Biophysics of underwater hearing in anuran amphibians

A standing wave tube apparatus was used to determine the biophysical basis of underwater hearing sensitivity in 3 species of Rana and in Xenopus laevis. A speaker inside the base of a vertical, water-filled 3 m steel pipe produced standing waves. Pressure and particle motion were measured with a hydrophone and geophone respectively and were spatially 90 degrees out of phase along the length of the tube. Microphonic responses were recorded from the inner ear of frogs lowered through pressure and particle motion maxima and minima. The air-filled lungs of whole frogs produced distortions of the sound field. Preparations of heads with only an air-filled middle ear produced little distortion and showed clear pressure tracking at sound intensities 10-20 dB above hearing thresholds from 200-3000 Hz. Filling the middle ear with water decreased or abolished microphonic responses. Severing the stapes reduced responses except at certain frequencies below about 1000 Hz which varied with body size and likely represent resonant frequencies of the middle ear cavity. We conclude that the frog species examined respond to underwater sound pressure from about 200-3000 Hz with the middle ear cavity responsible for pressure transduction.