Electrophysiological and psychophysical assessment of aerial hearing in pinnipeds

The hearing capabilities of the amphibious pinnipeds have fascinated scientists for some time, but have only recently garnered considerable public attention in light of increases in anthropogenic noise levels in marine ecosystems. Data regarding the hearing sensitivity of pinniped species are mostly based on studies with captive animals, and are unfortunately generally limited to sample sizes of one or two individuals representing a small number of species. It is therefore difficult draw conclusions concerning the extent of variability in hearing both among various pinniped species, and among individuals in free-ranging populations. This dissertation comprises a series of studies aimed at simultaneously developing efficient auditory evoked potential (AEP) measurements of pinniped hearing and increasing the availability of hearing data for pinnipeds. Chapters 1 and 2 provide data that establish a foundation for AEP audiometric methods. Results indicate that although pinnipeds do not possess the extent of auditory sophistication that provides optimal conditions for AEP measurements in dolphins and porpoises, the basic characteristics of pinniped auditory physiology make frequency-specific audiometry possible. Chapters 3 and 4 extend the foundations developed in previous chapters to the measurement of aerial hearing sensitivity in an increased number subjects. Results suggest that AEP measures provide a suitable estimate of psychophysically-determined hearing sensitivity in sea lions. Further measurements conducted with an increased sample size of sea lions from both captive and wild populations demonstrate that acute aerial hearing appears to be conserved in nearly all individuals. These studies represent a step towards understanding the hearing of pinnipeds on a level approaching that of a population. The data have the potential to address questions concerning the basic biology of pinniped hearing, and inform criteria designed to mitigate the effects of anthropogenic noise on free-ranging individuals.