Enumeration of briefly presented items by the chimpanzee (Pan troglodytes) and humans (Homo sapiens)

In this study, we compared the performances on an enumeration task (numerical labeling task) of 1 chimpanzee (Pan troglodytes) and 4 humans. In this task, two types of trials, with different exposure durations of the sample that was to be enumerated, were used. In the unlimited-exposure trials, the sample remained on until the subject made a choice. In the brief-exposure trials, the sample was presented for 100 msec and then was masked. The results show clear differences between the different species. The main differences had to do with accuracy during the unlimited trials and response times during the brief trials. Detailed analyses of the pattern of response times for the chimpanzee and of looking-back behavior during the task suggests that the enumeration process underlying the subject's performance was not counting but estimation.