Were Our Tetrapod Ancestors Deaf?
ScienceDaily (Nov. 8, 2010) — A research group led by Jakob Christensen-Dalsgaard, University of Southern Denmark, has shown that the closest living relatives of the tetrapods -- the lungfish -- are insensitive to sound pressure but sensitive to vibrations.
African lungfish. (Credit: Christian Brandt)
The discovery was published recently online in the journal Biology Letters.
Many changes in the sensory systems of tetrapods are associated with the water-to-land transition. In hearing, one of the crucial elements in detecting airborne sound is the tympanic ear. Surprisingly, the tympanic ear originated independently in the major tetrapod lineages and relatively late after the terrestrial tetrapods emerged -- in the Triassic, more than 100 million years after the origin of tetrapods.
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Hearing in the African lungfish (Protopterus annectens): pre-adaptation to pressure hearing in tetrapods?
Jakob Christensen-Dalsgaard1,*, Christian Brandt1, Maria Wilson2, Magnus Wahlberg1 and Peter T. Madsen2
+Author Affiliations
1Center for Sound Communication, Institute of Biology, University of Southern Denmark, DK-5230 Odense M, Denmark
2Department of Biological Sciences, Aarhus University, Denmark
*Author for correspondence (jcd@biology.sdu.dk).
Abstract
Lungfishes are the closest living relatives of the tetrapods, and the ear of recent lungfishes resembles the tetrapod ear more than the ear of ray-finned fishes and is therefore of interest for understanding the evolution of hearing in the early tetrapods. The water-to-land transition resulted in major changes in the tetrapod ear associated with the detection of air-borne sound pressure, as evidenced by the late and independent origins of tympanic ears in all of the major tetrapod groups. To investigate lungfish pressure and vibration detection, we measured the sensitivity and frequency responses of five West African lungfish (Protopterus annectens) using brainstem potentials evoked by calibrated sound and vibration stimuli in air and water. We find that the lungfish ear has good low-frequency vibration sensitivity, like recent amphibians, but poor sensitivity to air-borne sound. The skull shows measurable vibrations above 100 Hz when stimulated by air-borne sound, but the ear is apparently insensitive at these frequencies, suggesting that the lungfish ear is neither adapted nor pre-adapted for aerial hearing. Thus, if the lungfish ear is a model of the ear of early tetrapods, their auditory sensitivity was limited to very low frequencies on land, mostly mediated by substrate-borne vibrations.
lungfish, hearing, vibration, tetrapod, sound, evolution
Received July 13, 2010.
Accepted August 16, 2010.
© 2010 The Royal Society
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