Abstract
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'.
Highlights
In terrestrial mammals, structures such as the pinnae and ear canal serve to conduct sound to the middle and inner ear (Angell and Fite, 1901; Roffler and Butler, 1968)
Relative thresholds This work examined the hearing of two finless porpoises
Thresholds acquired from lower melon stimulation were substantially higher (>20 dB) than the areas of best sensitivity
Summary
Structures such as the pinnae and ear canal serve to conduct sound to the middle and inner ear (Angell and Fite, 1901; Roffler and Butler, 1968). Received 11 July 2013; Accepted 1 October 2013 the spectra, amplitude and arrival time of sounds (Müller, 2004). Variations in these cues aid animals in sound source localization and important biological activities, including locating predators and prey, navigating and communicating with conspecifics. Odontocetes (toothed whales, dolphins and porpoises) lack external pinnae. Most evidence indicates they receive sound using specialized adaptations around the lower jaw. Odontocete jaw hearing is supported by psychophysical echolocation research and detailed, non-invasive physiological studies of dolphin received sensitivity (Brill et al, 1989; Møhl et al, 1999). The mandibular fat hearing hypothesis is supported by physiological data, for the concept of an additional pathway near the bulla, which preferentially receives lower frequency sounds (Popov et al, 2008)
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