Numerical memory span in a chimpanzee
Nobuyuki Kawai, Tetsuro Matsuzawa
A female chimpanzee called Ai has learned to use Arabic numerals to represent numbers1. She can count from zero to nine items, which she demonstrates by touching the appropriate number on a touch-sensitive monitor2, 3, and she can order the numbers from zero to nine in sequence4, 5, 6. Here we investigate Ai's memory span by testing her skill in these numerical tasks, and find that she can remember the correct sequence of any five numbers selected from the range zero to nine.
Humans can easily memorize strings of codes such as phone numbers and postcodes if they consist of up to seven items, but above this number they find it much harder. This 'magic number 7' effect, as it is known in human information processing7, represents a limit for the number of items that can be handled simultaneously by the brain.
To determine the equivalent 'magic number' in a chimpanzee, we presented our subject with a set of numbers on a screen, say 1, 3, 4, 6 and 9. She had already displayed close to perfect accuracy when required to choose numerals in ascending order, but for this experiment all the remaining numbers were masked by white squares once she had selected the first number. This meant that, in order to be correct in a trial, she had to memorize all the numbers, as well as their respective positions, before making the first response. Chance levels with three, four and five items were 50, 13 and 6%, respectively.
Ai scored more than 90% with four items and about 65% with five items, significantly above chance in each case. In normal background trials, response latency was longest for the first numeral and much shorter for all the others, indicating that Ai inspected the numbers and their locations and planned her actions before making her first choice. In masking trials, response latency increased only for the choice directly after the onset of masking, but this latency was similar to those recorded in background trials, indicating that successful performance did not depend on spending more time memorizing the numbers.
Figure 1: The chimpanzee Ai performing the five-number ordering task in the 'masking' trial.
Five numbers (1, 3, 4, 6 and 9) are presented on the touch-sensitive monitor. a, b, Ai correctly chooses the number 1 as the lowest of the series (a), at which point the remaining numbers are automatically masked (b). c-f, She continues to identity the numbers one by one in ascending order (c-e), ending with the 9 (f).
Table 1: Performance in masking trials
In one testing session, after Ai had chosen the correct number and all the remaining items were masked by white squares, a fight broke out among a group of chimpanzees outside the room, accompanied by loud screaming. Ai abandoned her task and paid attention to the fight for about 20 seconds, after which she returned to the screen and completed the trial without error.
Ai's performance shows that chimpanzees can remember the sequence of at least five numbers, the same as (or even more than) preschool children. Our study and others8, 9, 10 demonstrate the rudimentary form of numerical competence in non-human primates.
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