller sooner options and larger later options (i.e. intertemporal choices), both humans and non-human animals discount future rewards. Apparently, only humans consistently show the magnitude effect, according to which larger options are discounted over time at a lower rate than smaller options. Most of the studies carried out in non-human animals led instead to negative results. Here, we tested ten tufted capuchin monkeys (Sapajus spp.) in a delay choice task to evaluate whether they show a magnitude effect when choosing between different quantities of the same food or when the options are represented by high- and low-preferred foods in different conditions. Whereas food quality did not play a role, we provided the first evidence of an effect of the reward amount on temporal preferences in a non-human primate species, a result with potential implications for the validity of comparative studies on the evolution of delay tolerance. In contrast with human results, but as shown in other animal species, capuchins' choice of the larger later option decreased as the amount of the smaller sooner option increased. Capuchins based their temporal preferences on the quantity of the smaller sooner option, rather than on that of the larger later option, probably because in the wild they virtually never have to choose between the above two options at the same time, but they more often encounter them consecutively. Thus, paying attention to the sooner option and deciding on the basis of its features may be an adaptive strategy rather than an irrational response.