Response strategies in list learning by orangutans (Pongo pygmaeus x P. abelii)

Rhesus monkeys (Macaca mulatta) develop strategies to acquire and execute serial lists (K. B. Swartz & S. A. Himmanen, 2001). Serial probe recognition studies of list memory have demonstrated similarities across monkeys and humans (S. F. Sands & A. A. Wright, 1980). The present study extended the investigation of list learning and memory to determine whether orangutans (Pongo pygmaeus x P. abelii) would show evidence of subjective organization of photographic lists in a manner similar to that shown by humans learning a list of unrelated words (E. Tulving, 1962). No evidence for the effective use of a subjective organization strategy was found, but the orangutans developed a right-to-left spatial response strategy, which emerged during the acquisition of 5-item lists. This strategy was an effective way to reduce the load on working memory when presented with a complex array of items.