When In Doubt, Chimpanzees Rely On Estimates Of Past Reward Amounts, Proc. Biol. Sc.
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. (...). 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. (...) 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.