Hearing pathways of the finless porpoise: Form and function in an 'unrepresentative' species

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TitleHearing pathways of the finless porpoise: Form and function in an 'unrepresentative' species
Publication TypeConference Paper
Year of Publication2011
AuthorsMooney, A. T., S. Li, D. R. Ketten, K. Wang, and D. Wang
Conference Name19th Biennial Conference on the Biology of Marine Mammals
Pagination209-210
Date Published11/2011
PublisherSociety for Marine Mammology
Conference LocationTampa, FL
Keywordsfinless porpoise, hearing pathways, marine mammal hearing, neophocaena phocaenoides, sound reception, sound transmission
AbstractThere are clear variations in the jaw and head morphologies of odontocetes suggesting subtle variation in sound reception.
Full TextThere are clear variations in the jaw and head morphologies of odontocetes suggesting subtle variation in sound reception. While prior studies have shown mandibular regions are important to odontocete hearing, sound transmission pathway studies have been confined to a few 'representative' species. How an animal receives sound may influence how it uses or is impacted by sound. Here we address how a divergent species, the Yangtze finless porpoise (Neophocaena phocaenoides asiaeorientalis), receives sound. Noise impacts on this subspecies are a concern as they inhabit waters with many acoustic sources. Hearing was measured in two animals using auditory evoked potentials. Broadband clicks and low-, mid-, and high-frequency (8, 54, 120 kHz) tone stimuli were presented at nine locations on the head and body using a jawphone transducer. Thresholds were compared to anatomical dissections, and computed tomography and magnetic resonance imaging of two finless porpoise. 'Acoustic fat' regions were confined to relatively small areas in the finless porpoise. Minimum thresholds and best hearing locations were from a cheek fat pad and distal to the porpoise bulla. However, mean thresholds were not substantially different at locations from the rostrum tip to the ear (11.6 dB). This is quite different from the bottlenose dolphin and beluga, in which 30-40 dB threshold differences were found across their heads. AEP response latencies were shortest from the cheek pad indicating a preferential sound pathway. Latencies were dependent on stimulus level suggesting hearing pathways which reduce transmission loss can result in both higher amplitude and faster auditory responses. The unique combination of anatomical and physiological data reinforces the importance of sound-conductive pathways. Finless porpoises have relatively less 'shading' of sounds and are potentially more susceptible to masking effects. The results show there are differences in how divergent odontocetes receive sound, supporting caution when applying anditory data across species.