Diaphragm muscle development in bottlenose dolphins (Tursiops truncatus)

Being born directly into the 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 ventilate the lungs, but their activities must also be coordinated with those of the locomotor muscles such that breathing takes place only at the water's surface. At least one major locomotory muscle of bottlenose dolphins (Tursiops truncatus) has been demonstrated to be well developed and, therefore, able to power the neonatal dolphin's early movements (Dearolf et al. [2000] J Morphol 244:203-215). Thus, because of the demands for coordinated behavior with the locomotor muscles, it is hypothesized that the breathing muscles of bottlenose dolphins, represented in this study by the diaphragm, will also demonstrate adult morphology at birth. However, histochemical and biochemical analyses demonstrate that neonatal dolphins have immature diaphragms, with only 52% of the adult slow fiber-type profile (neonates: 34% slow-twitch fibers; adults: 66% slow-twitch fibers). The developmental state of the dolphin diaphragm is compared to those of other neonatal mammals, using a muscle development index (% slow-twitch fibers in neonatal muscle / % slow-twitch fibers in adult muscle). Fiber-type profiles reported in the literature are used to calculate index values for the diaphragms of altricial rats, rabbits, and cats, intermediate baboons and humans, and precocial sheep and horses. The dolphin is not unique in having an immature diaphragm at birth; however, there is a positive relationship between the developmental state of the diaphragm and the overall developmental state of the neonate. The presence of type IIc ("undifferentiated") fibers in the diaphragms of altricial developers (e.g., rats, rabbits, and cats) is correlated with the slow contraction speeds recorded from their diaphragms. The diaphragms of neonatal horses and dolphins express little to no type IIc fibers and, thus, may have the ability to contract at the speeds required for their increased ventilation rates. These results lead to the modification of the criterion for evaluating the developmental state of a muscle at birth. Thus, the developmental state of a neonatal muscle should be based on both its value of Dearolf et al.'s (2000) developmental index, as well as the percentage of type IIc fibers found in that muscle.