Set a group of engineers a problem and each individual will often come up with their own solution, while on other occasions, several individuals may produce the same solution from different starting points; even evolution adopts both strategies. So, when Sabine Lague, Tony Farrell and Bill Milsom began wondering how birds have adapted to life at high altitude, it wasn't clear whether species with different lifestyles had opted for the same strategy, or other alternatives. ‘High-altitude environments pose a significant physiological challenge for animals; they are typically colder, drier and all of them have less available environmental oxygen’, says Lague. So, would birds that live permanently in thin air use the same strategy to supply sufficient oxygen as migratory species, which only venture to high altitudes during transit? Lague, Milsom and Beverly Chua ventured to the Andes and onto the Tibetan plateau to investigate the question.
Working with Yuxiang Wang and Yang Zhong in Tibet, and Kevin McCracken, Peter Frappell, Graham Scott and Luis Alza in Peru, Lague and her colleagues collected bar-headed geese from Lake Qinghai (3200 m), and Andean geese and crested ducks from altitudes over 4000 m in the Andean mountains before allowing the South American birds to adapt to the same altitude as the Tibetan birds. Once the birds had adjusted, the team gently restrained the animals before adding nitrogen to the birds’ air to reduce the amount of oxygen in order to simulate ascents from 3300 m to 4500, 7000, 8500 and 11,000 m while recording the birds’ oxygen consumption, how much air they inhaled, how fast and hard their hearts were beating and the amount of oxygen carried in their blood.
As the oxygen thinned, the bar-headed geese breathed harder, inhaling twice as much air at the highest simulated altitude; however, neither of the Andean species increased their breath rate or the volume of air inhaled. And when the team compared the performances of the hearts of the three species, they all managed to pump more blood as the oxygen became scarcer, but the bar-headed geese increased their heart rate while the South American birds pumped a larger volume of blood per heart beat. Yet, despite adopting different strategies, the Andean species and the bar-headed geese all managed to keep their blood equally well charged with oxygen, even at the lowest oxygen levels.
Lague says that the birds, ‘appear to have evolved two strikingly different strategies for coping with low oxygen’, adding that the discovery was a huge surprise to everyone involved. The bar-headed geese have evolved a flexible strategy where they breathe harder and their heart rate increases to provide sufficient oxygen during their high altitude migration. In contrast, the crested ducks and Andean geese, which spend their entire lives at altitudes in excess of 4000 m, have modified the surface of their lungs to allow them to pick up more oxygen without breathing harder. And when Lague and her colleagues compared the strategies of the high-altitude birds with those of the local high-altitude human populations that were already known, they were intrigued to see similarities in the responses: the Tibetan humans breathed harder, like the bar-headed geese, and the Andean humans breathed less hard, like the Andean geese and crested ducks.
However, she points out that not all of the strategies that have been adopted by humans that live at high altitudes are beneficial. ‘Overproduction of red blood cells in high-altitude residents makes the blood thicker, which places increased strain on the heart and can lead to heart failure’, she explains, but the birds have not fallen foul of the same problem.