International Journal of Comparative Psychology

Animals do not see the external world as it is. Different animals process information in different ways, even when looking at the same object. A visual illusion is a psychological phenomenon by which the eye perceives something as different from what it is. We tested whether a bottlenose dolphin produces the square-diamond illusion to see if it experiences the illusion in the same way as humans. In Experiment 1, two figures (square and diamond) of different sizes were presented in the training session and the subject had to choose the “smaller” figure. In the test session, 22 pairs of squares and diamonds of different areas were presented to see which the subject would choose. When the area difference is large, the percentage of correct responses is high, but when the area difference is small, the percentage of correct responses varies between pairs. When these results were then sorted into “small squares vs. large diamonds” and “small diamonds vs. large squares”, the percentages were significantly high in all pairs in the “small squares vs. large diamonds” group, whereas in the “small diamonds vs. large squares” group, the percentage of correct responses decreased as the difference between the areas of the two figures also decreased. In other words, this result suggests that the illusion may have come into play. Experiment 2 was a square-diamond illusion perception task. Two pairs of squares and diamonds of equal area (225 cm2 and 400 cm2, respectively) were presented and the subject’s choice was then tested. The results showed that the subject chose the square significantly more often than the diamond in both pairs. The square appeared smaller, and the diamond appeared larger to the subject, even though the fact that they had the same area (i.e., it was demonstrated that the square-diamond illusion had occurred), and this study showed that dolphins share the same visual characteristics as humans.