Anatomy and physics of the exceptional sensitivity of dolphin hearing (Odontoceti: Cetacea) |
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Authors: | Simo Hemilä Sirpa Nummela Tom Reuter |
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Institution: | (1) Department of Biosciences, University of Helsinki, Viikinkaari 1, P.O. Box 65, 00014 Helsinki, Finland; |
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Abstract: | During the past 50 years, the high acoustic sensitivity and the echolocation behavior of dolphins and other small odontocetes
have been studied thoroughly. However, understanding has been scarce as to how the dolphin cochlea is stimulated by high frequency
echoes, and likewise regarding the ear mechanics affecting dolphin audiograms. The characteristic impedance of mammalian soft
tissues is similar to that of water, and thus no radical refractions of sound, nor reflections of sound, can be expected at
the water/soft tissue interfaces. Consequently, a sound-collecting terrestrial pinna and an outer ear canal serve little purpose
in underwater hearing. Additionally, compared to terrestrial mammals whose middle ear performs an impedance match from air
to the cochlea, the impedance match performed by the odontocete middle ear needs to be reversed to perform an opposite match
from water to the cochlea. In this paper, we discuss anatomical adaptations of dolphins: a lower jaw collecting sound, thus
replacing the terrestrial outer ear pinna, and a thin and large tympanic bone plate replacing the tympanic membrane of terrestrial
mammals. The paper describes the lower jaw anatomy and hypothetical middle ear mechanisms explaining both the high sensitivity
and the converted acoustic impedance match. |
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Keywords: | |
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