Examining the Long-term Retention of Associative Stimulus Relations in Chimpanzees (Pan troglodytes)

Humans can retain memories for months to decades. In contrast, relatively few studies have examined such very long-term memory in nonhuman animals. We, therefore, investigated whether chimpanzees (Pan troglodytes) who had learned arbitrary relations between stimuli remembered these relations over a long period. In Experiment 1, we tested four chimpanzees, one of whom (Chloe) had learned a “symbolic” matching task between colors and geometric forms at age 9–10 years, and three who had not been previously trained, on their memory for these relations 19 years later. Chloe showed perfect accuracy in the test trials; the three other chimpanzees did not perform better than chance level. In Experiment 2, we tested Popo and Reo, who had learned the same set at the same time as Chloe, on their memory 27 years later. Although they relearned the identity-matching task very quickly (savings effect), they did not retain the stimulus relations learned 27 years ago. In Experiment 3, we tested Chloe’s retention of a different set of stimulus relations, which she had learned at the same time as those tested in Experiment 1 but with less intensive training than with the first set. She did not perform better than the chance level. Finally, in Experiment 4, we tested the memory retention of another chimpanzee, Ai, who had learned to “label” objects with lexigrams more than 20 years earlier by using the matching task. She did not perform significantly better than chance on test trials. Thus, our results were not consistent across the experiments, providing only limited evidence that chimpanzees retain associative stimulus relations over the long term. Several factors, such as the effect of cognitive environments, the levels of processing, and the context-dependent nature of memory retrieval, could have affected our results, but none of these factors can account for all the results from the four experiments.