Michael J. Beran, Ph.D.
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September 1, 2021



Beran, M. J. (2001). Summation and numerousness judgments of sequentially presented sets of items by chimpanzees (Pan troglodytes). Journal of Comparative Psychology, 155, 181-191.

This study investigated summation and relative numerousness judgments in chimpanzees when stimuli were presented sequentially and quantities were never viewed in their totality. Two chimpanzees (Pan troglodytes) chose one of a pair of sequentially presented quantities of M&Ms. Each M&M was visible only prior to placement in one of two cups. In Experiment 1, sets of 1 to 9 M&Ms were presented. In Experiment 2, the quantities in each cup were presented as two smaller sets (e.g., 1+3 versus 2+3). In Experiment 3, the quantities in each cup were presented as three smaller sets (e.g., 1+3+2 versus 2+3+2). In Experiment 4, an M&M was removed from one set prior to the chimpanzees making a selection. In Experiments 1 & 2, the chimpanzees selected the larger quantity on significantly more trials than would be predicted by chance. In Experiment 3 and Experiment 4, one chimpanzee performed at a level significantly better than chance. This study indicates that chimpanzees mentally represent quantity in such a way that multiple, non-visible, sequentially presented sets can be combined and compared to each other.

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