Morphometric analysis of ears in two families of pinnipeds

Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 2001

Pinniped (seal and sea lion) auditory systems operate in two acousticallydistinct environments, air and water. Piniped species differ in how much timethey typically spend in water. They therefore offer an exceptional opportunity toinvestigate aquatic versus terrestrial hearing mechanisms. The Otariidae (sealions and fur seals) generally divide their time evenly between land and waterand have several adaptations; e.g. external pinnae, related to this lifestyle.Phocidae (true seals) spend the majority of their time in water; they lack externalpinnae and have well developed ear canal valves.Differences in hearing ranges and sensitivities have been reported recently formembers of both of these familes (Kastak, D., Schusterman, RJ., 1998. Lowfrequency amphibious hearing in pinnipeds. J. Acoust. Soc. Am. 1303,2216-2228.; Moore, P.W.B., Schusterman, RJ., 1987. Audiometric assessment ofnorthern fur seals, Callorhinus ursinus. Mar. Mamm. Sci. 3,31-53.). In this project,the ear anatomy of three species of pinnipeds: an otariid, the California sea lion(Zalophus californianus), and two phocids, the northern elephant seal (Miroungaangustirostris) and the harbor seal (Phoca vitulina), was examined usingcomputerized tomography (CT scans) and gross dissection. Thee-dimensionalreconstructions of the heads and ears from CT data were used to determineinteraural dimensions and ossicular chain morphometrics. Ossicular weightsand densities were measured conventionally. Results strongly support a canalcentricsystem for pinniped sound reception and localization. Further, true sealsshow adaptations for aquatic high frequency specialization.