Evolution of sound localization in lizards

Abstract

Tympanic hearing may have developed independently in four major tetrapod groups, the anurans, lepidosaurs, archosaurs, and mammals. The emergence of a tympanic ear would have increased the frequency range and sensitivity of hearing. Furthermore, early tympana were acoustically coupled through the mouth cavity and therefore must have acted as pressure difference receivers, which are inherently directional, obviating a need for central computation of ITD and ILD. In lizards, this acoustical coupling generates a large directional difference at the tympanum and recordings from auditory nerve are strongly ITD‐ and ILD‐dependent. To separate neural from acoustical interactions, interaural transmission in a gecko was blocked to reveal true binaural neurons in nucleus magnocellularis, nucleus angularis, and superior olive. The function of these neurons with naturalistic, i.e., free‐field stimulation may be to sharpen the directional response, a mechanism similar to that suggested for teleost fish. The closure of the middle ear cavity in the mammals and some birds is a derived condition, and may have profoundly changed the operation of the ancestral tetrapod ear by decoupling the tympana, improving the low‐frequency response of the tympanum, and leading to a requirement for neural computation of directionality in the central nervous system.