European Research on Cetaceans (1988)


(…) The free-swinging bullae of odontocetes are very massive, their specific weight being twice that of the other skull bones. This feature led Yamada (1953) to his theory that the heavy bullae would function like seismographs, and that differences in oscillation between the lighter and heavier bones would be registered through the sense of touch. Yamada’s theory, however, was ignored since he could not find the required receptive organs.
In the harbour porpoise Phocoena phocoena, I have now discovered sense organs between the occipital bone and the bulla, which would enable odontocetes to perceive just such oscillations (Behrmann, 1987; see Fig. 2). (…)