DEAROLF, J.L.*; MCLELLAN, W.A.; PABST, D.A.; HERMANSON, J.W.: Diaphragm muscle development in bottlenose dolphins (Tursiops truncatus)

Being born directly into an aquatic environment creates unique challenges for the breathing muscles of neonatal cetaceans. Not only must these muscles be active at the instant of birth to allow for gas exchange, but their activities must also be coordinated such that breathing takes place at the surface. Thus, we hypothesize that the breathing muscles of bottlenose dolphins, represented in this study by the diaphragm, will demonstrate adult morphologies at birth, as measured by the histochemical (myosin ATPase) and biochemical activities of their component muscle fibers. The fiber-type profiles of adult (64% slow-twitch) and neonatal (33% slow-twitch) dolphin diaphragms are found to be statistically different (F=48.02, p=0.002, df=5, n=6). In addition, there is a strong positive correlation (R-sq=0.98) between the fiber-type profile (percent slow-twitch fibers) of the dolphin diaphragm and total body length (TBL), a character that is used as a relative measure of age in dolphins. The results of the biochemical analyses support the correlation between fiber-type profile and TBL, demonstrating an increase in the expression of slow (type I) myosin heavy chain isoform with increasing body length. Thus, the diaphragms of neonatal dolphins are not well developed at birth (92-132 TBL). However, as the dolphin grows and increases its swimming and diving activities, the diaphragm matures and achieves adult morphology. Therefore, unlike dolphin locomotor muscles, which are well developed at birth, the diaphragm requires an extended period of postnatal development to achieve the adult fiber-type profile.