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Live CT imaging of sound reception anatomy and hearing measurements in the pygmy killer whale, Feresa attenuata
Eric W. Montie, Charlie A. Manire, David A. Mann


In June 2008, two pygmy killer whales (Feresa attenuata) were stranded alive near Boca Grande, FL, USA, and were taken into rehabilitation. We used this opportunity to learn about the peripheral anatomy of the auditory system and hearing sensitivity of these rare toothed whales. Three-dimensional (3-D) reconstructions of head structures from X-ray computed tomography (CT) images revealed mandibles that were hollow, lacked a bony lamina medial to the pan bone and contained mandibular fat bodies that extended caudally and abutted the tympanoperiotic complex. Using auditory evoked potential (AEP) procedures, the modulation rate transfer function was determined. Maximum evoked potential responses occurred at modulation frequencies of 500 and 1000 Hz. The AEP-derived audiograms were U-shaped. The lowest hearing thresholds occurred between 20 and 60 kHz, with the best hearing sensitivity at 40 kHz. The auditory brainstem response (ABR) was composed of seven waves and resembled the ABR of the bottlenose and common dolphins. By changing electrode locations, creating 3-D reconstructions of the brain from CT images and measuring the amplitude of the ABR waves, we provided evidence that the neuroanatomical sources of ABR waves I, IV and VI were the auditory nerve, inferior colliculus and the medial geniculate body, respectively. The combination of AEP testing and CT imaging provided a new synthesis of methods for studying the auditory system of cetaceans.


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  • Data analysis and writing of this manuscript was supported under a subaward with the University Corporation of Atmospheric Research (UCAR) under grant no. NA06OAR4310119 (Training Tomorrow's Ecosystem and Public Health Leaders Using Marine Mammals as Sentinels of Oceanic Change) with the National Oceanic and Atmospheric Administration, US Department of Commerce. We would also like to acknowledge the partners of this training grant: University of California Davis Wildlife Health Center, The Marine Mammal Center and Northwest Fisheries Science Center. CT imaging was performed under a Letter of Authorization from NMFS, authorizing rehabilitation of marine mammals. Hearing tests were performed in accordance with National Marine Fisheries Service permit no. 1053-1825-00. These studies were performed with the approval of the University of South Florida Institutional Animal Care Use Committee (protocol no. 3638).


    auditory brainstem response
    auditory evoked potential
    computed tomography
    envelope following response
    fast Fourier transform
    Hounsfield units
    magnetic resonance imaging
    modulation rate transfer function
    root mean square
    sinusoidal amplitude modulated
    sound pressure level
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