Mechanical adaptations for echolocation in the cochlea of the bat Hipposideros lankadiva.

Abstract

The cochlear mechanics of bats with long constant-frequency components in their echolocation calls are sharply tuned to the dominant second harmonic constant frequency. Hipposiderid bats employ a shorter constant-frequency call component whose frequency is less stable than in long-constant-frequency bats. To investigate to what degree cochlear mechanics in hipposiderid bats are already specialized for the processing of constant frequencies, we recorded distortion-product otoacoustic emissions in Hipposideros lankadiva. Iso-distortion threshold curves for the 2f1-f2 distortion-product otoacoustic emission reveal a threshold maximum close to the second harmonic constant frequency, between 65.0 and 70.0 kHz, and a second insensitivity close to the first harmonic constant frequency. The group delay of the 2f1-f2 distortion is prolonged for both frequency ranges, indicating that a specialized cochlear resonance may act to absorb the constant-frequency call components. Compared to long-constant-frequency bats, the threshold maximum at the second harmonic constant frequency is less pronounced and the optimum cochlear frequency separation is larger. Distortion-product otoacoustic emission suppression tuning curves and neuronal tuning curves recorded from neurons in the cochlear nucleus display an increase of tuning sharpness close to the second harmonic constant-frequency range which is smaller than that reported for long-constant-frequency bats. Our data suggest that the cochlea of hipposiderid bats represents an intermediate state between that of non-specialized bats and long-constant-frequency bats.