Abstract
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.
Highlights
Odontocete cetaceans use high frequency echolocation sounds for navigation and foraging and are wellknown for possessing high frequency hearing abilities [1,2]
Auditory evoked potential (AEP) methods, which are commonly used to measure hearing in human infants [7], have been used to measure hearing in captive and wild cetaceans [8,9,10]
This finding is in stark contrast to results of population-level hearing evaluations of bottlenose dolphin in Sarasota Bay, Florida, during capture-release health assessments [16] that have shown a general absence of hearing loss across more than 60 dolphins of a variety of ages and both sexes [17]
Summary
Odontocete cetaceans (the toothed whales) use high frequency echolocation sounds for navigation and foraging and are wellknown for possessing high frequency hearing abilities [1,2]. We show profound high-frequency hearing loss in three species of cetaceans that had stranded, which suggests that hearing impairment could be a causative factor for some stranding events. Hearing measurements in captive cetaceans are often performed with behavioral conditioning, where the dolphin is trained to press a response paddle or vocalize when it hears a test signal [1,4,5,6]. Behavioral hearing tests cannot be performed on stranded cetaceans. AEPs provide a rapid means to assess hearing and yield comparable results to behavioral measures [11,12,13,14]. A further advantage is that AEPs provide the means to screen free-ranging populations of cetaceans if individuals are able to be handled briefly
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