Bottlenose dolphins use auditory (or echoic) information to recognise their environments, and many studies have described their echolocation perception abilities. However, relatively few systematic studies have examined their visual perception. We tested dolphins on a visual-matching task using two-dimensional geometric forms including various features. Based on error patterns, we used multidimensional scaling to analyse perceptual similarities among stimuli. In addition to dolphins, we conducted comparable tests with terrestrial species: chimpanzees were tested on a computer-controlled matching task and humans were tested on a rating task. The overall perceptual similarities among stimuli in dolphins were similar to those in the two species of primates. These results clearly indicate that the visual world is perceived similarly by the three species of mammals, even though each has adapted to a different environment and has differing degrees of dependence on vision.

We are very grateful to M. Soichi, the director of the Port of Nagoya Public Aquarium (PNPA); I. Uchida, the former director of the PNPA; and H. Nitto, Y. Akune, K. Kobayashi, M. Yoshii, M. Dozaki, K. Kanda, and the other all staff of the PNPA for their generous support of the present study. Special thanks are due to M. Ninomiya, Y. Uehara, J. Kawakami, C. Minami, and T. Kamiya who conducted the experiments. Drs. T. Matsuzawa, M. Tanaka, H. Misato, and I. Adachi gave us invaluable comments. Ms. M. Nakashima helped data analyses.

Bottlenose dolphins are known to use signature whistles to identify conspecifics auditorily. However, the way in which they recognize individuals visually is less well known. We investigated their visual recognition of familiar human individuals under the spontaneous discrimination task. In each trial, the main trainer appeared from behind a panel. In test trials, two persons (one was the main trainer) appeared from the left and right sides of the panel and moved along the poolside in opposite directions. Three of the four dolphins spontaneously followed their main trainers significantly above the level of chance. Subsequent tests, however, revealed that when the two persons wore identical clothing, the following response deteriorated. This suggests that dolphins can spontaneously discriminate human individuals using visual cues, but they do not utilize facial cues, but body area for this discrimination.