Hearing pathways in the finless porpoise, Neophocaena phocaenoides, and implications for noise impacts.

Abstract

How an animal receives sound will influence how it uses or is impacted by sound. While the “jaw hearing” hypothesis is well supported, work has been limited to a few “representative” species. There are clear variations in the jaw and head morphologies of odontocete species suggesting subtle variation in sound reception. Here we address how a divergent cetacean 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 using auditory evoked potentials. Click and amplitude modulated tone stimuli were presented at nine locations on the head and body using a jawphone transducer. Thresholds were compared to anatomical dissections and CT scans of porpoise heads. Minimum thresholds and best hearing locations were at the cheek‐fat‐pad and distal to the porpoise bulla. However, thresholds were not substantially different at locations from the rostrum tip to the ear (11.6 dB). This minimal variation is quite different from the 30–40 dB differences found across the head of bottlenose dolphins and belugas suggesting differences in how divergent odontocetes receive sound. Porpoises may have relatively less “shading” of sounds and are potentially more susceptible to masking effects.