Metacognition refers to the ability to monitor and control one's own cognitive activities such as memory. Although recent studies have raised an interesting possibility that some species of nonhuman animals might possess such skills, subjects often required a numerous number of training trials to acquire the effective use of metacognitive responses. Here, five orangutans (Pongo pygmaeus) were tested whether they were able to escape spatial memory tests when they did not remember the location of preferred reward in a relatively small number of trials. The apes were presented with two identical cups, under one of which the experimenter hid a preferred reward (e.g., two grapes). The subjects were then presented with a third container, "escape response", with which they could receive a less preferred but secure reward (e.g., one grape). The orangutans as a group significantly more likely selected the escape response when the baiting of the preferred reward was invisible (as compared to when it was visible) and when the hiding locations of the preferred reward were switched (as compared to when they remained unchanged). Even when the escape response was presented before the final presentation of the memory test, one orangutan successfully avoided the test in which she would likely err. These findings indicate that some orangutans appear to tell when they do not remember correct answers in memory tests.

The ability to monitor and control one's own cognitive states, metacognition, is crucial for effective learning and problem solving. Although the literature on animal metacognition has grown considerably during last 15 years, there have been few studies examining whether great apes share such introspective abilities with humans. Here, we tested whether four gorillas could meet two criteria of animal metacognition, the increase in escape responses as a function of task difficulty and the chosen-forced performance advantage. During testing, the subjects participated in a series of object choice memory tests in which a preferable reward (two grapes) was placed under one of two or three blue cups. The apes were required to correctly select the baited blue cup in this primary test. Importantly, the subjects also had an escape response (a yellow cup), where they could obtain a secure but smaller reward (one grape) without taking the memory test. Although the gorillas received a relatively small number of trials and thus experienced little training, three gorillas significantly declined the memory tests more often in difficult trials (e.g., when the location of the preferred reward conflicted with side bias) than in easy trials (e.g., when there was no such conflict). Moreover, even when objective cues were eliminated that corresponded to task difficulty, one of the successful gorillas showed evidence suggestive of improved memory performance with the help of escape response by selectively avoiding trials in which he would be likely to err before the memory test actually proceeded. Together, these findings demonstrate that at least some gorillas may be able to make optimal choices on the basis of their own memory trace strength about the location of the preferred reward.

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