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Dolphins are able to successfully coordinate their behavior using audio signals. Therefore, the current study tested whether or not belugas can use sound to exchange information in a cooperative task and verified the mechanisms of the transmission of information during trials. The subjects are two male belugas, Nack and Duke. Nack was trained to rotate its body when the experimenter turns a hand in a circular motion (rotation cue) or to take a vertical position on the spot of the pool when the experimenter turned on a light toward the subject (headstand cue). Duke was required to do the same behavior as Nack but was blindfolded. During the test trials, the experimenter presented both cues toward Nack in random order. The result indicated that Nack responded correctly to both cues, and Duke behaved similarly to Nack. Moreover, Nack emitted different sounds in response to the cues, which indicates that Nack spontaneously transmitted information to Duke by emitting sounds, which led Duke to provide correct responses. Although Duke displayed the same behaviors as Nack did despite the lack of training to do so, Duke’s performance was due to learning by hearing sounds instead of training.

We tested the ability of a beluga (Delphinapterus leucas) to imitate sounds presented to it. During the training session, we presented the subject three recorded sounds that were emitted by the subject, and the subject was trained to imitate them. The subject learned to correctly imitate the sounds. During the test session, two novel computer-generated artificial sounds were presented through an audio speaker. In addition, nine arbitrary vocal sounds produced by the experimenter were presented to the subject, and the subject was required to imitate them. Seven persons, who were not involved in the experiment, were presented the sample sounds and imitated calls; subsequently, they judged whether both sounds were similar to each other. In addition, sound spectrums of the sample sounds and imitated calls were analyzed. As a result, some components of the sound spectrums were similar, and most of imitated calls possess spectral features similar to the sample sounds. These results demonstrated that the beluga was able to correctly imitate novel sounds and spontaneously displayed aptitude for imitation.

A beluga was tested to label objects using vocal symbols in order to form a bidirectional relationship between the visual symbols and sounds. In the Training session, the subject was first trained todistinguish four objects by four separate calls that he made. He learned to emit different calls corresponding to the sample stimuli. Next, these three recorded calls were played back to the subject,and he was required to select the objects (comparative stimuli) corresponding to the presented sound. He succeeded in correctly choosing the objects corresponding to the sounds played back. In the Test session, when a completely new recorded sound was presented to the subject, he could choose the correct object by matching the sample sound with the object. It is suggested that a beluga have realized bidirectional relationships or symmetrical relationship between visual symbols and sounds.This is a preliminary study that shows both production and comprehension of symbols in marine mammals.