This study investigated the perceptions of Ebbinghaus and Delboeuf illusions in captive bottlenose dolphins (Tursiops truncatus), aimed at understanding their visual information processing. Four female bottlenose dolphins were used for the Ebbinghaus illusion, and three for the Delboeuf illusion tests. The dolphins varied in age and were housed in the Kagoshima City Aquarium, Japan. A series of trials were performed for two types of Ebbinghaus and Delboeuf illusion experiments. The results showed that a few dolphins perceived the Ebbinghaus and Delboeuf illusions in a manner similar to humans. In addition, essential optical cognitive histological mechanisms and evolutionary background were discussed by comparing this study’s results with previous studies' results. One or more evolutionary ecological factors might affect optical information processing.

Bottlenose dolphin, Delboeuf illusion, Ebbinghaus illusion, Neuroethology