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 examine the cardiovascular responses to progressive hypoxia in four high-altitude Andean duck species compared to 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 concentration [Hb]. 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], increasing the O₂ content of arterial blood, and potentially providing a greater reserve for enhancing O₂ delivery during hypoxia. Hct and [Hb] 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] compared to their low-altitude relatives, revealing yet another avian strategy for coping with high altitude.