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"The magical number 5" in a chimpanzee

Nobuyuki Kawai and Tetsuro Matsuzawa, Primate Research Institute, Kyoto University

     A female chimpanzee named Ai has learned to use Arabic numerals to represent number. She can at present correctly label 0 through 10 items shown to her by touching the corresponding numeral on a touch-sensitive monitor. She can also choose the numerals 0 through 9 in succession, arranging them in an ascending order. Utilizing Ai's skill in such numerical tasks, we explored her memory span. We presented our subject with a set of numerals on screen, say, 1, 3, 4, 6, and 9. As mentioned above, she had already displayed close to perfect accuracy when required to choose numerals in an ascending order. Our memory task, however, entailed a manipulation where immediately following the selection of the first numeral, all remaining items were transformed into- or "masked" by - identical white squares. This meant that in order to score correct in a trial, the subject was required to memorize all numerals as well as their respective positions before making the first response. Ai reached high levels of accuracy in such masking trials, with over 90 % correct in the 4-item condition, and about 65 % correct with 5 items. Her performance was in each case significantly above chance level. Analysis of response latency of non-"masking" trial suggests that Ai inspected numerals and their locations, planning her action leading to a correct response in advance, that is before making her first choice. In addition, analysis of response latency of masking trials suggests her successful performance did not depend upon a longer observation for memorizing. Taken together, we estimate the "magical number" of the chimpanzee to be at least 5, which has been regarded as " 7" in human information processing.

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