The aquatic tympanic ear: convergent adaptations for underwater hearing in three tetrapods

All groups of tetrapods have members that adopt aquatic lifestyles with physiological adaptations also of their auditory system. Auditory sensitivity is affected by the different characteristic impedances in air and water caused by the differences in density and sound speed. 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, since sound is transmitted from water to inner ear tissue with little loss. However, the sensitivity of such an ear is limited by the very low particle motion in water. We report on underwater hearing in 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) studied by laser vibrometry and auditory evoked responses. All have tympanic middle ears with an air-filled middle ear cavity. The eardrum vibration peak frequency is correlated with the resonance frequency of the middle ear cavity air volume and the eardrum is modified (cartilaginous or partly cartilaginous). In all three species, the lowest threshold to underwater sound is at this peak frequency and is around 80 dB re 1 µPa. The 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.