Acoustic fatheads: Parallel evolution of underwater sound reception mechanisms in dolphins, turtles, and sea birds

Abstract

Dolphins lack conventional external ear canals, seal ear canals vary widely in size and patency, and sound conduction mechanisms in aquatic birds and turtles are virtually unknown. In this study, computerized tomography (CT) and magnetic resonance imaging (MRI) were used to map densities of tissues associated with the outer, middle, and inner ears of three dolphin, two seal, two turtle, and two sea-bird species. Three-dimensional reconstructions of scan data were used to determine species-specific geometry of tissue groups connected to the middle ear or surrounding the ear canal. The analyses show bundles of coherent fatty tissues emerging from the middle ear in all species examined. Densities of these fats are similar across species and are consistent with sound speeds near that of sea water. In seals and birds, these fats sheathed the external auditory canal. In turtles, the fats formed a discrete column communicating with the surface of the head. In dolphins, the fats formed three distinct bundles: two directed anteriorly along the lower jaw with a third projecting laterally. These findings suggest that all four groups evolved parallel soft tissue mechanisms that act as low impedance channels for underwater sound. [Work supported by Mellon Foundation, Seaver Institute, and Office of Naval Research.]