The slippery slope of a Johnsonian ear: Natural variability versus natural loss

Abstract

The normal audiogram of the bottlenosed dolphin, first established by Johnson [C. S. Johnson, in Marine Bio-Acoustics, edited by W. N. Tavolga (1967), pp. 247–260], has been pivotal to nearly three decades of underwater echolocation research, most of which focused on the mechanisms and analytical limits of dolphin sonar. One important aspect of dolphin hearing that has escaped investigation is what are normal audiometric ranges, or, more important, how to determine whether differences seen in audiometric responses of a new animal represent normal variants or a pathologically impaired ear. In this study, radiologic data and audiograms from animals tested by Brill et al. (this session) were compared with results from postmortem histologic analyses of ears from dolphins with known hearing losses. Structural alterations in the ear x rays of the older, male dolphins were consistent with obstructive disease processes and demineralization of temporal bones from dolphins with conductive and progressive sensorineural loss. These findings imply that specific features of the older male’s audiogram, such as the relatively flat, elevated, midfrequency thresholds and the rapid decrement of high-frequency responses, are diagnostic correlates for chronic infection and age-related retrograde neural loss. [Work supported by ONR N000149310940.]