The animal is the Andean leaf-eared mouse, Phyllotis vaccarum. A living specimen was photographed on the summit of Llullaillaco, on the Chile–Argentina border, and researchers have traced members of the same species down all the way to sea level — giving it both the highest altitude and the widest elevational range of any known mammal.
At that height, the air contains only about a third of the oxygen available at sea level, and temperatures plunge well below freezing. A human unacclimatized would quickly suffer altitude sickness; most mammals simply cannot generate enough heat to stay alive. The leaf-eared mouse does not just survive — it thrives.
The newly published study, led by evolutionary biologist Jay Storz of the University of Nebraska–Lincoln with an international team, traced the answer down to the mitochondria — the tiny energy plants inside every cell. Compared with their lowland relatives, high-altitude leaf-eared mice carry mitochondria that produce far more heat per unit of oxygen, the same kind of efficient energy economy found in marathon runners. This thermogenic capacity lets them stay warm without needing to eat constantly, even when food is scarce on a windswept volcanic ridge.
The mouse also shows broader metabolic flexibility: it can draw more oxygen into the bloodstream when needed and adjust how its tissues use that oxygen. Rather than relying on a single dramatic adaptation, it combines several smaller shifts — in oxygen-carrying proteins, cellular respiration, and heat production — that together make extreme altitude manageable.
Why does this matter beyond the record books? Because it shows evolution can solve the problem of low oxygen in more than one way. High-altitude humans and Tibetan populations, for example, often adapt by carrying more red blood cells. The leaf-eared mouse took a different route, leaning on how its cells burn fuel. Studying it could inform how life endures in thin-air environments — including the design of life-support for high-altitude work and, by analogy, the very different physics of spaceflight.
Knowledge takeaway: The Andean leaf-eared mouse lives above 6,700 meters — higher than Everest. High-altitude individuals carry mitochondria with exceptional heat-production efficiency and flexible oxygen use. The study, published in Science, shows that surviving extreme altitude is about optimizing cellular energy, not just breathing more.