Centre for Ornithology, School of Biosciences, University of Birmingham, Birmingham, B152TT, England, UK.
Comp Biochem Physiol A Mol Integr Physiol. 2010 Jul;156(3):325-9. doi: 10.1016/j.cbpa.2010.01.016. Epub 2010 Jan 28.
Up to half the world's population of bar-headed geese (Anser indicus) migrate between central Asia and India and fly between 5000 m and 9000 m above sea level as they cross the Himalayas. The partial pressures of oxygen at these altitudes are, respectively, about 50% and 30% those at sea level. Flapping flight is energetically expensive, so how are bar-headed geese able to migrate at such altitudes? The haemoglobin of bar-headed geese has a greater affinity for oxygen than those of lowland birds, and birds are able to hyperventilate to a greater extent than mammals during severe hypoxia. Together, these mean that the concentration of oxygen in the arterial blood at a given altitude is greater in bar-headed geese than in lowland birds and mammals. The low partial pressure of CO(2) in arterial blood (hypocapnia) that accompanies hyperventilation does not cause reduction of cerebral blood flow in birds as it does in mammals, thus there is greater oxygen delivery to the brain in hypoxic birds, including bar-headed geese, than in mammals. Captive bar headed geese could not maintain elevated aerobic metabolism during exercise at a simulated altitude of 8500 m and their cardiac stroke volume was much lower than that during exercise at sea level. This suggests that if some individuals of this species of geese do really manage to fly over Mt Everest, they may only do so if they receive assistance from vertical air movements, for example from lee waves downwind from the mountains.
世界上多达一半的斑头雁（Anser indicus）在中亚和印度之间迁徙，在飞越喜马拉雅山时，飞行高度在 5000 米至 9000 米之间。在这些海拔高度上，氧气的分压分别约为海平面的 50%和 30%。拍翅飞行的能量消耗很大，那么斑头雁是如何在这样的高度迁徙的呢？斑头雁的血红蛋白比低地鸟类对氧气的亲和力更大，而且鸟类在严重缺氧时比哺乳动物更能过度通气。这些共同意味着，在给定海拔高度下，斑头雁动脉血液中的氧气浓度比低地鸟类和哺乳动物更高。伴随过度通气的动脉血液中二氧化碳分压（低碳酸血症）降低不会像在哺乳动物中那样导致脑血流量减少，因此在缺氧鸟类中，包括斑头雁在内，大脑的氧气输送量比哺乳动物更大。在模拟海拔 8500 米的高度进行运动时，圈养的斑头雁无法维持有氧代谢的升高，其心搏量比在海平面运动时低得多。这表明，如果该物种的某些个体真的设法飞越珠穆朗玛峰，它们可能只有在获得垂直空气运动的帮助下才能做到，例如从山脉下风的背风涡中获得帮助。
