Quantity matching by an orangutan (Pongo abelii)

An adult male orangutan (Pongo abelii) was presented with a series of delayed matching-to-sample (DMTS) tasks in which he was to match images based on (a) the number of individual animals depicted in the photograph (from 1 to 4), (b) the number of abstract shapes presented in the stimulus (from 1 to 4), or (c) the number of dots presented in the stimulus (from 1 to 4, 4–7, or 7–10). The spatial arrangement of the dots and the background color of the stimuli varied, and the size of the dots was manipulated to control for overall ratio of foreground to background. The subject’s performance was not affected by these perceptual features, but was affected by the absolute difference and ratio between number of elements in the comparison stimuli. However, the relationship between these variables and his performance was not always linear as predicted by the analog magnitude model. In addition, the subject showed a high degree of transfer to novel numerosities up to ten, indicating that orangutans are capable of estimating quantity for a greater number of items than can presumably be subtilized by humans.