Delay of gratification in capuchin monkeys (Cebus apella) and squirrel monkeys (Saimiri sciureus)

In two separate series of experiments four capuchin monkeys (Cebus apella) and four squirrel monkeys (Saimiri sciureus) were given demonstration trials in which a human transferred six pieces of food, one by one, from out of each monkey's reach to within reach. On test trials the monkey could reach for the transferred food at any time, an action that ended the trial. Therefore, it was in the monkey's interest to allow food items to accumulate before reaching for food. No capuchin monkey showed delay of gratification in the first phase of testing. An attempt to facilitate performance by presenting a single free food item immediately before the transfer failed (Phase 2). In Phase 3, when the transferred food items increased progressively in size, two capuchins maintained delays, and frequently waited for all 6 items to accumulate. One squirrel monkey started to delay gratification in Phase 1, and another did so in Phase 3. A return to single-sized food items did not impair the monkeys' ability to delay. Short (1 s) interitem delays were generally easier to maintain than longer delays (3 or 5 s). In both species the delaying individuals bridged the delays idiosyncratically.