Twists and turns, in cochlear anatomy: Curvatures related to infra vs ultrasonic hearing

Abstract

Microchiropteran bats and odontocete cetaceans are sophisticated echolocators with acute ultrasonic hearing operating in radically different media. Similarly, elephants and mysticetes share the ability to generate and respond to infrasonics. In this study, the heads, outer, middle, and inner ears of 32 specimens from 11 species of bats, dolphins, elephants, and whales were analyzed with microCT (11 to 100 micron isotropic voxel imaging; Siemens Volume Zoom and X-Tek CT units). Canal length, basilar membrane dimensions, and cochlear curvatures varied widely among all species. Length correlates with body mass, not hearing ranges. High and low frequency limits correlate with basilar membrane ratios and radii ratios, which are a measure of the radius of curvature. The ears of the known echolocators were significantly different from the mid to low frequency ears, with increased stiffness, thicker membranes, and outer osseous laminae supporting up to 60% of the basilar membrane. Anatomical correlates of "foveal" regions with stretched representation for peak echolocation spectra were found in both bat and porpoise ears. Radii and membrane ratios are consistent despite media and are predictive of high and low frequency hearing limits in all ear types. [Work supported by NIH, JIP, N45/LMRS -US Navy Environmental Division, and ONR Global.].