On the summits of Andean volcanoes, above 6,700 metres, the temperature can fall to minus 60 degrees Celsius, the air holds about 44 percent of the oxygen available at sea level, and there is next to nothing to eat. It is not a place biology expected to find a mammal. Yet a mouse smaller than a human hand lives there – and a study published on 9 July in Science now explains how it manages.
The Andean leaf-eared mouse, Phyllotis vaccarum, became the record holder in 2020, when researchers found one at the top of Llullaillaco, a 6,739-metre volcano on the Argentina–Chile border. Before that, large-eared pikas in the Himalayas were thought to occupy the mammalian ceiling, some 600 metres lower. The mouse also lives at sea level on Chile's desert coast and at every elevation in between, which gives it the widest altitudinal range of any mammal on Earth – and, conveniently for scientists, a built-in comparison group.
That is what the team exploited. Researchers including evolutionary biologist Jay Storz of the University of Nebraska–Lincoln, comparative physiologist Naim Bautista of the University of Oklahoma and biologist Grant McClelland of McMaster University collected 167 of the mice from highland and lowland sites in the Central Andes between 2020 and 2023, then placed them in chambers that reproduced the cold, thin air of a summit.
Engineered for the cold
The highland animals burned through oxygen faster than their lowland relatives and generated markedly more body heat. The advantage traces to their cells: mitochondria in skeletal muscle showed greater respiratory capacity, letting oxygen-hungry processes keep running where oxygen is scarce. Shivering muscle and brown fat supply the warmth. Genetic analysis pointed to changes tuning energy metabolism and blood-vessel regulation under chronic low oxygen, and hinted that the animals' blood cells may hold extra carbon dioxide – which would reduce the risk of hyperventilating in thin air. "They're basically engaging all of their metabolic machinery toward the goal of maintaining a constant body temperature," Storz says.
The genetics turned up something unrelated to altitude, too: both highland and lowland mice appear equipped to detoxify compounds from poisonous plants, the kind of diet desert animals fall back on when little else grows.
Why the mice went up in the first place remains unanswered. Mummified remains found on three volcano summits, reported in 2023, suggest they are not accidental tourists but genuine residents. For McClelland, the lesson is broader than one rodent. "Evolution never ceases to surprise us," he told CBC News: look anywhere on Earth, including places that appear flatly hostile to life, and something has probably worked out how to thrive there. Storz puts the physical reality more plainly – unlike the mountaineers who go looking for them, mice cannot wear Gore-Tex jackets.