Consistent changes in muscle metabolism underlie dive performance across multiple lineages of diving ducks

Abstract

Diving animals must sustain high activity with limited O₂ stores to successfully capture prey. Studies suggest that increasing body O₂ stores supports breath-hold diving, but less is known about metabolic specializations that underlie underwater locomotion. We measured maximal activities of 10 key enzymes in locomotory muscles (gastrocnemius and pectoralis) to identify biochemical changes associated with diving in pathways of oxidative and substrate-level phosphorylation and compared them across three groups of ducks-the longest diving sea ducks (eight spp.), the mid-tier diving pochards (three spp.) and the non-diving dabblers (five spp.). Relative to dabblers, both diving groups had increased activities of succinate dehydrogenase and cytochrome c oxidase, and sea ducks further showed increases in citrate synthase (CS) and hydroxyacyl-CoA dehydrogenase (HOAD). Both diving groups had relative decreases in capacity for anaerobic metabolism (lower ratio of lactate dehydrogenase to CS), with sea ducks also showing a greater capacity for oxidative phosphorylation and lipid oxidation (lower ratio of pyruvate kinase to CS, higher ratio of HOAD to hexokinase). These data suggest that the locomotory muscles of diving ducks are specialized for sustaining high rates of aerobic metabolism, emphasizing the importance of body O₂ stores for dive performance in these species.

Keywords: aerobic metabolism; breath-hold diving; electron transport system; lipid oxidation; mitochondrial energetics; oxidative phosphorylation.

Figure 1.

Phylogeny for the 16 species studied here, generated using 10 731 overlapping autosomal ddRAD-seq loci in BEAST v. 2.5.2. Branch lengths were assigned using treePL. Asterisks indicate corresponding species graphic. Tribes are indicated by colour: sea ducks (dark blue), pochards (teal), dabblers (gold). Dive times were obtained from several sources in the literature, represented by the following superscripts: a, [23]; b, [24]; c, [25]; d, (Boone JL, LaRue EA 1998, unpublished data); e, [26]; f, [27]; g, [28]; h, [29]; i, [30].

Figure 2.

Schematic of the metabolic pathways investigated here. The 10 enzymes assayed are highlighted in green (glycolysis and lactate production), brown (beta-oxidation), pink (citric acid cycle), purple (electron transport system; succinate dehydrogenase is complex II; cytochrome c oxidase is complex IV), and orange (substrate-level phosphorylation).

Figure 3.

Maximal activity in the gastrocnemius for mitochondrial enzymes (a) citrate synthase (CS) and (b) cytochrome c oxidase (COX), and beta-oxidation enzyme (c) 3-hydroxyacyl-CoA dehydrogenase (HOAD), along with maximal activity in the pectoralis for mitochondrial enzyme (d) succinate dehydrogenase (SDH) for three tribes of ducks: sea ducks (diving, eight spp., n = 72), pochards (diving, three spp., n = 30), and dabblers (non-diving, five spp., n = 50). Activity was measured as μmol g⁻¹ tissue min⁻¹ (U g⁻¹). Box plots show the median and quartile ranges of the data, with the mean indicated by the dashed line. In the gastrocnemius, values in the sea ducks are significantly higher than those in the dabblers, and in the pectoralis, values in both diving tribes are significantly higher than those in the dabblers using Bonferroni post hoc tests (*p < 0.05; **p < 0.01) after a phylogenetic ANOVA (for which p-values are shown on each panel).

Figure 4.

Enzyme activity ratios in three tribes of ducks: sea ducks (diving, eight spp., n = 72), pochards (diving, three spp., n = 30) and dabblers (non-diving, five spp., n = 50). Box plots show the median and quartile ranges of the data, with the mean indicated by the dashed line. There were significant decreases in the ratios in the sea ducks compared with the dabblers for ATPSyn:CS in both the (a) pectoralis and (b) gastrocnemius; in LDH:CS in the (c) pectoralis and (d) gastrocnemius; and in PK:CS in the (e) gastrocnemius. (f) HOAD:HK in the gastrocnemius is higher in the sea ducks than in the pochards. Bonferroni post hoc tests (*p < 0.05) were used after a phylogenetic ANOVA (for which p-values are shown on each panel).

Figure 5.

Principal components analysis (PCA) of the 10 enzymes assayed across all individuals in the gastrocnemius, coloured by (a) tribe, and (b) species. Ellipses represent the 95% confidence ellipse for each group. The three tribes are clearly visible along PC1, with the dabblers most distinct from the more deeply diverged sea ducks, and pochards intermediate between the two. Enzymes include hexokinase (HK); pyruvate kinase (PK); lactate dehydrogenase (LDH); hydroxyacyl-CoA dehydrogenase (HOAD); citrate synthase (CS); succinate dehydrogenase (SDH); cytochrome c oxidase (COX); ATP synthase (ATPSyn); creatine kinase (CK); adenylate kinase (AK). Species include long-tailed duck (LTDU); harlequin duck (HARD); surf scoter (SUSC); red-breasted merganser (RBME); common merganser (COME); bufflehead (BUFF); common goldeneye (COGO); barrow's goldeneye (BAGO); ring-neck duck (RNDU); greater scaup (GRSC); lesser scaup (LESC); northern shoveler (NSHO); American wigeon (AMWI); American green-wing teal (AGWT); mallard (MALL); northern pintail (NOPI).