| Summary: | Bar-headed geese migrate over the Himalayas at up to 9000m elevation where they must sustain the high metabolic rates needed for flight despite being severely hypoxic. The present thesis studied the respiratory physiology of this species to better understand the basis for this impressive feat. Evolutionary changes important for high altitude flight were deduced by comparing bar-headed geese to several low altitude species (greylag geese, pink-footed geese, barnacle geese, mallard ducks, or pekin ducks).
Theoretical modelling of the O2 transport pathway was first used to determine traits with the greatest influence over O2 transport in hypoxia. This suggested that a heightened capacity to increase ventilation, a high haemoglobin O2 affinity, and an enhanced O2 diffusing capacity in the muscle should be most influential for enhancing O2 supply. Therefore, the remainder of this thesis tested the general hypothesis that these traits have evolved in bar-headed geese.
Total ventilation was higher in bar-headed geese than in low altitude waterfowl during severe environmental hypoxia. This was entirely due to larger tidal volumes, which should have further enhanced effective ventilation of the gas exchange surface and thus increased O2 loading and arterial O2 tension. Two primary mechanisms accounted for this difference, (i) a reduction in metabolic depression during hypoxia and (ii) a blunted chemosensitivity to respiratory hypocapnia. These studies also supported previous research showing that bar-headed geese have a higher haemoglobin O2 affinity.
Oxygen diffusing capacity in the flight muscle was also enhanced in bar-headed geese. This was due to an increased number of capillaries surrounding each muscle fiber and a redistribution of mitochondria within these fibers towards the cell membrane, closer to capillaries. There were also slight increases in aerobic capacity and alterations in the control of mitochondrial respiration, which could help sustain ATP turnover during prolonged flight in hypoxia.
This thesis has shown that high altitude adaptation in bar-headed geese has involved a suite of evolutionary changes at multiple steps in the O2 transport pathway. This work provides important insights into how respiratory systems evolve, and helps explain the incredible ability of bar-headed geese to fly at extremely high altitudes.
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