Colossal ears? How baleen whales hear low-frequency sound.

Baleen whales produce and receive underwater sounds with wavelengths much longer than their bodies, despite having ears approximately the size of a human fist. How do they hear long wavelength sounds with relatively small ears? In 2015, we produced a computational model to simulate low-frequency hearing in the fin whale. That study predicted bone conduction as the most likely hearing mechanism. The whale's enormous skull acts as an external ear, capturing sounds like an acoustic antenna and transmitting them to other parts of the ear. In the current study, we tested the bone conduction hypothesis with physical, vibroacoustic experiments using partially denuded gray whale skulls. These experiments validated that long wavelength sounds excite skull vibrations, which are amplified and transferred to the dynamic components of each bony ear, known as the tympanoperiotic complex. Vibrations of the dynamic components include the bony pedicles, tympanic bullae and middle ear ossicles, resulting in displacement of fluid within the cochlea of the inner ear. The pedicles are important components of this mechanism, thin flexible bones that suspend the bullae from the periotic, amplifying the low-frequency vibrations from the skull. We contend that this skull-driven pathway of sound reception and amplification within the bony ear complexes is key to understanding low-frequency hearing capabilities and mysticete natural history.