Distribution and development of the highly specialized lipids in the sound reception systems of dolphins

Fat bodies in the heads of toothed whales, which serve to transmit and receive sound, represent extraordinary examples of physiological specialization in adipose tissues among mammals, yet we know surprisingly little about their biochemical composition. We describe the spatial distributions and development of unusual endogenous lipids (branched-chain ["iso"] molecules and wax esters) in the mandibular fat bodies of bottlenose dolphins (Tursiops truncatus) using an ontogenetic series (fetus to adult; n = 10). Although concentrations of iso-acids, iso-alcohols and waxes were lower in younger dolphins than in adults, the same relative spatial arrangement was present in all age classes, implying a set "pattern" of acoustic lipid distribution that is established very early in life. In all age classes, a small region of blubber overlying the lateral region contained unusually high concentrations of iso-acids, exhibiting a tenfold increase over "normal" adjacent blubber. Being chemically more similar to the acoustic fat bodies, this region may serve as an entry point for sound into the head. Developmental accumulations of some iso-acids and iso-alcohols occurred more rapidly than others, implying that not only are the spatial distributions of branched-chain molecules under extremely fine-scale control, but the regulatory mechanisms controlling acoustic lipid synthesis are also highly complex.