Evolution and function of specialized adipose tissue in whales
Heather N. Koopman
Biology & Marine Biology, University of North Carolina Wilmington, 601 S. College Road, Wilmington, NC 28403, USA
The cetaceans (whales) represent a very interesting group in which to consider the evolution and function of adipose tissue, as they possess two types of specialized fat. Like pinnipeds, all whales have a layer of superficial blubber, which serves to insulate, streamline, and provide buoyancy and energy reserves. However, some species of toothed whales (beaked and sperm whales) deposit large amounts of wax esters (fatty acid + alcohol), rather than typical triacylglycerols (three fatty acids), in blubber, which is unusual. Waxes have very different physical and physiological properties and function as superior insulators, with higher energy density. The acoustic fat depots of toothed whales, which are located in the forehead (melon) and around the mandibles, serve to focus sound during echolocation and hearing. These acoustic fats have unique morphologies, but they are even more specialized biochemically as they are entirely endogenous, composed of a mix of waxes and triacylglycerols with unusual branched elements (derived from amino acids), not seen in other mammals. Both waxes and branched elements alter how sound travels through a fat body, and these are arranged in a 3D topographical pattern to focus sound. Further, the specific branched chain acid/alcohol synthesis mechanisms and products vary phylogenetically (e.g. dolphins make lipids from leucine; beaked whales use valine). Despite these disparate processes for branched-chain synthesis, the overall structure of waxes in the acoustic fats is more conserved across species than the triacylglycerols. As blubber is common across all whales, and waxes are present in almost all acoustic fats, I propose that these specialized lipids evolved first in the head: wax synthesis first emerged to serve an acoustic function in toothed whales, with branched chain synthesis adding additional acoustic focusing power, and some species secondarily retained wax synthesis pathways for blubber.