Precocial development of axial locomotor muscle in bottlenose dolphins (Tursiops truncatus).

At birth, the locomotor muscles of precocial, terrestrial mammals are similar to those of adults in both mass, as a percent of total body mass, and fiber-type composition. It is hypothesized that bottlenose dolphins (Tursiops truncatus), marine mammals that swim from the instant of birth, will also exhibit precocial development of locomotor muscles. Body mass data from neonatal and adult dolphins are used to calculate Grand's (1992) Neural and Muscular Indices of Development. Using these indices, the bottlenose dolphin is a Condition "3.5" neonate, where Condition 4 is the documented extreme of precocial development in terrestrial mammals. Moreover, myosin ATPase (alkaline preincubation) analyses of the epaxial locomotor m. extensor caudae lateralis show that neonatal dolphins have fiber-type profiles very similar to those of adults. Thus, based on mass and myosin ATPase activity, muscle development in dolphins is precocial. However, succinic dehydrogenase and Nile red histochemistry demonstrate that neonatal dolphin muscle has mitochondrial and lipid distributions different from those found in adults. These data suggest that neonates have a lower aerobic capacity than adults. Dolphin neonates may compensate for an apparent lack of aerobic stamina in two ways: 1) by being positively buoyant, with a relatively increased investment of their total body mass in blubber, and 2) by "free-riding" off their mothers. This study investigates quantitatively the development of a dolphin locomotor muscle and offers suggestions about adaptations required for a completely aquatic existence.