Hearing pathways in the Yangtze finless porpoise,Neophocaena asiaeorientalis asiaeorientalis

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 suctioncup 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'.

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

“…Mooney et al (2008) stated that there might be acoustic channels beginning at the tip of the rostrum for a beluga whale (Delphinapterus leucas) when they measured a good hearing sensitivity at this region. Relative high hearing sensitivities on lower jaw tip were also reported in Risso's dolphins (Grampus griseus) and a Yangtze River finless porpoise (Neophocaena phocaenoides asiaeorientalis) (Mooney et al 2015, Mooney et al 2014.…”

“…They produce high frequency echolocation clicks with narrow bandwidths to detect targets (Li et al 2007). Their audiogram shape is similar to that of many odontocetes species (Popov et al 2005, Mooney et al 2008, 2014. The best hearing sensitivity of a Yangtze finless porpoise (N. a. asiaeorientalis) was found adjacent to the mandibular fat pad (Mooney et al 2014).…”

“…Mooney et al (2008) stated that there might be acoustic channels beginning at the tip of the rostrum for a beluga whale (Delphinapterus leucas) when they measured a good hearing sensitivity at this region. Relative high hearing sensitivities on lower jaw tip were also reported in Risso's dolphins (Grampus griseus) and a Yangtze River finless porpoise (Neophocaena phocaenoides asiaeorientalis) (Mooney et al 2014(Mooney et al , 2015. Even the teeth overlying the lower jaw are considered as a periodic structures system to play roles in sound reception of the odontocetes and as a passive resonator system (Goodson and Klinowska 1990, Dobbins 2007, Graf et al 2008, Dible et al 2009.…”

“…But this was challenged in physiological, anatomical and behavioral studies (McCormick et al 1970, Brill et al 1988b, 1991, Møhl et al 1999, which showed the external auditory meatus might be vestigial and lower jaw region was important for sound reception. New ideas in sound reception theory and new information for sounds reception pathways have arisen in recent years (Mooney et al 2008, 2014, Cranford et al 2008b. Cranford et al (2008b) refined the notion of jaw hearing theory and proposed a 'gular pathway', which describes sounds enter the internal mandibular fat channel through the ventral margin of the mandible in a Cuvier's beaked whale (Ziphius cavirostris).…”

“…Most studies related to odontocetes hearing are presented on delphinids, and only a few focus on finless porpoises (Popov et al 2005, Mooney et al 2014. Finless porpoises are small odontocetes, distributed in both fresh and marine habitats (Pilleri and Gihr 1975).…”