Bottlenose dolphins use auditory (or echoic) information to recognize their environments, and many studies have described their echolocation perception abilities. However, relatively few systematic studies have examined their visual perception. A team of scientists now tested dolphins on a visual-matching task using two-dimensional geometric forms including various features. Based on error patterns, they used multidimensional scaling to analyze perceptual similarities among stimuli. In addition to dolphins, they 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.
Because dolphins have adapted to an underwater environment, they have developed a perceptual system that differs considerably from that of terrestrial mammals such as primates. One strikingly different aspect of the perceptual system of dolphins is echolocation1,. They can recognize shapes, materials, and the texture of objects using this form of biological sonar. Many echolocation studies on cetaceans have been conducted both in the laboratory and in the wild4. A few studies have investigated dolphins' ability to use cross-modal integration through vision–echolocation matching5, 6,. In these studies, dolphins were very accurate in matching three-dimensional complex objects using information gathered via echolocation. On the other hand, these results indirectly suggest that dolphins may also visually discriminate complex objects. Dolphins (e.g., bottlenose dolphins) have poorer in-air and underwater visual acuity (12.6 min of visual angle from a distance of 2.5 m) than that of primates10. Nevertheless, they still visually recognize and discriminate human gestural signs11, 12, 13, mirror images of themselves14, 15, numbers of objects16, three-dimensional objects4, 17, and two-dimensional forms17, 18. Moreover, researchers have used visual stimuli to study the basic features of the vision and various cognitive abilities of dolphins17, 18.