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Additional file 1: Figure S1. Graphical overview of the experimental design of our study. Deer mice from high- (H) and low- (L) altitude populations were crossed in captivity to produce F1 interpopulation hybrids that were then mated with siblings to produce the F2 interpopulation hybrids that were used in our experiments before and after a 6-wk acclimation to hypobaric hypoxia (12 kPa O2). These hybrids were grouped based on the altitudinal origin of their α- and β- globin genotype.

Additional file 2: Figure S2. Body mass of F2 inter-population hybrids both before and after a 6-wk acclimation to hypobaric hypoxia. Each individual’s mass was measured before normoxic and hypoxic V̇O2max trials, with the mean of these values used to create each individual’s data point in the figure. Different α- and β- globin genotypes are shown as superscripts with ‘L’ representing the lowland haplotype and ‘H’ representing the highland haplotype. There was no effect of genotype (P = 0.2977), acclimation (P = 0.4018), or their interaction (P = 0.3362) on body mass. Bars display mean ± SEM (n = 3-8) with individual data superimposed (circles).

Additional file 3: Table S1. Effects of inspired PO2 and acclimation to hypoxia on cardiorespiratory physiology of F2 inter-population hybrid deer mice at V̇O2max, without accounting for effects of genotype. Table S2. Effects of acclimation to hypoxia and globin genotype on cardiorespiratory physiology in hypoxia of F2 inter-population hybrid deer mice. Table S3. Effects of inspired PO2 and hemoglobin genotype on cardiorespiratory physiology of F2 inter-population hybrid deer mice acclimated to normoxia. Table S4. Effects of acclimation to hypoxia and globin genotype on cardiorespiratory physiology in normoxia of F2 inter-population hybrid deer mice. Table S5. Parameters used to generate the initial solution in the model of the oxygen transport pathway representing the "ancestral" condition with the most lowland P50.

Additional file 5: Figure S4. Physiological parameters for F2 inter-population hybrids acclimated to normoxia, measured at V̇O2max in normoxia (21 kPa O2) and hypoxia (12 kPa O2). Different α- and β- globin genotypes are shown as superscripts with ‘L’ representing the lowland haplotype and ‘H’ representing the highland haplotype. *P < 0.05 vs. normoxia value within a genotype. P < 0.05 between genotypes for hypoxic values not sharing a letter. Bars display mean ± SEM (n = 3-8) with individual data superimposed (circles).

Additional file 7: Figure S6. Physiological parameters for F2 inter-population hybrids measured at V̇O2max in hypoxia (12 kPa O2) both before and after a 6-wk acclimation to hypobaric hypoxia. Different α- and β- globin genotypes are shown as superscripts with ‘L’ representing the lowland haplotype and ‘H’ representing the highland haplotype. †P < 0.05 vs. pre-acclimation value within a genotype. P < 0.05 between genotypes within an acclimation condition for values not sharing a letter. Bars display mean ± SEM (n = 3-8) with individual data superimposed (circles).

Additional file 8: Figure S7. Physiological parameters for F2 inter-population hybrids measured at V̇O2max in normoxia (21 kPa O2) both before and after a 6-wk acclimation to hypobaric hypoxia. Different α- and β- globin genotypes are shown as superscripts with ‘L’ representing the lowland haplotype and ‘H’ representing the highland haplotype. †P < 0.05 vs. pre-acclimation value within a genotype. P < 0.05 between genotypes within an acclimation condition for values not sharing a letter. Bars display mean ± SEM (n = 3-8) with individual data superimposed (circles).

Additional file 6: Figure S5. Hypoxic V̇O2max before and after a 6-wk acclimation to hypobaric hypoxia (12 kPa O2). Different α- and β- globin genotypes are shown as superscripts with ‘L’ representing the lowland haplotype and ‘H’ representing the highland haplotype. †P < 0.05 vs. pre-acclimation value within a genotype. Bars display mean ± SEM (n = 3-8) with individual data superimposed (circles).

Additional file 4: Figure S3. Hematology of F2 inter-population hybrids measured before and after a 6-wk acclimation to hypobaric hypoxia (12 kPa O2). Hct, hematocrit; [Hb], blood hemoglobin content. Different α- and β- globin genotypes are shown as superscripts with ‘L’ representing the lowland haplotype and ‘H’ representing the highland haplotype. †P < 0.05 vs. pre-acclimation value within a genotype. Bars display mean ± SEM (n = 3-8) with individual data superimposed (circles).

The cardiovascular system is critical for delivering O2 to tissues. Here, we examined the cardiovascular responses to progressive hypoxia in four high-altitude Andean duck species compared with four related low-altitude populations in North America, tested at their native altitude. Ducks were exposed to stepwise decreases in inspired partial pressure of O2 while we monitored heart rate, O2 consumption rate, blood O2 saturation, haematocrit (Hct) and blood haemoglobin (Hb) concentration. We calculated O2 pulse (the product of stroke volume and the arterial–venous O2 content difference), blood O2 concentration and heart rate variability. Regardless of altitude, all eight populations maintained O2 consumption rate with minimal change in heart rate or O2 pulse, indicating that O2 consumption was maintained by either a constant arterial–venous O2 content difference (an increase in the relative O2 extracted from arterial blood) or by a combination of changes in stroke volume and the arterial–venous O2 content difference. Three high-altitude taxa (yellow-billed pintails, cinnamon teal and speckled teal) had higher Hct and Hb concentration, increasing the O2 content of arterial blood, and potentially providing a greater reserve for enhancing O2 delivery during hypoxia. Hct and Hb concentration between low- and high-altitude populations of ruddy duck were similar, representing a potential adaptation to diving life. Heart rate variability was generally lower in high-altitude ducks, concurrent with similar or lower heart rates than low-altitude ducks, suggesting a reduction in vagal and sympathetic tone. These unique features of the Andean ducks differ from previous observations in both Andean geese and bar-headed geese, neither of which exhibit significant elevations in Hct or Hb concentration compared with their low-altitude relatives, revealing yet another avian strategy for coping with high altitude.

Whole-body cortisol (ng/g BW) separated by sex within normoxia- and anoxia-treated zebrafish

Quantification of behaviours separated by sex within normoxia- and anoxia-treated zebrafish