Previous studies on dolphin electrocardiograms have shown that they are mainly composed of increased negative waves, similar to ungulates. The electrocardiogram waveform was determined by the distribution of the Purkinje fibers. Based on the waveform of the dolphin electrocardiogram, Hamlin predicted that the Purkinje fibers would be distributed within the ventricular muscle, as in ungulates. The purpose of this study was to confirm the histological distribution of Purkinje fibers in dolphins. In the present study, bottlenose dolphin hearts were observed both grossly and histologically, and the effects of Purkinje fiber distribution and cardiac morphology on electrocardiogram waveforms were examined. This study showed that the Purkinje fibers of dolphins run just below the endocardium, as in humans, dogs, and cats, whose electrocardiograms mainly show positive waves. When the cardiac morphology of dolphins was observed carefully, the right ventricle was found to be extremely dilated compared to that of terrestrial mammals. In human recreational divers, right ventricular dilatation is induced by diving. We hypothesized that the dolphin’s heart is in a state similar to that of the right heart dilatation in terrestrial animals. The dolphin electrocardiogram waveform was considered to be due to right axis deviation. Based on the above, we concluded that the dolphin electrocardiogram waveform was due to its ability to live in water. We found that the dolphins are genetically related to ungulates, particularly the hippopotamus, but that their hearts have evolved differently.