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The concept of number in the chimpanzee (Pan troglodytes): The introduction of "Zero"

Dora Biro and Tetsuro Matsuzawa, Department of Zoology, University of Oxford and Primate Research Institute Kyoto University

     A number of studies have demonstrated numerical competence in non-human subjects, including rats, pigeons, monkeys, and a parrot. Chimpanzees (Pan troglodytes) have a long history as participants in such studies, past experiments having revealed a variety of counting and other numerical abilities long considered unique to humans. Here we report the introduction of "zero" in three different numerical tasks, with special emphasis on the distinct phases of acquisition that became apparent. Our subject was a 21-year-old female chimpanzee, named Ai, with prior training in a variety of symbolic tasks, including the recognition and productive as well as receptive use of Arabic numerals 1-9 as applied to everyday objects and computer-generated stimuli (Matsuzawa, 1985; Murofushi, 1997; Biro & Matsuzawa, 1999). "Zero" was introduced in the following three tasks. (1) Productive use: subject was required to label a collection of white dots appearing on a computer screen by selecting, from among a set of alternatives, the numeral corresponding to the total number of dots. (2) Receptive use: subject was required to select a set of white dots whose total corresponded to a sample numeral appearing on screen. (3) Ordering: subject was required to select numerals presented to her on a computer screen in an ascending order.
     Our subject mastered the recognition of the meaning of zero in all three tasks. However, details of her usage of the symbol reveal that her representation of the concept of zero may not have been analogous to that of humans. Ai's understanding of zero in relation to the rest of the number symbols was not consistent with an "absence of items vs. presence of items" scheme. Over the course of acquisition and toward the high levels of accuracy we eventually observed, the newly-introduced zero appeared to shift along the length of a continuous numerical scale toward the lower end, while confusions with 1 remained the most frequently encountered errors.

Matsuzawa, T. (1985, May 2-8). Use of numbers by a chimpanzee. Nature, 315, 57-59.
Murofushi, K. (1997). Numerical matching behavior by a chimpanzee (Pan troglodytes): Subitizing and analogue magnitude estimation. Japanese Psychological Research, 39, 140-153.
Biro, D., & Matsuzawa, T. (1999). Numerical ordering in a chimpanzee (Pan troglodytes): Planning, executing, and monitoring. Journal of Comparative Psychology, 113, 178-185.