Abstract: Dolphins are known to show similarities to humans with respect to a range of visual perceptual tasks, but it is not well understood how dolphins perceive objects through vision. In this study, we tested the relative size discrimination of visual stimuli in a bottlenose dolphin (Tursiops truncatus) in order to examine whether dolphins could solve size discrimination problems using relative size concepts. In addition, we investigated whether dolphins are susceptible to the Ebbinghaus illusion. In Experiment 1 (a size discrimination task), the subject was trained to select the larger of two black circles during the training session. After the subject mastered this task, four types of figures of different sizes that were unfamiliar to the subject were paired and presented as the probe trial. The subject chose the larger figure, suggesting that the subject could solve the problem based on relative size concepts that were learned during the training session. In Experiment 2 (the Ebbinghaus Illusion Perception Task), after the subject learned to select the larger target circle from two circles of different sizes that were surrounded by inducer circles, the Ebbinghaus figures were presented—that is, two black circles of the same area, one surrounded by six small inducer circles and the other surrounded by six large inducer circles. The subject selected the target circle that was surrounded by six small inducer circles significantly more often than the one surrounded by six large inducer circles. This suggested that dolphins, like humans, are receptive to the Ebbinghaus illusion and that the dolphins mechanism is thought to be the same as that of humans—that is, the subject displayed a global-oriented perceptual tendency.

Key Words: Ebbinghaus illusion, relative size discrimination, global-oriented perception, bottlenose dolphin, Tursiops truncatus

Document Type: Research article

DOI: 10.1578/AM.38.4.2012.333

Page Numbers: 333-342