ABSTRACT
As air-breathing vertebrates, all marine mammals are obligated to periodically return to the water surface to breathe. This interruption in aquatic activities, one of the most important being foraging, is due to an evolutionary history that involved the r e-invasion of the oceans by the terrestrial ancestors of cetaceans (Thewissen et al. 1994), pinnipeds (Berta 2012), sea otters (Riedman and Estes 1990), and sirenians (Barnes et al. 1985). As a result, the internal building blocks required for exercising while diving by ancestral marine mammals included tolerance or modication of morphological, physiological, biochemical, and molecular mechanisms originally intended for locomotion on land (Williams 1999).
