Molecular basis for formation of lipid sound lens in echolocating cetaceans

Abstract

MOST echolocating porpoises and small whales (odontocete) possess a fatty protrusion on the forehead and a peculiarly formed lower jaw which flares into a posterior flange. Overlying this flange is a region of highly fatty tissue referred to by Norris¹ as an acoustic window. The fatty tissues (>80% lipid) of the mandible and the melon are believed to be associated with sound transmission^1–3. It is proposed^4,5 that the melon functions as a sound lens, a concept consistent with the view that it is heterogeneous in terms of sound velocity and refraction⁶. Recent studies^7–13 revealed that the lipids of acoustic tissues are different from those of the surrounding tissues. For example, the acoustic tissues contain triacylglycerols and wax esters rich in short and medium chain (C₅–C₁₂) acids, whereas blubber tissues are composed primarily of triacylglycerols containing long chain (C₁₄–C₂₂) acids. In the families Delphinidae^7–9 and Monodontidae¹², isovaleric acid (3-methyl butyric acid) and longer iso (gem-dimethyl) acids are present in large amounts in acoustic tissues. Furthermore, certain areas, such as the acoustic window and the inner melon, contain almost exclusively l,3-diisovaleroyl-2-acylglycerols and isovaleroyl wax esters (Fig. 1). These compounds are replaced by medium and long chain triacylglycerols in adjacent areas. Thus, specific lipid compositions with various proportions of branched structures are found in the putative areas of sound transmission^14,15. The intriguing association between proposed acoustic pathways and unique lipid composition led us to study the acoustic properties of these lipids.