Hearing threshold measurements of five stranded short-finned pilot whales (Globicephala macrorhynchus).

On May 5, 2011, 23 short-finned pilot whales, Globicephala macrorhynchus, were stranded along the coastline near Cudjoe Key, FL. Five animals (two adult females, two juvenile females, and an adult male) were transported to a rehabilitation facility in Key Largo, FL. Auditory evoked potentials (AEPs) were recorded in response to amplitude modulated tone pips modulated at 1000 Hz. AEP thresholds were determined at 10, 20, 40, 80, and 120 kHz for the four females. However, the adult male was euthanized prior to testing. Short-finned pilot whales had peak sensitivity at lower frequencies than other odontocetes such as bottlenose dolphins. Greatest sensitivity was around 40 kHz for all whales, while thresholds for the two adult females were 25-61 dB higher at 80 kHz than the juveniles. Click evoked potentials were similar between the four whales and comparable to other echolocating odontocetes. Click evoked potential data from a fifth short-finned pilot whale that had stranded in Curacao showed no response. These findings add to the limited database of pilot whale (short- and long-finned) hearing studies, of which there are only two others [Schlundt et al. (2011). J. Acoust. Soc. Am. 129, 1111-1116 and Pacini et al. (2010). J. Exp. Biol. 213, 3138-3143].

Hearing loss in stranded odontocete dolphins and whales.

The causes of dolphin and whale stranding can often be difficult to determine. Because toothed whales rely on echolocation for orientation and feeding, hearing deficits could lead to stranding. We report on the results of auditory evoked potential measurements from eight species of odontocete cetaceans that were found stranded or severely entangled in fishing gear during the period 2004 through 2009. Approximately 57% of the bottlenose dolphins and 36% of the rough-toothed dolphins had significant hearing deficits with a reduction in sensitivity equivalent to severe (70-90 dB) or profound (>90 dB) hearing loss in humans. The only stranded short-finned pilot whale examined had profound hearing loss. No impairments were detected in seven Risso's dolphins from three different stranding events, two pygmy killer whales, one Atlantic spotted dolphin, one spinner dolphin, or a juvenile Gervais' beaked whale. Hearing impairment could play a significant role in some cetacean stranding events, and the hearing of all cetaceans in rehabilitation should be tested.

Auditory evoked potentials in a stranded Gervais' beaked whale (Mesoplodon europaeus).

Efforts to identify the specific causal mechanisms responsible for beaked whale strandings coincident with naval exercises have been hampered by lack of data concerning the hearing abilities of beaked whales and their physiological and behavioral responses to sound. In this study, auditory capabilities of a stranded Gervais' beaked whale (Mesoplodon europaeus) were investigated by measuring auditory evoked potentials. Click-evoked potentials, auditory thresholds as a function of frequency, and the modulation rate transfer function were determined. The evoked potentials and modulation rate transfer function were similar to those measured in other echolocating odontocetes; the upper limit of functional hearing was 80-90 kHz.