Lizard ears and simple solutions to auditory coding.

Abstract

The variety of lizard auditory organs provides an optimal substrate for examining structure‐function relationships. What does an auditory organ need to perform the basic tasks in the coding of acoustic stimuli? The single‐ossicle tympanic middle ears of lizards are connected through the head, providing for sensitive directional hearing with little neural processing. Lizards evolved hair cells whose frequency responses were determined by the morphological details of their stereovillar bundles and tectorial material, enabling a tonotopic arrangement of frequency sensitivity. Modern lizards achieve this with few hair‐cell rows, showing that tonotopicity does not demand an extensive epithelium. Finally, different lizard families achieved dissimilar frequency selectivity through manipulations of anatomical features, especially of the tectorial membrane. There is a pay‐off between the size of the auditory papilla and its ability to code different frequencies. Long papillae allow very local coupling of hair‐cell bundles with very selective frequency tuning. Short papillae generally abandon tectorial coupling to allow for useful tuning but this results in poorer sensitivity and poorer tuning selectivity. Lizards demonstrate that sensitive and selective hearing organs need only a few hundred hair‐cells organized along a straight axis. [work supported by the German Research Foundation DFG (MA 871‐10.)]