ABSTRACT
Diving animals must sustain high muscle activity with finite oxygen (O2) to forage underwater. Studies have shown that some diving mammals exhibit changes in the metabolic phenotype of locomotory muscles compared with non-divers, but the pervasiveness of such changes across diving animals is unclear, particularly among diving birds. Here, we examined whether changes in muscle phenotype and mitochondrial abundance are associated with dive capacity across 17 species of ducks from three distinct evolutionary clades (tribes) in the subfamily Anatinae: the longest diving sea ducks, the mid-tier diving pochards and the non-diving dabblers. In the gastrocnemius (the primary swimming and diving muscle), mitochondrial volume density in both oxidative and glycolytic fiber types was 70% and 30% higher in sea ducks compared with dabblers, respectively. These differences were associated with preferential proliferation of the subsarcolemmal subfraction, the mitochondria adjacent to the cell membrane and nearest to capillaries, relative to the intermyofibrillar subfraction. Capillary density and capillary-to-fiber ratio were positively correlated with mitochondrial volume density, with no variation in the density of oxidative fiber types across tribes. In the pectoralis, sea ducks had greater abundance of oxidative fiber types than dabblers, whereas pochards were intermediate between the two. These data suggest that skeletal muscles of sea ducks have a heightened capacity for aerobic metabolism and an enhanced ability to utilize O2 stores in the blood and muscle while diving.
Footnotes
Author contributions
Conceptualization: E.R.S., G.R.S., N.J.D., K.G.M.; Methodology: G.R.S.; Validation: E.R.S.; Formal analysis: E.R.S.; Investigation: E.R.S.; Resources: G.R.S., K.W., K.G.M.; Data curation: E.R.S.; Writing - original draft: E.R.S.; Writing - review & editing: G.R.S., N.J.D., K.W., K.G.M.; Visualization: E.R.S.; Supervision: G.R.S., K.G.M.; Project administration: E.R.S.; Funding acquisition: K.W., K.G.M.
Funding
This work was supported by funds from the Kushlan Endowment for Waterbird Biology and Conservation at the University of Miami and the University of Miami Department of Biology. N.J.D. was also supported by an ISSF ECR Catalyst Grant (#310331-01/Wellcome Trust) and an Advanced Grant from the European Research Council (ERC; no. 834653) to Neil B. Metcalfe. G.R.S. was supported by the Canada Research Chairs program.
Data availability
Data are available from duckDNA.org: https://duckdna.org/data/jeb247550/.