The effect of reward value on the performance of long-tailed macaques (Macaca fascicularis) in a delay of gratification exchange task

In the context of a global research initiative called ManyPrimates, scientists from around the world collaborated to collect data aimed at comparing the ability of various primate species to delay gratification. Our contribution to this project involved collecting data from long-tailed macaques (Macaca fascicularis). Our findings indicated that these macaques rarely opted to exchange a given food item for a larger food reward at a later time. However, we suspected that the experimental protocol might not accurately capture the macaques' true capacity to delay gratification. Specifically, we hypothesized that when the macaques possessed a highly desirable food item, it might discourage their engagement in any form of participation in a food exchange task or a delay of gratification paradigm. To explore whether this potential mental block was affecting their performance more than their actual delay of gratification skills, we conducted experiments on six long-tailed macaques under two different conditions. In these conditions, we examined how the value of the exchange item influenced the frequency of exchanges. In one condition, subjects received a high-value food item, while in the other, they received a low-value food item, both of which could be exchanged for three high-value food items. Our results revealed that the macaques were more inclined to exchange low-value food items than high-value ones across three different delay periods. Furthermore, the likelihood of an exchange decreased as the delay period increased. Remarkably, when we reduced the value of the exchange item, the long-tailed macaques displayed significantly improved abilities to delay gratification within the exchange task. These findings strongly suggest that the possession of a high-value item significantly contributed to the lower performance of these monkeys in the original delay of gratification exchange protocol.