The ontogeny of skeletal muscle adaptations that enable long deep dives in Weddell seals

Our objective was to determine the ontogenetic changes in the skeletal muscles of Weddell seals that transform a non‐diving pup into an elite diving adult. Muscle biopsies were collected from pups, juveniles and adults and analyzed for changes in fiber type, mitochondrial density, myoglobin concentrations, and aerobic, lipolytic and anaerobic enzyme activities. The fiber type results showed a decrease in type I fibers with a significant increase in type IIA fibers as the animals mature. In addition, the volume density of mitochondria and the activity of lipolytic enzymes significantly decreased as the seals matured. To our knowledge, this is the first quantitative account describing a decrease in aerobic fibers shifting towards an increase in fast‐twitch oxidative fibers with a significant decrease in mitochondrial density as the animals mature. These differences in muscle physiology are due to their three stages of life history. During the first few weeks of life, pups are a non‐diving terrestrial mammal that must rely on natal fur for thermoregulation in the harsh conditions of Antarctica. The increased aerobic capacity of pups, associated with increased mitochondrial volumes, acts to provide additional thermogenesis. As these future elite divers mature, their skeletal muscles transform to a more sedentary state in order to maintain the low levels of aerobic metabolism associated with long duration diving.