Control of Breathing in Birds: Implications for High-Altitude Flight

For birds that fly at high altitude, breathing must increase substantially to accommodate the dual oxygen transport requirements of exercise and hypoxia. Here we review the literature on control of breathing in birds, with particular emphasis on the adaptive trends seen in high-altitude flying species. Increases in breathing during high-altitude flight result from neurally mediated reflexes arising from multiple sites. The locomotor system stimulates breathing directly during exercise via both feedforward stimulation from brainstem locomotor centers and feedback stimulation from exercising muscles. O2-sensitive chemoreceptors in the carotid body also stimulate breathing during hypoxia, whereas CO2/pH-sensitive chemoreceptors can restrain breathing if the hypoxic ventilatory response produces a secondary hypocapnia. Theoretical modeling suggests that an enhanced capacity to increase breathing should be adaptive for high altitude flight. Empirical research suggests that the high-altitude flying bar-headed goose can indeed increase breathing significantly more than low-altitude birds during hypoxia at rest, loading more oxygen into the blood. This is probably caused by a reduction in the sensitivity of CO2/pH-sensitive chemoreceptors to hypocapnia, and/or a reduction in hypoxic metabolic suppression and its depressive effects on breathing. Although this suggests that alterations in respiration control are an important component of the suite of adaptations to high altitude in birds, future studies are needed on control of breathing during flight, especially at altitude. A greater appreciation of the genetic basis for differences in the oxygen transport pathway that occur in high-altitude species will lead to a greater understanding of the evolution of physiological performance.