Choosing to wait for a better outcome (delay choice) and sustaining the delay prior to that outcome (delay maintenance) are both prerequisites for successful self control in intertemporal choices. However, most existing experimental methods test these skills in isolation from each other, and no significant correlation has been observed in performance across these tasks. In this study we introduce a new paradigm, the hybrid delay task, which combines an initial delay choice with a subsequent delay maintenance stage. This allows testing how often choosing to wait is paired with the actual ability to do so. We tested 18 capuchin monkeys (Cebus apella) from two laboratories in various conditions, and we found that subjects frequently chose the delayed reward but then failed to wait for it, due to poor delay maintenance. However, performance improved with experience and different behavioral responses for error correction were evident. These findings have far reaching implications: if such a high error rate was observed also in other species (possibly including Homo sapiens), this may indicate that delay choice tasks that make use of salient, prepotent stimuli do not reliably assess generalized self control, insofar as choosing to wait does not entail always being able to do so.

Self-control is defined as foregoing an immediate reward to gain a larger delayed reward. Methods used to test self-control comparatively include intertemporal choice tasks, delay of gratification tasks, and accumulation tasks. To date, capuchin monkeys have shown different levels of self-control across tasks. This study introduced a new task that could be used comparatively to measure self-control in an intuitive context that involved responses that required no explicit training. Capuchin monkeys (Cebus apella) were given a choice between two food items that were presented on a mechanized, revolving tray that moved those foods sequentially towards the monkeys. A monkey could grab the first item or wait for the second, but was only allowed one item. Most monkeys in the study waited for a more highly preferred food item or a larger amount of the same food item when those came later, and they inhibited the prepotent response to grab food by not reaching out to take less preferred foods or smaller amounts of food that passed directly in front of them first. These data confirm that the mechanisms necessary for self-control are present in capuchin monkeys, and indicate that the methodology can be useful for broader comparative assessments of self-control.

Prospective memory involves the encoding, retention, and implementation of an intended future action. Although humans show many forms of prospective memory, less is known about the future oriented processes of nonhuman animals, or their ability to use prospective memory. In this experiment, a chimpanzee named Panzee, who had learned to associate geometric forms called lexigrams with real-world referents, was given a prospective memory test. Panzee selected between two foods the one she wanted to receive more immediately. That food was scattered in an outdoor yard where she could forage for it. Also outdoors were lexigram tokens, one of which represented the food item that remained indoors throughout a 30 minute period, and that could be obtained if Panzee brought in the token that matched that food item. After foraging for the selected food item, Panzee consistently remembered to retrieve and return the correct token when food was available indoors, whereas on control trials involving no indoor food she rarely returned a token. This indicated that Panzee encoded information relevant to the future action of token retrieval after extended delays for one type of food, even when a more immediately preferred food was available.

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.

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