Cardiovascular responses to progressive hypoxia in ducks native to high altitude in the Andes

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.