Topographical Distribution of Lipids Inside the Mandibular Fat Bodies of Odontocetes: Remarkable Complexity and Consistency

Abstract

Odontocetes possess unusual and specialized mandibular fat bodies in and around their lower jaws. These tissues have been proposed to facilitate sound reception and are composed of unusual endogenously synthesized lipids. Little is known about how the topographical arrangement of the lipid molecules in these tissues influences sound reception. We examined the lipid composition of the mandibular fat bodies, using a fine-scale approach, on six specimens (representing four odontocete families). We show that odontocete jaw lipids exhibit a complex structural three-dimensional topography. Different odontocetes synthesize and deposit slightly different molecules, but the relative arrangement of the lipids within each head showed marked consistency. Mandibular fats of beaked whales were uniquely dominated by isolauric acid ( -12:0). In contrast, the dolphin and porpoise biosynthesized isovaleric acid ( -5:0), while the pygmy sperm whale deposited medium-length (10-14 carbons) straight-chain lipids. In all heads examined, the shortest and branched-chain (" ") fatty acids were concentrated in the center of the jaw fats, which connect intimately with the ears. We hypothesize that in odontocete jaws, this arrangement may serve to channel an incoming sound to the ears because sound travels slower through shorter branched-chain fatty acids than through longer straight-chain fatty acids.Index Terms-Branched-chain fatty acid, delphinid, hearing, isolauric acid, isovaleric acid, kogiid, mandibular fat, odontocete, phocoenid, ziphiid.

Topographical variation in lipid content and morphological structure of the blubber in the striped dolphin

“…In odontocetes, the melon and the large fat bodies found in and around the mandibular region serve as part of the acoustic pathway and are known as the acoustical window for sound transmission (Norris et al 1961, Norris 1968, Norris and Harvey 1974. The biochemical composition of these "acoustic fats bodies" is different to that of body blubber (Varanasi and Malins 1970, Ackman et al 1971, Litchfield et al 1975, Koopman et al 2006. Acoustic fat bodies contain a high quantity of lipids with high concentrations of unusual endogenous lipids, and their biochemical composition does not seem to be influenced by diet.…”

“…Acoustic fat bodies contain a high quantity of lipids with high concentrations of unusual endogenous lipids, and their biochemical composition does not seem to be influenced by diet. These fat bodies do not change in lipid content or composition during fasting and starvation, are quite metabolically stable (Pond 1998, Cranford et al 1996, and do not exhibit biochemical stratification (Zahorodny Duggan et al 2009), although acoustic fat depots showed a biochemical composition gradient to channel sound toward ears (Koopman et al 2006). Some studies have addressed the morphology of melon and acoustic fat depots in the striped dolphin (Scano et al 2005, Maxia et al 2007), but performed no comparison with morphological structure from body blubber.…”

Shape analysis of odontocete mandibles: Functional and evolutionary implications

“…The mandibular fat body fills the mandibular foramen and extends somewhat medially beyond the bounds of the mandible. The lipid components of this structure have properties that may refract sound and assist in transmitting, or channeling, sound to the ear complex (Varanasi and Malins, 1972;Koopman et al, 2006). Mandibular fat bodies have a consistent arrangement of lipids based on specimens representing four main odontocete lineages (Koopman et al, 2006).…”

“…The lipid components of this structure have properties that may refract sound and assist in transmitting, or channeling, sound to the ear complex (Varanasi and Malins, 1972;Koopman et al, 2006). Mandibular fat bodies have a consistent arrangement of lipids based on specimens representing four main odontocete lineages (Koopman et al, 2006). It is possible that the shape of the foramen, housing the fat body, limits the geometry and, therefore, the acoustic function of the fat body.…”

Life history constrains biochemical development in the highly specialized odontocete echolocation system

“…Thus biochemical development of the entire acoustic reception apparatus proceeds according to a pre-determined blueprint. The existence of an acoustic lipid bauplan is supported by previous anecdotal observations of a young Sowerby's beaked whale (Mesoplodon bidens; Koopman et al 2006) and a sperm whale calf (Physeter macrocephalus; Morris 1975) exhibiting similar spatial distributions of unusual endogenous lipids in cranial acoustic tissues as adult conspecifics.…”