Fig. 5. Summary reveals the ontogeny of skeletal muscle adaptations that enable long deep dives in Weddell seals. As newborn pups, Weddell seals have an extremely high aerobic capacity, similar to that found in terrestrial animal athletes and short-duration divers. However, this enhanced aerobic capacity is not an adaptation towards diving but is due to their high fat diet and the need to offset thermoregulatory costs associated with using their lanugo (natal fur) for insulation in the extremely harsh environment of Antarctica. As the pups begin to dive and mature into juveniles, their skeletal muscles begin to transform. As juveniles, they initiate the development of fast-oxidative fibers and significantly increase their intramuscular stores of oxygen in the form of oxymyoglobin. As they continue to mature and increase their diving capacity, Weddell seals increase their percentage of Type IIA fast-oxidative fibers in their skeletal muscles. In addition, their skeletal muscles transform to a more sedentary state in order to maintain low levels of aerobic metabolism under the hypoxic conditions associated with long-duration diving. Similar to what has been found in terrestrial mammals; the results of our subtractive hybridization analysis indicate that these changes in skeletal muscle metabolic potential are regulated by calcium signaling and its downstream mediator, calcineurin.