![]() ![]() The extensible grooves and the ridges are reinforced with stiff collagen fibres that provide the ultimate limit to tissue extensibility, resulting in a steeply non-linear stress–strain curve for this material, and a much lower extensibility in the longitudinal axis.īased on the ease with which the VGB could be stretched in tensile tests, Orton and Brodie ( Orton and Brodie, 1987) proposed that if a fin whale's forward speed heading into a lunge was high enough, the dynamic pressure of the water contacting the mouth cavity could be sufficient to completely extend the VGB and inflate the ventral cavity. This change is accomplished by stretching of the elastin-rich tissue in the grooves between the relatively inextensible ridges ( Orton and Brodie, 1987) (see Fig. ![]() When expanded, the VGB grooves unfold and can expand by as much as 300% in the circumferential direction (i.e. Elastin fibres in the blubber are large (1–2 mm diameter) and run at a 45 deg angle to the body long axis ( Orton and Brodie, 1987). The muscle strata have been described as highly extensible and elastic, due to a high proportion of elastin and collagen ( Sokolov, 1960 Orton and Brodie, 1987). The primary constituents of the VGB are blubber (an outer lipid-rich connective tissue layer) and two underlying muscle strata, one with fibre bundles oriented parallel to the body axis and another with oblique fibre bundles in multiple layers with alternating angles of ~45 deg to the LS and body axis ( Fig. It extends from the anterior tip of the mandibles near the chin to the umbilicus, a distance that typically spans 50–60% of the entire body length ( Goldbogen et al., 2010). The VGB is a thick elastic structure composed of several tissue layers, as described by previous investigators ( Pivorunas, 1977 Orton and Brodie, 1987). 1), bounded by the ventral groove blubber (VGB) superficially and the body wall medially. This amazing feat is achieved by the rapid expansion of a capacious ventral cavity ( Fig. ![]() physalus) whales, each lunge takes approximately 6 to 8 s, during which a volume of water as large as or exceeding the entire body volume is engulfed. In species such as blue ( Balaenoptera musculus) and fin ( B. This method of bulk filter feeding is one of the most impressive biomechanical events among vertebrates, especially given the extremely large body size of rorquals ( Goldbogen et al., 2007 Goldbogen, 2010). Lunge feeding by rorquals (baleen whales of the family Balaenopteridae) is a process that may also involve multiple demands on muscles. VGB transverse strains in routine-feeding rorquals were substantially less than those observed in dead ones, where decomposition gas stretched the VGB to its elastic limit, evidence supporting the idea that eccentric muscle contraction modulates the rate of expansion and ultimate size of the ventral cavity during engulfment. This allows the muscles to accommodate large tissue deformations of the VGB yet still operate within the known strain limits of vertebrate skeletal muscle. Our analyses of the functional properties and mechanical behaviour of VGB muscles revealed a crimped microstructure in an unstrained, non-feeding state that is arranged in parallel with dense and straight elastin fibres. The great expansion of the cavity wall presents a mechanical challenge for the physiological limits of skeletal muscle, yet its role is considered fundamental in controlling the flux of water into the mouth. In rorqual whales, feeding and locomotion are integrated in a dynamic process called lunge feeding, where an enormous volume of prey-laden water is engulfed into a capacious ventral oropharyngeal cavity that is bounded superficially by skeletal muscle and ventral groove blubber (VGB). Muscle serves a wide variety of mechanical functions during animal feeding and locomotion, but the performance of this tissue is limited by how far it can be extended. ![]()
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