Hearing in the African lungfish, Protopterus annectens

The lungfishes are believed to be the closest living relatives to the tetrapods and important for the understanding of evolution of hearing in the tetrapods. We review results from a non-invasive study of hearing and vibration sensitivity in the West African lungfish (Protopterus annectens) using auditory brainstem responses (ABR) to sound and vibration stimulations. The experiments show that the sensitivity to particle motion generated by sound stimulation in water is similar to the sensitivity to direct vibration of the head. The hearing range is limited to frequencies below 300 Hz. Therefore, the West African lungfish is an auditory generalist that detects the particle motion component of sound and not sound pressure. The air volumes in lungfish, most notably the lungs, are therefore not functionally connected to the inner ear as is the swimbladder in auditory specialists among ray-finned fishes. If the hearing of the African lungfishes resembles that of the tetrapod ancestors, tetrapods may have been very insensitive to airborne sound before the emergence of the tympanic ear, and hearing mediated chiefly through bone conduction (sound-induced skull vibrations). However, it is also possible that the African lungfishes are secondarily reduced with concomitant changes to the auditory system.