Auditory sensitivity to local stimulation of the head surface in a beluga whale (Delphinapterus leucas)

Abstract

Using the auditory evoked response technique, sensitivity to local acoustic stimulation of the ventro-lateral head surface was investigated in a beluga whale (Delphinapterus leucas). The stimuli were tone pip trains of carrier frequencies ranging from 16 to 128 kHz with a pip rate of 1 kHz. For higher frequencies (90-128 kHz), the low-threshold point was located next to the medial side of the middle portion of the lower jaw. For middle (32-64 kHz) and lower (16-22.5 kHz) frequencies, the low-threshold point was located at the lateral side of the middle portion of the lower jaw. For lower frequencies, there was an additional low-threshold point next to the bulla-meatus complex. Based on these data, several frequency-specific paths of sound conduction to the auditory bulla are suggested: (i) through an area on the lateral surface of the lower jaw and further through the intra-jaw fat-body channel (for a wide frequency range); (ii) through an area on the ventro-lateral head surface and further through the medial opening of the lower jaw and intra-jaw fat-body channel (for a high-frequency range); and (iii) through an area on the lateral (near meatus) head surface and further through the lateral fat-body channel (for a low-frequency range).

Investigation on acoustic reception pathways in finless porpoise (Neophocaena asiaorientalis sunameri) with insight into an alternative pathway

“…Generally, the middle parts of lower jaw (mandible) had the best hearing sensitivities. The rostrum tip just showed a relatively high sensitivity for middle-frequency (32 kHz-64 kHz) acoustic stimuli (Popov et al 2016). The simulations here introduced an alternative reception pathway for the target animal, but whether this pathway is responsible for the high hearing sensitivity at the rostrum tips of Beluga whale, Risso's dolphin and Yangtze finless porpoise still needs future studies to address.…”

“…While the authors ascribe these lower hearing thresholds to hearing responses generated by both ears, an additional hypothesis is that the good hearing sensitivity at rostrum tip might be related to the bone conduction in current paper. In a recent study, Popov et al (2016) concluded the sound conduction to the auditory system is frequency dependent. The areas of best sensitivity shifted when the frequency of the acoustic stimuli changed.…”

Sound Reception in the Yangtze Finless Porpoise and Its Extension to a Biomimetic Receptor

“…Cranford et al developed a numerical model of a Cuvier’s beaked whale ( Ziphius cavirostris ) and found an additional sound entrance through the ventral margin of the mandible [ 15 ]. The solid mandible was thought to be an important guide to conduct sounds to the bony complexes as well, as demonstrated in auditory examinations on beluga whales ( Delphinapterus leucas ), indicating a high sensitivity to sound stimulation from the rostrum tip [ 22 , 23 ]. This location might also be the entrance of the effective acoustic channels for the Risso’s dolphin ( Grampus griseus ) and the Yangtze finless porpoise ( Neophocaena phocaenoides asiaeorientalis ) [ 24 , 25 ].…”

“…These aggregations showed the complexity of acoustic windows in odontocetes. Though the most sensitive areas (the positions of acoustic windows) may have been demonstrated in many species [ 22 , 23 , 24 , 25 , 27 ], there might be other sound-reception paths that have not been addressed in detail as well as the roles of the various structures in these paths. The head as a whole may be considered a volume antenna [ 15 , 27 ], and thus, the roles of the various structures, including the mandible, the mandibular fats, soft tissue, and skull, in conducting sounds to the ear complex require additional work to elucidate.…”