Near the summit of Chile's Andes Mountains, where the air is so thin it feels like breathing through a straw and temperatures plunge far below freezing, a tiny mouse is doing something scientists once thought impossible.
The Andean leaf-eared mouse has been found living nearly 7,000 meters (23,000 feet) above sea level — that's higher than any mountain climber would dare climb without oxygen tanks, and more than a kilometer (0.6 miles) beyond what scientists believed was the absolute limit for mammalian survival.
"It was completely unexpected," says Graham Scott, a biology professor at McMaster University who co-authored a new study about the discovery. "People did not think mammals could survive at these altitudes, but they're there."
For decades, scientists assumed that about 5,500 meters was the ceiling for mammal life. That is roughly the altitude of the highest permanent human settlements in the Himalayas. But the new research, published in the journal Science, reveals that these little mice not only survive at nearly 7,000 meters — they thrive.
Scott and colleague Grant McClelland joined an international team to study how the mice manage such an extreme existence. The environment at that height is brutal: freezing cold, barely any oxygen, almost no liquid water, and virtually no plants growing. Scientists often compare it to the conditions on Mars.
What they found was remarkable. The mice have evolved an entire toolkit of adaptations that work together. Their muscle cells are packed with extra mitochondria — the tiny powerhouses that fuel cells — making these creatures more like marathon runners than sprinters. This lets them keep generating heat continuously instead of just in short bursts.
"Their muscle cells are packed with mitochondria that allow them to sustain heat-producing activity for longer periods," Scott explains.
The mice also burn fat more efficiently than their lowland relatives, which provides energy for both shivering and specialized heat-making tissues.
But perhaps the biggest surprise came from the mice's diets. At those heights, food is incredibly scarce. The mice survive on lichens — simple organisms that grow on rocks — and possibly seeds or insects blown up by the wind. Genetic analysis revealed the mice have evolved special changes to digest these unusual foods and even detoxify compounds that would make other animals sick.
"We were initially focused on the most obvious environmental challenges, things like low oxygen and cold, but there were important factors we didn't expect, including how these animals deal with what they're eating," says Scott.
The researchers say the discovery shows evolution is more creative than scientists gave it credit for.
"Sometimes our assumptions about the most extreme environments animals can live in can be questioned," says McClelland. "Evolution has a lot of room to experiment."
That message may matter beyond just mouse science. As climate change forces species to adapt to new conditions, this tiny mountaineer offers a reminder that life is more resilient — and more surprising — than we often assume.