Role of the mental foramens in dolphin hearing

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.