Evoked-potential audiogram of the Yangtze finless porpoise Neophocaena phocaenoides asiaeorientalis (L)

Abstract

Evoked-potential audiograms were obtained in two ͑one male and one female͒ Yangtze finless porpoises, Neophocaena phocaenoides asiaseorientalis. Sinusoidal amplitude-modulated 20-ms tone bursts were used as probes with recording envelope-following evoked potentials. A frequency range of 8 to 152 kHz was investigated. The range of greatest sensitivity covered frequencies from 45 to 139 kHz, and the lowest thresholds of 47.2 and 48.5 dB re: 1 Pa were found at a frequency of 54 kHz in the two subjects, respectively. At lower frequencies, threshold increased with a rate of around 14 dB/octave, and threshold steeply increased at 152 kHz.

Audiogram of a formerly stranded long-finned pilot whale (Globicephala melas) measured using auditory evoked potentials

“…The U-shaped audiogram with a region of best hearing up to 50kHz and threshold values in the 50dB range emphasize the quality of threshold measurements in a quiet environment. The high frequency hearing region was found to be relatively poor compared with the hearing capabilities of other toothed whales that can hear up to 150kHz (Johnson, 1967;Nachtigall et al, 2005;Popov et al, 2005). Although these results represent the first audiogram for this species, one should interpret these data carefully.…”

“…The EFR obtained from the animal followed the typical response obtained with other species Popov et al, 2005;Mooney et al, 2008) with a delay of 4-6ms which corresponded to the latency of the neurophysiological response. When the sound stimulus SPL was well above the threshold level, a completely formed response was recorded and as the SPL decreased, the response disappeared in the ambient biological noise.…”

Hearing pathways in the Yangtze finless porpoise,Neophocaena asiaeorientalis asiaeorientalis

“…The basic audiograms of these animals were measured in water several years prior to this research (Popov et al, 2005) and probe hearing tests related to this study reflected normal odontocete hearing capabilities (Mooney et al, 2011). All received sensitivity hearing examinations were obtained out of the water.…”

“…The second was the latency of the AEP responses. For thresholds, SPLs started at an estimated 20-30 dB above previous audiogram thresholds of the finless porpoise (Popov et al, 2005). With a detectable stimulus, an envelope following response (EFR) was visible as part of the custom-written AEP program.…”

“…Noise impacts on this subspecies are of concern because these animals inhabit waters with substantial acoustic sources including shipping, dredging and underwater construction. While hearing ranges have been established for this subspecies (Popov et al, 2005), there are no data regarding how they receive sound. The goal of this work was to examine the relative sensitivity of finless porpoise hearing across its head and lower jaw.…”