Scientists in Denmark believe that a study of hearing and ear structures in present-day vertebrates makes it possible to understand the hearing of early terrestrial vertebrates, and trace the evolution of hearing. According to new research from investigators at Aarhus University, Aarhus University Hospital, and the University of Southern Denmark, a study of lungfish and salamanders reveals that they can hear, despite not having an outer ear or tympanic middle ear. Lungfish and salamanders are considered to be similar to early terrestrial vertebrates, and the new research reveals that prehistoric terrestrial vertebrates or tetrapods existing more than 250 million years ago were likely able to hear prior to developing a tympanic middle ear.
The team of Danish scientists, whose work recently appeared in an article in the February 4, 2015 edition of The Royal Society Journal Proceedings B, and in another article in the February 1, 2015 edition of The Journal of Experimental Biology, report that their new findings provide information about the development of aerial hearing from water-borne hearing. The research team, led by Christian Bech Christensen, PhD, Department of Bioscience, Aarhus University, reports that the ears of lungfish and salamanders are good models for different stages of ear development in early terrestrial vertebrates.
Lungfish and salamanders are able to detect both water-borne and airborne sound by sensing vibrations that travel along sound waves, the researchers discovered. The team measured the auditory nerve signals and neural signals in the brainstems of lungfish and salamanders, examining the function of sound stimulation at different frequencies and levels. The resulting measurement showed that the lungfish, the terrestrial adult salamanders, and the fully aquatic juvenile salamanders were all able to detect airborne sound despite not having a tympanic middle ear.
According to the researchers, the tympanic middle ear improves the transfer of sound energy through the air and surroundings to the sensory cells in the inner ear. This capability is very important for hearing in terrestrial vertebrates, such as present-day terrestrial mammals, birds, reptiles, and amphibians. The researchers say the results of their lungfish and salamander studies show that even vertebrates without outer and middle ears are capable of detecting airborne sound. They believe that adaptation to aerial hearing following the transition of early terrestrial vertebrates from aquatic to terrestrial habitats was a gradual process, but during that transition the early tetrapods were not deaf to airborne sound.
Source: The Royal Society Journal Proceedings B, and The Journal of Experimental Biology
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