Fatty sound reception in minke whales: the lipid composition and potential function of fats associated with mysticete ears

Abstract

Cetaceans possess highly derived auditory systems because a conventional pinna and air-filled ear canal are ineffective at collecting and guiding sound towards the middle ears in an aquatic environment. Odontocetes, or toothed whales, receive sound through specialized “acoustic fats” associated with the mandibles and ears. Although sound reception mechanisms of mysticetes (baleen whales) are unknown, we found that some mysticete species also have large, discrete fat bodies associated with their auditory bullae. In this study, we investigated the biochemical composition of this fat in the minke whale (Balaenoptera acutorostrata) and used Finite Element Modeling to determine sound propagation through the whale head. Our results indicate that the mysticete fats are composed of lipids common in mammalian tissues, lacking the short, branched chain fatty acids and wax esters characteristic of odontocete acoustic fats. However, the mysticete and odontocete fats share some characteristics, including having fewer dietary signatures than blubber and being conserved under starvation. FE models were created using morphological data from CT scans of whole minke whale heads and material property data from measurements on minke whale tissues as well as published data for typical mammalian fatty tissues. The model indicates that the presence of the fat body causes a slight increase in peribullar sound pressures, which is attributed to the low sound speeds and densities of the fat body. Fatty tissues are known to have lower densities and sound speeds than other types of soft tissues, which may explain why they are found in the auditory systems of cetaceans.