Credit: Graham Yuile

Chimpanzees may have better numerical memory than humans, according to a recent report by Sana Inoue and Tetsuro Matsuzawa of the Primate Research Institute at Kyoto University, Japan (Curr. Biol. 17, R1004–R1005; 2007). Their results challenge the widely held belief that humans have superior cognitive function across the board compared with nonhuman primates.

Their study included three mother-offspring chimpanzee pairs. One of the mothers (named Ai) had been previously trained to use numerals to label real-life objects; the other chimps had no prior exposure to tasks involving numerals. In the first test, the numerals 1 through 9 were displayed on a touch screen in various positions, and the objective was to touch each numeral in order. Chimps received a peanut upon completing each test successfully. They were then given a new version of the test in which only five numerals were included, with the same objective. All the chimps learned to complete both tasks successfully.

Inoue and Matsuzawa then introduced a masking test. This was similar to the first task, except that once the subject touched the first numeral, all remaining numerals were replaced by white squares. The chimp was thus required to remember the locations of the remaining numerals in order to touch them all in numerical sequence. All the chimps also mastered this task. The researchers note that the probability of completing the test successfully by chance is very low (1/24 with four numerals, 1/120 with five and 1/362,880 with nine). The young chimps performed better on this task than did their mothers or the human test subjects.

The final test (limited-hold memory) built on the masking test by introducing a defined 'hold' interval in which the numerals were visible before being replaced by squares. Three different hold intervals were tested using two chimps (Ai, the best-performing mother, and Ayumu, her son and the best-performing offspring) and nine university students. Among the humans, accuracy decreased with shorter hold times. Ai's performance similarly decreased with shorter hold intervals, and her accuracy was overall lower than theirs. Ayumu's accuracy was consistent regardless of hold interval, however, and he completed the trials more quickly and accurately than did the human subjects.

The chimp's ability may be similar to photographic memory, the retention of a detailed image of a complex pattern. This ability is not uncommon among human children but seems to fade with age, perhaps in exchange for the development of language recognition.