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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Control of breathing and adaptation to high altitude in the bar-headed goose

Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada

Submitted 5 March 2007 ; accepted in final form 8 May 2007

The bar-headed goose flies over the Himalayan mountains on its migratory route between South and Central Asia, reaching altitudes of up to 9,000 m. We compared control of breathing in this species with that of low-altitude waterfowl by exposing birds to step decreases in inspired O₂ under both poikilocapnic and isocapnic conditions. Bar-headed geese breathed substantially more than both greylag geese and pekin ducks during severe environmental (poikilocapnic) hypoxia (5% inspired O₂). This was entirely due to an enhanced tidal volume response to hypoxia, which would have further improved parabronchial (effective) ventilation. Consequently, O₂ loading into the blood and arterial PO₂ were substantially improved. Because air convection requirements were similar between species at 5% inspired O₂, it was the enhanced tidal volume response (not total ventilation per se) that improved O₂ loading in bar-headed geese. Other observations suggest that bar-headed geese depress metabolism less than low-altitude birds during hypoxia and also may be capable of generating higher inspiratory airflows. There were no differences between species in ventilatory sensitivities to isocapnic hypoxia, the hypoxia-induced changes in blood CO₂ tensions or pH, or hypercapnic ventilatory sensitivities. Overall, our results suggest that evolutionary changes in the respiratory control system of bar-headed geese enhance O₂ loading into the blood and may contribute to this species' exceptional ability to fly high.

hypoxic ventilatory response; birds; respiration; metabolism; carbon dioxide