Review

The external ear canal of cetaceans: vestigial or not?

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Abstract

Cetaceans descend from land mammals. Consequently, their hearing apparatus basically consists of the same anatomical components. Whales, as land mammals, feature an outer, middle and inner ear. However, these structures show strong evolutionary adaptations to underwater hearing. In addition, other morphological elements, such as the mandibles and the associated acoustic fat, have acquired an acoustical function in the propagation of sound waves to the middle ear. The original function of other structures such as the external auditory canal is therefore questioned. How cetaceans hear is not yet fully understood. However, it is essential to clarify this in order to assess the impact of anthropogenic underwater sound, since high-energy underwater sound may lead to physical trauma and hearing loss, physiological stress and behavioral changes. In this study, the external ear canal of a white-beaked dolphin (Lagenorhynchus albirostris) and a common minke whale (Balaenoptera acutorostrata) are described and compared based on histological cross sections. These external ear canals seemed rudimentary, with a small to absent lumen. However, the blood supply, well-developed muscles, active glands and numerous nerve fibres could indicate some functionality. Moreover, the presence of numerous structures with a morphological similarity to lamellar corpuscles also could advert to a functional structure. Although the function of these potential mechanoreceptors is not known, they could play a part in pressure perception in these diving mammals.