Click emission in Dall's porpoise Phocoenoides dalli, focusing on physical properties of tissues

Dall's porpoise (Phocoenoides dalli) is one of most common North Pacific porpoise species, for which information on sound-emitting processes is limited. To evaluate the mechanism of click emission in the head of this porpoise, the distribution of acoustic impedance in head tissues was calculated using density and Young's modulus`which is a measure of linear resistance to linear compression. Two Dall's porpoise heads were examined: one for macroscopic dissection, and one for investigating the distribution of acoustic impedance calculated from CT-measured density, and Young's modulus measured by creep meter. Acoustic impedance increased from the dorsal bursae to the melon's emitting surface, with impedance matching observed at the boundary between the emitting surface and seawater, and was more similar in distribution to Young's modulus than it was to density. The distribution of acoustic impedance differed from that of harbor porpoise (Phocoena phocoena), despite similarities in the sound-producing organs in the heads of Dall's and harbor porpoises. A comparison of the physical properties of Dall's and harbor porpoise head tissues suggests that hypertrophic vestibular sacs and an oval emitting surface are common characteristics in Phocoenidae.