Morphometric analyses of hearing in two families of pinnipeds

Pinniped (seal and sea lion) auditory systems operate in two acoustically distinct environments, air and water. Otariids (sea lions and fur seals) generally divide their time evenly between land and water while phocids (true seals) spend the majority of their time in water. These pinniped families therefore offer an exceptional opportunity to investigate aquatic versus terrestrial hearing mechanisms. Recent reports indicate differences in hearing ranges and sensitivities among species from these families [D. Kastak and R. Schusterman, J. Acoust. Soc. Am. 1303, 2216–2228 (1998); P. W. B. Moore and R. Schusterman, Marine Mammal Sci. 3, 31–53 (1987)]. In this project, ear anatomy of three representative pinniped species (Otariidae: California sea lion, Zalophus californianus; Phocidae: northern elephant seal, Mirounga angustirostris; and harbor seal, Phoca vitulina) was examined using computerized tomography (CT scans) and gross dissection. Three-dimensional reconstructions of heads and ears from CT data were used to determine inter-aural dimensions and ossicular chain articulations. Ossicular weights and densities were measured using conventional and displacement techniques. Results strongly support a canal-centric system for pinniped sound reception and localization. Further, true seals show adaptations for aquatic high-frequency specialization. [Work supported by ONR.]