A Proposed Echolocation Receptor for the Bottlenose Dolphin (Tursiops mincatus): Modelling the Receive Directivity from Tooth and Lower Jaw Geometry
Perception and production of underwater sound in Odontocetes is known to extend over a wide band of frequencies but the transmit and receive pathways employed for echolocation appear unconventional. The lower jaw has been demonstrated to be an important component in the echolocation receive mechanism, but the mode of signal coupling to the auditory sense organs is still unclear. The resolving power of sonar signals is severely restricted by physical acoustics and to explain the excellent performance of the dolphin sonar during target tracking and food capture manoeuvres presupposes the presence of an efficient matched receiver fully exploiting the highest frequency components within the pulse transmissions.
The lower jaw is examined as an acoustic construct using the tooth spacing and jaw geometry to compare various detection enhancement hypotheses. Frequency/ sensitivity receive ‘beam patterns’ have been computed to test for similarities with the transmitted signals. Of several hypotheses examined, one simple and apparently efficient model has been isolated which implicates the tooth/mandibular nerve structure as part of a high frequency echo pulse receptor which can accurately match the transmitted signal parameters. The model would also provide an explanation for the evolution of homodonty and polydonty, for the variability of tooth numbers between individuals in a given species, and for the alternate spacing of the teeth in opposite sides of the mandible.
This paper examines the component parts of this hypothetical echo receptor and collates the supporting evidence noted so far. The theory suggests that discriminatory sensing of a target echo range and bearing, particularly during the final hunting phase leading to food capture, exploits a dedicated high frequency receptor operating in parallel with the conventional wide band auditory sense organ.
KeywordsTrigeminal Ganglion Bottlenose Dolphin Beam Pattern Mandibular Nerve Sonar System
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