Chimpanzees’ spontaneous temporal sorting of stimuli on the basis of physical identity

Abstract

This study asked the question “Would chimpanzees’ choice of stimuli in a sorting task follow any consistent sequence?”. Four stimuli (e.g., two plastic grapes and two hairpins) were presented on each trial. The two chimpanzees were reinforced after placing all four stimuli, one at a time, in a receptacle. The order that stimuli were chosen was classified into one of three sequences: two pairs (e.g., grape, grape, hairpin, hairpin), one pair (e.g., grape, hairpin, hairpin, grape), or no pairs (e.g., grape, hairpin, grape, hairpin). The hypothesis that the number of different sequences would be equal was rejected (p<.001) for both subjects. The sequence of two pairs occurred most frequently. This result was accounted for by two factors: (1) some stimuli were chosen first significantly more often than others, indicating the subjects preferred certain stimuli; (2) when the data reflecting preferences was removed, the hypothesis that the number of different sequences would be equal was rejected again (p<.001) and the sequence of two pairs still occurred most frequently. The results indicate chimpanzees’ choice of stimuli in a sorting task is an organized rather than a random process.