When in doubt, chimpanzees rely on estimates of past reward amounts.

Many animals can repeatedly judge the larger of two sets of food items. However, it remains unclear as to what information might accrue regarding the relative rates of return from these repeated responses. Information about overall rates of return is, in fact, unnecessary to perform well at the task itself. However, if an uncertain situation arose, such as when the quantity in one set was unknown, that information would be useful in determining whether to select a known quantity or an unknown quantity. We gave chimpanzees this test. First, they made multiple judgements between two visible food sets that varied in the number of items across trials. Then, they were faced with the same combinations of set sizes, but only one set was revealed while the other remained unknown. Rather than using a specific quantity as a threshold for choosing the known or the unknown set, the chimpanzees' choice of the unknown set varied in relation to the rate of return from responses in the first phase (when both sets were known). This indicated that the chimpanzees' decisions in the face of uncertainty were guided by a sense of how well they were rewarded overall during the session.