Abstract
Diving animals must sustain high rates of aerobic respiration in the working muscle on a single breath while foraging underwater, creating a potential mismatch in oxygen (O2) supply and demand. Studies have shown that diving animals correct for this mismatch through increases in whole-body O2 storage capacity and changes in locomotory muscle structure and metabolism. Here, I quantified blood and muscle O2 stores, locomotory muscle (gastrocnemius, primary diving muscle) ultrastructure characteristics, and changes in metabolic pathways related to oxidative and substrate-level phosphorylation to explore changes associated with diving in 16 species of diving (sea ducks, pochards) and non-diving ducks (dabblers). Across all three levels of O2 transport and utilization studied, I found distinct differences between the sea ducks and dabblers, with the pochards generally showing an intermediate phenotype. Compared to the dabblers, sea ducks had significantly higher hemoglobin and myoglobin concentrations, preferential proliferation of mitochondria close to the cell membrane and capillary blood supply, and increased activities of citrate synthase and cytochrome c oxidase, both key mitochondrial enzymes for aerobic respiration. Further analyses showed that muscle myoglobin, citrate synthase activity, and average muscle mitochondrial density were the most highly correlated with mean dive times in these species. These data suggest that diving ducks increase total onboard O2 stores in the blood and muscle, and that the gastrocnemius is specialized for sustaining high rates of aerobic metabolism. These observed changes would work to maximize O2 extraction from blood and muscle stores as O2 tension decreases during later stages of a breath-hold dive, as well as augmenting aerobic ATP generation to sustain intense underwater exercise while foraging.