The use of numerical symbols and concepts by a chimpanzee (Pan troglodytes)

Dora Biro and Tetsuro Matsuzawa

University of Oxford, UK

The nature and extent of numerical competence in non-human animals has been investigated in a variety of species. A considerable bulk of this work has concentrated on the chimpanzee (Pan troglodytes) whose evolutionary proximity to humans may allow us to speculate the existence of human-like "counting" and other numerical skills. Here we report a series of experiments carried out with a single subject designed to elucidate the extent to which a chimpanzee was able to comprehend and use abstract numerical symbols. In the initial stages of training, Ai, a 21-years-old female chimpanzee was tested using real-life objects as items to be counted and Arabic numerals as labels. Later a fully computer-controlled experimental-set-up was adopted, and the subject's range was expanded to incorporate numbers up to and including 9. At present, she is able to label sets of 0 to 9 items presented on a computer screen as well as arrange the same symbols according to an ascending numerical scale. The recent addition of "zero" to Ai's repertoire provided an opportunity to examine transfer of the meaning of newly-acquired numerical symbol between the cardinal and ordinal domains. We concluded that although the subject had reached near-perfect levels in the use of zero in individual numerical tasks, transfer of the meaning between different kinds of tasks (productive use, receptive use, and ordering of numerals) was incomplete, suggesting that the level of abstraction characteristic of human numerical ability was not attained in the chimpanzee. Patterns of errors instead revealed a tendency towards specific confusions between 0 and 1, thereby suggesting that an “absence of items vs. presence of items” scheme was not operating. In an additional study, we examined the perceptual and motor skills involved in the ordering of numerals into ascending series. Our results pointed towards a sophisticated strategy in which the chimpanzee pre-planned her entire motor sequence required for a correct response. The use of numerical symbols was in this case thus comparable to that performance by humans.

Biro, Dora

Department of Zoology, University of Oxford

Oxford, OX1 3PS, UK

dora.biro@merton.oxford.ac.uk