Author response: Reduced metabolism supports hypoxic flight in the high-flying bar-headed goose (Anser indicus)

The bar-headed goose is famous for reaching extreme altitudes during its twice-yearly migrations across the Himalayas. These geese have been tracked flying as high as 7,270 meters up, and mountaineers have anecdotally reported seeing them fly over summits around Mount Everest (that are over 8,000 meters tall). At these heights, the air is so thin that it contains only about 30–50% of the oxygen available at sea-level. Bar-headed geese have several adaptions that help them exercise in low oxygen conditions. For example, they have larger lungs than most other birds their size, and their red blood cells contain a version of hemoglobin that binds oxygen much more tightly. To date, however, there has been no work that has comprehensively measured how the bar-headed goose adapts its physiology to fly under low oxygen conditions. As such, it remains unclear whether these birds would even be able to fly where the oxygen is as limited as it is above the summits of the world’s highest mountains. This is partly because it is extremely challenging to make these kinds of recordings from flying geese, and partly because there are few wind tunnels in the world suitable to carry out such experiments. To better understand how the bar-headed goose accomplishes its remarkable, high altitude migration, Meir et al. raised bar-headed geese from eggs, with experimenters acting as the birds’ foster parents. The birds took their first flights either in a 30-meter wind tunnel at an engineering department in the University of British Columbia or, if the wind tunnel was unavailable, alongside a bicycle or a motor scooter. Once trained, the geese then flew in the wind tunnel wearing a backpack that contained the sensors needed to record their physiology. The birds also wore a breathing mask that could simulate the limited oxygen availability at altitudes of roughly 5,500 and 9,000 meters, and measure the oxygen consumed and the carbon dioxide produced by the geese. Meir et al. found that bar-headed geese could indeed fly at these simulated extreme altitudes in the wind tunnel, and that the birds largely achieved this by reducing their metabolism to match low oxygen conditions. The recordings show that the geese did not increase their heart rate when flying in reduced oxygen compared with normal flights, suggesting that their hearts were not working at maximum capacity despite the extreme conditions. Meir et al. also discovered that the blood in the birds’ veins cooled when flying, and in some cases by more than 2°C. Since hemoglobin’s affinity for oxygen changes with temperature, this may help increase the amount of oxygen that these birds can load into their blood at the lung when in flight. These measurements suggest that the anecdotes of bar-headed geese flying over some of the highest mountains in the world are indeed physiologically plausible. The findings will be valuable to researchers studying animals living at extreme altitudes. They may also be relevant to those looking to understand how humans respond to situations where oxygen is limited, such as during medical conditions like a heart attack or stroke, or procedures like organ transplants.

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