Fiber-type profile and metabolic comparison of the bottlenose dolphin (Tursiops truncatus) and harbor porpoise (Phocoena phocoena) scalenus muscles
Erin Guy and Jennifer L. Dearolf, Biology Department, Hendrix College, Conway, AR
Cetaceans (whales, dolphins, porpoises) must surface to breathe, thereby breaking the interface between air and water. Consequently, these marine mammals exhibit explosive exhalation and inhalation in order to minimize their time spent at the surface. It is thereby necessary that the muscles contributing to ventilation in marine cetaceans be fast enough to provide the elastic explosiveness needed for surface exchange. In mammals, the diaphragm is thought to be the principal muscle of ventilation. However, it has been found that the diaphragm is not built to solely power the quick inhalation exhibited by dolphins. Therefore, other muscles must be utilized in these marine mammals to power ventilation. One such muscle is the scalenus muscle, which is proportionally larger (muscle/ body mass) in cetaceans than in terrestrial mammals. The scalenus muscle has been identified as a primary muscle of inspiration in humans and is also recruited by hamsters during labored breathing.
The purpose of this study was to elucidate the role of the scalenus in cetacean ventilation by comparing the fiber-type profiles (% fast-twitch fibers) of this muscle in two cetaceans, the harbor porpoise (Phocoena phocoena) and the bottlenose dolphin (Tursiops truncatus), with different ventilatory frequencies. If the scalenus is involved in their ventilation, we would expect the harbor porpoise to have a higher percentage of fast twitch fibers than the bottlenose dolphin, because it inspires at a faster rate (5.0 breaths/minute vs. 2.1 breaths/minute). Sections of the scalenus muscle of both species were stained for their myosin ATPase activities and metabolic enzyme levels. Results indicate that the harbor porpoise scalenus has a slightly higher, but insignificant, percentage of fast twitch fibers (60.18% ± 4.04) than this muscle in the bottlenose dolphin (57.16% ± 4.08). Further research will investigate the hypothesis that the scalenus muscle of each species will contain low densities of oxidative enzymes, because their breath-hold times allow the muscles to recuperate from contractions without requiring oxidative respiration.
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