The receptor and neuron distribution in the cochlea of the bat, Taphozous kachhensis.

Cochlear receptors and spiral ganglion neurons in the inner ear of the bat, Taphozous kachhensis of the family Emballonuridae, were quantitatively analyzed on the basis of surface specimen technique, histological sections, and three-dimensional computer reconstruction. The length of the basilar membrane was 12.1 (+/- 0.4) mm. There were about 23,000 neurons in the spiral ganglion and about 1430 inner hair cells and 4635 outer hair cells in the organ of Corti. Cochlear neurons had two density maxima, one basal and one apical. Inner hair cells had basically the same distribution but less pronounced. Outer hair cells had only one density maximum in the apical region corresponding to the respective maxima of inner hair cells and cochlear neurons. Basal density maxima of inner hair cells and neurons are most probably involved in the analysis of echolocation signals, and presumably correlated with maximum sensitivity of the auditory system. A comparison with the bat, Nyctalus noctula of the family Vespertilionidae revealed a conspicuous similarity in some quantitative aspects of the cochlear structure. These particular similarities may be looked upon as convergent adaptations given by similar flight, hunting, and echolocation strategies in both bat species.