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.
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, and may even die.
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."
She added: " The wider implications of these findings are for low oxygen medical conditions in humans, such as heart attack and stroke - suggesting what adaptations might help prevent problems in the first place and learning how animals have managed to cope with really extreme environments."
The scientists tested bar-headed geese by getting them to run on a treadmill inside a box with oxygen levels similar to those found on Mount Everest.
The geese had an astonishing tolerance of low oxygen conditions, both at rest and when exercising for 15 minutes at top speed.
Other experiments with barnacle geese, which migrate at sea level, showed they did not have the same ability to cope with lack of oxygen.
The research is reported in the latest edition of the online journal Public Library of Science ONE.
Bar-headed geese have been shown to possess a number of physiological adaptations that may assist their survival in low oxygen conditions.
In particular, their heart and locomotor muscles contain extra blood vessels.
Altitudes above 8,000 metres, or 26,000ft, occupy what climbers call the "death zone".
Mount Everest is 29,000ft high, well within the zone. In 1978, Reinhold Messner and Peter Habeler made the first ascent of the mountain without additional oxygen.
Since then, the feat has been repeated by a number of climbers. But all were specially trained and acclimatised, and none stayed on the summit for long.