Auditory Structures that Impact Sensitivity and Acuity in Cetaceans:Location, Location, Location

Microchiropteran bats and odontocete cetaceans have sophisticated echolocation capabilities dependent upon good high frequency hearing, but they operate in radically different media. By contrast, larger terrestrial mammals and mysticetes share the ability to generate and respond to low frequency sounds, although little is known about their hearing abilities. The similarity of tasks and information that odontocetes and microchiropterans obtain acoustically suggest their auditory systems have some commonalities for ultrasonic signal processing mechanisms and differences related to media dependent elements such as interaural time differences, peak spectra of echolocation signals, and latencies. In this study, the heads, outer, middle, and inner ears of 12 specimens from five species of bats, dolphins and whales (Eptesicus fuscus, Pipistrellus abramus, Phocoena phocoena, Tursiops truncatus, Balaenoptera acurostrata) were analyzed based on microimaging at resolutions of 11 to 100 micron isotropic voxels obtained from a Siemens helical (CT) scanner (http://csi.whoi.edu) and X-Tek micro CT. Outer and middle ears anatomies varied widely amongst all species in terms of canal length, middle ear stiffness and volume, and ossicular anatomy. The minke whale inner ear was most similar to that of mid to low frequency terrestrial mammals, particularly human and pig inner ears. The ears of the echolocators were significantly different from the mid to low frequency ears, with increased stiffness, thicker membranes and outer osseus laminae supporting up to 60% of the basilar membrane. The high frequency species also had “foveal” regions with “stretched” frequency representation for peak spectra of echolocation signals. Stapedial inputs to the cochlea in cetacean echolocators differed from the bat and baleen ears, with more complex canal configurations and higher placement that suggest a novel anatomy and mechanics that improves high frequency acuity. Supported by NIH, JIP/OGP, CNO/N45-US Navy Environmental Division, and the Office of Naval Research.