Abstract:
Pinniped (seal and sea lion) auditory systems operate in two acoustically
distinct environments, air and water. Piniped species differ in how much time
they typically spend in water. They therefore offer an exceptional opportunity to
investigate aquatic versus terrestrial hearing mechanisms. The Otariidae (sea
lions and fur seals) generally divide their time evenly between land and water
and have several adaptations; e.g. external pinnae, related to this lifestyle.
Phocidae (true seals) spend the majority of their time in water; they lack external
pinnae and have well developed ear canal valves.
Differences in hearing ranges and sensitivities have been reported recently for
members of both of these familes (Kastak, D., Schusterman, RJ., 1998. Low
frequency amphibious hearing in pinnipeds. J. Acoust. Soc. Am. 1303,2216-
2228.; Moore, P.W.B., Schusterman, RJ., 1987. Audiometric assessment of
northern 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 (Mirounga
angustirostris) and the harbor seal (Phoca vitulina), was examined using
computerized tomography (CT scans) and gross dissection. Thee-dimensional
reconstructions of the heads and ears from CT data were used to determine
interaural dimensions and ossicular chain morphometrics. Ossicular weights
and densities were measured conventionally. Results strongly support a canalcentric
system for pinniped sound reception and localization. Further, true seals
show adaptations for aquatic high frequency specialization.
