Basilar membrane resonance in the cochlea of the mustached bat

The mustached bat (Pteronotus parnellii) detects the wing beats of prey insects by resolving small frequency modulations in the return echoes to the constant frequency component of its echolocation call at about 61 kHz. The fine frequency discrimination required for this task is due largely to the mechanical properties of the cochlea, as indicated indirectly by the presence of large otoacoustic emissions at 61 kHz. A laser diode interferometer was used to measure basilar membrane displacements in the basal turn of the cochlea at a location that is 4-6 mm basal to the 61-kHz place on the basilar membrane. The region of the basilar membrane from which the measurements were made was tuned not only to those characteristic of this location (88-98 kHz) but also very sharply tuned to 61 kHz. The 61-kHz tuning was labile and could be modeled as a simple resonance. The resonance is not restricted to the 61-kHz place but involves more basal regions. We also provide direct evidence to show that otoacoustic emissions from the ear propagate along the basilar membrane.