A study of the world's highest-flying bird has shown how it manages to survive at extreme altitudes as it soars above the Himalayas.
The bar-headed goose literally puts its heart into the challenge of breathing air consisting of just 7% oxygen, scientists found.
Available oxygen is channelled to the bird's heart, which beats fast to pump oxygenated blood to the rest of its body.
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In this way, the bar-headed goose is able to cross the Himalayan mountain range on its migratory flights between India and China, reaching heights of almost 24,000ft.
At this altitude, the oxygen level of the air is only a third of what it is at sea level. Most people exposed to such conditions would quickly pass out. Scientists believe lessons learned from the bar-headed goose could help prevent heart attacks and strokes in humans.
Lead researcher Dr Lucy Hawkes, from the centre for ecology and conservation at the University of Exeter, said: "It all seems to come down to how much oxygen bar-headed geese can supply to their heart muscles. The more they can supply, the faster they can beat their hearts and keep the supply of oxygen to the rest of the body going.
"This suggests that other species, including humans, are limited more by what our hearts can do than by how fit the rest of our muscles are at altitude."