Modeling whale audiograms: effects of bone mass on high-frequency hearing

In a previous paper (Hemila et al., Hear. Res. 133 (1999) 82-97) we have pr esented a mechanical model, based on species-specific anatomical data, for the toothed whale middle ear. For five odontocete species of six we found t hat the model quite well predicted published behavioral audiograms. Here we report that new published data indicate that the audiogram of the sixth an d deviating species, the killer whale Orcinus orca, was from a specimen wit h deficient high-frequency hearing. A new published killer whale audiogram is similar to other odontocete audiograms and does fit our four-bone model. With certain general conditions, a model with isometric (middle) ears resu lts in uniform audiograms for different species, when presented in a log-lo g plot; with larger ears the audiogram curves are just moved towards lower frequencies. The audiograms coincide in case all frequencies are scaled by a Factor 1/3 rootm, where m is the mass of the ear ossicles. Odontocete ear s are isometric enough to show that the corresponding audiograms are indeed similar after such mass scaling. Specifically, this scaling factor can be used to predict the high-frequency hearing limits of all odontocete species . Our anatomical data and models support the notion that ossicular mass is a crucial factor limiting high-frequency hearing in both terrestrial mammal s and toothed whales. (C) 2001 Elsevier Science B.V. All rights reserved.