Abstract
While bar-headed geese are renowned for migration at high altitude over the Himalayas, previous work on captive birds suggested that these geese are unable to maintain rates of oxygen consumption while running in severely hypoxic conditions. To investigate this paradox, we re-examined the running performance and heart rates of bar-headed geese and barnacle geese (a low altitude species) during exercise in hypoxia. Bar-headed geese (n = 7) were able to run at maximum speeds (determined in normoxia) for 15 minutes in severe hypoxia (7% O2; simulating the hypoxia at 8500 m) with mean heart rates of 466±8 beats min−1. Barnacle geese (n = 10), on the other hand, were unable to complete similar trials in severe hypoxia and their mean heart rate (316 beats.min−1) was significantly lower than bar-headed geese. In bar-headed geese, partial pressures of oxygen and carbon dioxide in both arterial and mixed venous blood were significantly lower during hypoxia than normoxia, both at rest and while running. However, measurements of blood lactate in bar-headed geese suggested that anaerobic metabolism was not a major energy source during running in hypoxia. We combined these data with values taken from the literature to estimate (i) oxygen supply, using the Fick equation and (ii) oxygen demand using aerodynamic theory for bar-headed geese flying aerobically, and under their own power, at altitude. This analysis predicts that the maximum altitude at which geese can transport enough oxygen to fly without environmental assistance ranges from 6,800 m to 8,900 m altitude, depending on the parameters used in the model but that such flights should be rare.
Highlights
Bar-headed geese (Anser indicus) and barnacle geese (Branta leucopsis) undertake similar long distance migratory flights between breeding and wintering grounds, usually covering thousands of kilometres, during the autumn and spring [1,2]
When running at their maximum sustainable speed, heart rate increased to 385.465.5 beats min21 during normoxia and to a significantly higher value of 453.2615.0 beats min21 during severe hypoxia (Table 1, Fig. 2A)
In the present study we have shown that bar-headed geese are capable of maximum sustained running exercise in conditions of hypoxia similar to the summit of Mount Everest
Summary
Bar-headed geese (Anser indicus) and barnacle geese (Branta leucopsis) undertake similar long distance migratory flights between breeding and wintering grounds, usually covering thousands of kilometres, during the autumn and spring [1,2]. Bar-headed geese must typically fly at over 4,000 m elevation for many hundreds of kilometres while traversing the Himalayan mountains and Tibetan Plateau [2]. At such high altitudes, barometric pressure is substantially reduced along with the partial pressure of oxygen (PO2), leading to progressive environmental hypoxia and, a decrease in the oxygen available to fuel flapping flight. Sustaining any form of exercise when at high altitude is extremely challenging but bar-headed geese have been shown to possess a number of specific physiological adaptations that may increase their performance, relative to other species of geese when exposed to severe environmental hypoxia (reviewed in [9,10,11]). Studies on captive birds have demonstrated that bar-headed geese at rest have remarkable hypoxia tolerance compared to other similar sized waterfowl [12,13,14,15], including barnacle geese, and even remain conscious and upright at an inspired PO2 equivalent to that at an altitude of 12,190 metres [16]
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