How an animal receives sound may influence its use of sound. While ‘jaw hearing’ is well supported for odontocetes, work examining how sound is received across the head has been limited to a few representative species. The substantial variation in jaw and head morphology among odontocetes suggests variation in sound reception. Here, we address how a divergent subspecies, the Yangtze finless porpoise (Neophocaena asiaeorientalis asiaeorientalis) hears low-, mid- and high-frequency tones, as well as broadband clicks, comparing sounds presented at different locations across the head. Hearing was measured using auditory evoked potentials (AEPs). Click and tone stimuli (8, 54 and 120 kHz) were presented at nine locations on the head and body using a suction-cup transducer. Threshold differences were compared between frequencies and locations, and referenced to the underlying anatomy using computed tomography (CT) imaging of deceased animals of the same subspecies. The best hearing locations with minimum thresholds were found adjacent to a mandibular fat pad and overlaying the auditory bulla. Mean thresholds were not substantially different at locations from the rostrum tip to the ear (11.6 dB). This contrasts with tests with bottlenose dolphins and beluga whales, in which 30–40 dB threshold differences were found across the animals' heads. Response latencies increased with decreasing response amplitudes, which suggests that latency and sensitivity are interrelated when considering sound reception across the odontocete head. The results suggest that there are differences among odontocetes in the anatomy related to receiving sound, and porpoises may have relatively less acoustic ‘shadowing’.
↵‡ Present address: Marine Mammal and Marine Bioacoustics Laboratory, Sanya Institute of Deep-sea Science and Engineering of the Chinese Academy of Sciences, 62 Fenghuang Road, Sanya 572000, China.
T.A.M., S.L. and D.R.K. participated in the conception, design and execution of the experiment(s), interpretation of the findings being published, and drafting and revising the article. K.W. and D.W. assisted with execution of the experiment(s), interpretation of the findings being published, and drafting and revising the article.
The authors declare no competing financial interests.
The work was funded by the Office of Naval Research, a Mellon Joint Initiatives Award, the Knowledge Innovation Program of Chinese Academy of Sciences [grant no. KSCX2-EW-Z-4] and the National Natural Science Foundation of China [grant no. 31170501], and we thank them for their support. Deposited in PMC for immediate release.
- © 2014. Published by The Company of Biologists Ltd
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