Humans, apes, and rhesus monkeys demonstrate memory awareness by collecting information when ignorant and acting immediately when informed. In this study, five capuchin monkeys searched for food after either watching the experimenter bait one of four opaque tubes (seen trials), or not watching (unseen trials). Monkeys with memory awareness should look into the tubes before making a selection only on unseen trials because on seen trials they already know the location of the food. In Experiment 1, one of the five capuchins looked significantly more often on unseen trials. In Experiment 2, we ensured that the monkeys attended to the baiting by interleaving training and test sessions. Three of the five monkeys looked more often on unseen trials. Because monkeys looked more often than not on both trial types, potentially creating a ceiling effect, we increased the effort required to look in Experiment 3, and predicted a larger difference in the probability of looking between seen and unseen trials. None of the five monkeys looked more often on unseen trials. These findings provide equivocal evidence for memory awareness in capuchin monkeys using tests that have yielded clear evidence in humans, apes, and rhesus monkeys.
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We thank David Ide of Research Services Branch, NIMH for designing and fabricating the apparatus. We also thank Ruth Woodward of Research Animal Management Branch, NICHD for excellent veterinary care. This research was supported by the NIMH Intramural Research Program. Additional support for preparation of this manuscript was provided by Yerkes Center base grant No. RR-00165 awarded by the Animal Resources Program of the National Institutes of Health, and by the Center for Behavioral Neuroscience under the STC Program of the National Science Foundation under Agreement No. IBN-9876754. These experiments comply with US law.
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