A female chimpanzee (Pan troglodytes), who had been trained both in enumeration and in numerical ordering tasks, was tested to reveal the cognitive processes underlying the way to solve a serial recognition task. The chimpanzee called Ai had learned to touch Arabic numerals (0 through 9) on a computer monitor in an ascending order. In the present experiment, test trials were occasionally interspersed among background trials of ordering numerals. In the test trials, two to four identical numerals were included in a sequence (e.g., 1-3-4-4-5). There was a total of five patterns of test trial, which varied in the number of identical numerals in the sequence. Under background condition, the subject's reaction time was longest at the first response in a given sequence followed by equal and shorter reaction times for the remaining responses. Such an L-shaped pattern of reaction times was also maintained under test conditions. The reaction times in the first response under test condition, however, were longer than those of background conditions. The test trials in which four identical numerals were included in a sequence of five items produced longest reaction time in the first response, and it was even longer than that of another test condition, in which all the numerals were the same (e.g., 3-3-3-3-3). These results strongly suggest that the chimpanzee not only ordered and memorized all the presented numerals, but also she planned a motor sequence for correct responses before making the initial response in a sequence. The present study successfully isolated a planning process from pre-planning strategy by means of the manipulation of identical numerals in a sequence, which produced other possible motor sequences for a correct response.

Key words: number, chimpanzee, planning, ordering, cognitive processes