Basilar membrane tuning properties in the specialised cochlea of the CF-bat, Rhinolophus ferrumequinum.

The greater horseshoe bat has greatly expanded frequency mapping, and morphological specialisations, in the first half turn of its cochlea and a sudden transition to normal mapping. Amplitude and phase of vibration have been measured on various structures in the expanded and normal regions and have not revealed any sharply tuned responses. Amplitudes are much lower than those found in other species and frequently show a deep notch in the 77-84 kHz region. The high-frequency cut-off frequencies are tonotopically organised but deviate from the Bruns map, so that hair-cell tuning appears to occur at a frequency at which basilar membrane vibration is small. In the basal region, phase differences were frequently found between the inner and outer parts of the basilar membrane. These appear to be due to interaction between two components of motion and are probably not indicative of a further filtering mechanism. There is no evidence for reflection of the travelling wave at the transition.