Astronaut raised geese to study their hearts. The birds stole hers.

Before Jessica Meir became a NASA astronaut who will blast into space from Russia this month, she made a career of studying animals that live at extremes. She followed elephant seals and emperor penguins as they swam through frigid waters. She raised a dozen bar-headed geese, capable of flying three miles high, from the moment they hatched out of their eggs. They treated her as their mother.

Few creatures dare to fly over the tallest mountains on Earth. Bar-headed geese are an exception. Above the Himalayas, where the atmosphere is so thin that helicopters struggle to fly and human exertion is nearly impossible, the birds beat their wings as they migrate from India to Mongolia.

Meir led the geese - they followed her anywhere - into a wind tunnel designed to test submarines and sports equipment. A year later, Julia York, then a student at the University of British Columbia, helped another group of geese fly in the machine. When scientists lowered the oxygen the geese breathed, the animals chilled their blood and slowed their metabolism, Meir, York and their colleagues reported in a study published Tuesday in the journal eLife.

The geese "would be absolutely fine" in conditions that would "probably kill us and many other animals," said Graham Scott, an expert in low-oxygen biology at McMaster University in Ontario who did not participate in this study.

Bar-headed geese and their "extraordinary migration" have attracted decades of scientific interest, said Douglas Altshuler, who studies flight behavior at the University of British Columbia. (Altshuler has worked closely with the study authors but was not a member of the research team behind the new paper.) In an earlier study, scientists trained geese to run on treadmills.

To get birds to fly in a wind tunnel, the study authors took advantage of the avian behavior known as imprinting. Zoologist Konrad Lorenz, who was awarded the Nobel Prize in 1973, discovered that freshly hatched birds will bond with the first large shape they see. But few researchers use imprinting as a scientific technique, because raising birds requires so much time. Study author William Milsom, a University of British Columbia expert in the respiratory systems of animals, knew of only one other research group, based in Britain, where bar-headed geese had imprinted on researchers. Before this work, no biologists had flown the geese in wind tunnels while reducing their oxygen.

Two sets of geese imprinted on the scientists, one group on Meir in 2010 and another gaggle on York in 2011. Meir met her goslings in summer 2010, as they cracked out of their eggs at the Sylvan Heights Bird Park in Scotland Neck, North Carolina.

The scientist was the first thing the animals saw. "It was one of the most amazing things I've ever experienced in my life," Meir said. "I say this jokingly, but there's a little bit of truth to it, too: I was a woman in my mid-30s when I was imprinting these geese . . . so there's a lot going on, with the imprinting of my 12 baby goslings."

Meir spent almost all of her waking hours with the goslings at the bird park. They chirped and cried at the sound of her voice as she approached their pens each morning. She took baby birds for walks. When she sat on a blanket to read, they smothered her in a fluffy, gray goose-pile. "They would just nap and snuggle and cuddle up inside," she said.

When the birds were old enough to travel, Meir took her geese in carrier crates on a plane to Seattle. They drove through customs across the Canadian border to the University of British Columbia, one of the few universities with a wind tunnel that could accommodate the geese. On the Vancouver campus, she taught them to fly.

She began by biking away from the geese on a secluded farm. Eager to stay with her, they started to run. "And then they realize that they can keep up better if they fly," she said. After two days, the birds flew faster than she could bike, so Meir switched to a motor scooter and led them along a quiet street on the university campus.

"It would be flying so close to me that its wing tip would be sometimes brushing my arm. I am looking right into the eye of this flying bird, not to mention my baby," Meir said. When she stopped on her scooter, they would stop with her.

Except when they didn't. The road partly runs along a nude beach, from which the scientists once rescued a wayward goose, Milsom said. One goose landed on an athletic field during a field hockey game and scurried after the players. Another confused bird, lost in a supermarket parking lot, began to follow people in and out of the store, as if searching for Meir.

While study authors developed this experiment, other researchers told Altshuler that the trials would fail, he said. The devices required to study the geese were Rube Goldberg-like contraptions: masks fitted to the geese bills, connected by tubes to tanks of oxygen and nitrogen, which enabled the scientists to control the air the birds breathed.

The 30-yard-long tunnel, open to the outdoors, was so chilly the scientists wore heavy jackets. York kept her hands free to clap and encourage the birds. (Geese are not motivated by food; no matter how delicious a piece of lettuce proffered, geese cannot be bribed to fly.)

Seven birds flew in the tunnel, one at a time, while wearing the masks. "If you adjust the wind speed of the wind tunnel, then the birds are almost motionless," Milsom said. The scientists fitted the birds with sensors to gauge their heart rate as well as the temperature and gases in their blood.

Meir and her colleagues tested the birds at three oxygen concentrations: normal oxygen, equivalent to sea level; diminished oxygen, comparable to 3.4 miles up; and very little oxygen, equal to about 5.6 miles high. A few birds flew in all conditions.

When humans exercise, our muscles warm up and our blood heats with it. This is true for most animals, but not these geese.

"The fact that their blood cooled during flight has really important implications for how birds can fly in low-oxygen conditions," Scott said. In lower temperatures, hemoglobin molecules bind more tightly to oxygen - meaning each red blood cell can carry more oxygen. The amount of oxygen pumped in the blood per heartbeat soared.

Despite the demands of flight, the birds' metabolism slowed as the external oxygen dropped. "They proved quite conclusively that it's through a reduction in metabolic rate" that these animals can fly in oxygen-poor conditions, Altshuler said. What Meir and her colleagues accomplished in the wind tunnel "tells us more about the physiology of bar-headed geese and flight than almost all of the research that came before it, combined," he said.

Meir suspects the animals shut down some body functions, such as digestion, when the air thins. "If you don't need to keep delivering blood to the digestive system," she said, "you can save some oxygen there."

The geese's ability to cope with extreme environments cannot be directly mimicked in humans, but Scott said it could inspire potential therapies for humans who encounter low-oxygen conditions.

Meir will travel to the International Space Station in late September. "It is not at all surprising to me to know that she is an astronaut," Altshuler said. "These are people who are willing to take on ambitious projects that should not be able to be accomplished in the time frame they give themselves."

Though she'll be in the extreme environment of space, Meir won't be conducting her own research - she'll be a test subject.

"I'm finally paying my dues," Meir said. "I'm going to be the one poked and prodded."