Waiting for what comes later: capuchin monkeys show self-control even for nonvisible delayed rewards

Abstract

Self-control tasks used with nonhuman animals typically involve the choice between an immediate option and a delayed, but more preferred option. However, in many self-control scenarios, not only does the more impulsive option come sooner in time, it is often more concrete than the delayed option. For example, studies have presented children with the option of eating a visible marshmallow immediately, or foregoing it for a better reward that can only be seen later. Thus, the immediately available option is visible and concrete, whereas the delayed option is not visible and more abstract. We tested eight capuchin monkeys to better understand this potential effect by manipulating the visibility of the response options and the visibility of the baiting itself. Monkeys observed two food items (20 or 5 g pieces of banana) each being placed either on top of or inside of one of the two opaque containers attached to a revolving tray apparatus, either in full view of monkeys or occluded by a barrier. Trials ended when monkeys removed a reward from the rotating tray. To demonstrate self-control, monkeys should have allowed the smaller piece of food to pass if the larger piece was forthcoming. Overall, monkeys were successful on the task, allowing a smaller, visible piece of banana to pass from reach in order to access the larger, nonvisible banana piece. This was true even when the entire baiting process took place out of sight of the monkeys. This finding suggests that capuchin monkeys succeed on self-control tasks even when the delayed option is also more abstract than the immediate one-a situation likely faced by primates in everyday life.

Delayed gratification in New Caledonian crows and young children: influence of reward type and visibility

“…These findings were consistent across various short delay lengths and reward magnitudes (Bramlett et al 2012). When reward visibility (though not reward type) was manipulated in a further study, capuchins still performed well, even when only the immediate, less preferred option was visible and the delayed option was hidden, or the baiting process took place out of sight of subjects (Perdue et al 2015). The authors highlight that temporal delays are intuitive in this design as the subjects can directly observe the rewards moving closer to them (Perdue et al 2015).…”

“…In children, when the immediate reward is visible (and delayed reward not visible), performance typically suffers, whereas when the delayed reward is visible (and immediate reward is not), this sometimes improves self-control (Mischel et al 1972). However, in capuchin monkeys, subjects continued to perform well even when the delayed option was not visible (Perdue et al 2015).…”

“…Both aspects are likely to be ecologically relevant for humans and non-human species. For example, delayed options in everyday life may be more abstract than immediate ones, and require memory representation rather than direct perceptual contact (Perdue et al 2015). This can be observed in tool using species, who show the ability to delay gratification using tools to access out of reach food items instead of only eating immediately available food.…”

Self-control assessments of capuchin monkeys with the rotating tray task and the accumulation task

“…One monkey, Nkima, had no previous experience in self-control tasks such as those used in the present study. All remaining monkeys had experience in computerized self-control tasks (Evans and Beran, 2014; Evans, Perdue, Parrish, and Beran, 2014), as well as with the accumulation task (Addessi et al, 2013) and the rotating tray task (Bramlett et al, 2012; Perdue et al, 2015). …”

“…Some researchers have argued that quantitative reward differences are less likely to promote or sustain delay of gratification than qualitative differences (e.g., Hillemann, Bugnyar, Kotrschal, and Wascher, 2014), but in this case, many of the monkeys we tested also proved successful in waiting for the larger piece of food. In a follow-up study, we presented new conditions in which the food items were first shown to the monkeys and then placed into opaque containers rather than being visible throughout the trial (Perdue, Bramlett, Evans, and Beran, 2015). …”