Abstract
The role of mental foramens in dolphin hearing was studied in the present work. To this effect the mental foramens’ morphology features which are essential from acoustical viewpoint have been studied. The patterns of relationship between the location of mental foramens and their sizes are found. The affinity of the mental foramens’ morphology and acoustics that the nature had created testify acoustical expediency of the mental foramens’ architecture. This natural inference in the main is confirmed in this work by the experimental data. The mean values of detection thresholds of short broadband stimuli with spectral maximum on frequencies 8, 16, 30 and 100 kHz at acoustical shielding the mental foramens increased on 30, 34, 40 and 50 dB, respectively. Results obtained testify that the mental foramens are the unique sound-conducting pathway into the fat body of the mandibular canal for the sounds of all frequencies used in the experiment, approximately 6?150 kHz (in view of stimuli broadbandness). The left and right row of the mental foramens together with respective mandibular canal plays the role of pinna and external auditory meatus if to use conventional terminology of a land mammal ear. But it is already qualitatively the new external ear implemented by the nature as the receive array and acoustical horn. The new external ear has apparently appeared in result of the dolphins’ ancestors’ adaptation to new environment conditions, as evolutionary adaptation of the ear to the water and as functional adaptation of the ear in order that to fulfill the new more sophisticated functions in the structure of sonar. The findings give good reason to suppose existence of the same external ear in Odontoceti.
Highlights
The mechanisms of Odontoceti hearing attract researchers for many years, a lot of works have been performed in this study area and different hypotheses about sound-conduction mechanisms to a cochlea were suggested in the works
The cross section size of the neurovascular bundle is much smaller than the sounds wavelengths propagating along the mental foramens and the mandibular canal
The sound conduction via mental foramens and mandibular canal has been analyzed on the basis of lower jaw's canals geometry [21, 22,23,25]
Summary
The mechanisms of Odontoceti hearing attract researchers for many years, a lot of works have been performed in this study area and different hypotheses about sound-conduction mechanisms to a cochlea were suggested in the works. Some researchers assume that the sound passes to the cochlea through the external auditory meatus and the middle ear [1,2,3]. Norris [10] supposed that the sound can be conducted into the fat body of mandibular canal through the mental foramens. Though, little later he suggests different hypothesis [11] which is being generally accepted so far, to the effect that the sound passes into the mandibular fat body directly through the postero-lateral wall of mandible in the area which he called the “acoustic window”
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