Beached dolphins are often deaf

Washington Post : David Mann of the University of South Florida and his collaborators have adapted a technique used to test the hearing of infants and applied it to beached or stranded dolphins. By measuring the electrical impulses evoked by an auditory stimulus, Mann and his team discovered that some beached or stranded dolphins are deaf. As the researchers report in a paper in PLoS One, the deafness appears to depend on species:

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 (7090 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.

Mann notes there are five possible causes of hearing loss in dolphins: intense chronic noise, transient intense noise, age-related hearing loss, congenital hearing impairment, and the side effects of certain antibiotics that are given to sick, stranded dolphins. Because dolphins rely on hearing to find food, Mann argues “that hearing screening should be part of the standard veterinary examination of stranded cetaceans.” A deaf dolphin, even if it recovers from being stranded, is unlikely to fare well when returned to the sea.