A Comparison of Sequential Learning Errors Made by Apes and Monkeys Reveals Individual but not Species Differences in Learning

Abstract

Using methods comparable to those used previously to test closely related taxa (Pan troglodytes and Macaca mulatta), our aim was to better understand how gorillas (Gorilla gorilla gorilla) and Japanese macaques (M. fuscata) learn sequences. Using a disappearing-type simultaneous chain, we trained 5 gorillas and 8 macaques on a 2-item list of colored stimuli presented via touchscreens. There was no difference across species in the number of trials required to learn the 2-item list. We added a third item to the list as each subject reached criterion. We then analyzed the subjects' first 30 trials with the 3-item list and found that the rate of successfully sequencing the list varied by subject but not by species. In their first 30 trials of the 3-item list, subjects selected the second item correctly only at chance, suggesting they had only encoded the first symbol when learning the 2-item list. One gorilla, tested on longer sequences, showed similar responses: When first presented with a newly-lengthened list, he only selected the penultimate item at chance levels. Thus, the primates' errors with newly-lengthened lists are suggestive of the chaining theory of learning. These results highlight similarities in list learning of two distantly related primate species as well as clear intraspecies variations in learning.