Abstract
Lizards have highly sensitive ears, but most lizard heads are small (1-2 cm in diameter) compared to the wavelengths of sound of frequencies to which they are most sensitive (1-4 kHz, wavelengths 34-8.5 cm). Therefore, the main cues to sound direction that mammals use - binaural time and intensity cues due to arrival-time differences and sound shadowing by the head--will be very small in lizards. The present work shows that acoustical coupling of the two eardrums in lizards produces the largest directionality of any terrestrial vertebrate ear studied. Laser vibrometric studies of tympanic motion show pronounced directionality within a 1.8-2.4 kHz frequency band around the best frequency of hearing, caused by the interference of ipsi- and contralateral inputs. The results correspond qualitatively to the response of a simple middle ear model, assuming coupling of the tympana through a central cavity. Furthermore, observed directional responses are markedly asymmetrical, with a steep gradient of up to 50-fold (34 dB) response differences between ipsi- and contralateral frontal angles. Therefore, the directionality is easily exploitable by simple binaural subtraction in the brain. Lizard ears are the clearest vertebrate examples of directionality generated by tympanic coupling.
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