The Tympanic Ear as an Efficient Underwater Sound Transducer
* Presenting author
All groups of tetrapods have members that adopt aquatic lifestyles with adaptations also of their auditory system. Water is a high pressure, low particle motion medium, and the consequence is that an efficient underwater ear is sensitive to sound pressure. It is often stated that underwater hearing can work efficiently without a middle ear apparatus by bone conduction, but the sensitivity of such an ear is limited by the very low particle motion in water. A comparison of tetrapods ranging from totally aquatic (the clawed frog Xenopus laevis) and mostly aquatic (the red-eared slider Trachemys scripta) to mostly terrestrial (the cormorant Phalacrocorax carbo sinensis) show similar features. All have tympanic middle ears with an air-filled middle ear cavity. The eardrum vibration peaks at the resonance frequency of the middle ear cavity air volume and the eardrum is modified (plate-like). In all three species, the lowest threshold to underwater sound is at this peak frequency and is around 80 dB re 1 µPa. Sensitivity to sound pressure is slightly lower in water than in air, making underwater hearing much more efficient in terms of sound energy. Consequently, the slightly modified tympanic ears of these species are efficient aquatic sound receivers.