Teaching Ordinals to a Cardinal-Trained Chimpanzee

An adult female chimpanzee, named Ai, who had experience in a cardinal-labeling task with Arabic numerals, was trained on a sequential-responding task with these numerals. Initially, she was given a free sequential-responding test (test for ordinality of numerals), which showed no transfer of performance from the cardinal training; she responded almost randomly. Then, Ai was shifted to differential-reinforcement training of 2-item sequences. After learning the 36 possible 2-item sequences, Ai successfully transferred her performance to 3-, 4-, and 5-item sequences. Reaction-time analyses further revealed a serial position effect and a symbolic distance effect during her sequential-responding performance; these effects are commonly observed in comparative judgement by humans.