In February, scientists found a mouse living at the 22,110 foot high summit of Llullaillaco, the highest historically active volcano in the world. And a study published this week reports that mouse is the highest-dwelling mammal in the world. The record comes less than a year after it was previously set—by another mouse of the same species, at the same volcano, National Geographic reports. That yellow-rumped leaf-eared mouse was found last summer at 20,340 feet, in the upper reaches of the volcano that straddles Argentina and Chile. "It’s hard to overstate the hostility of the environment" at that elevation, says the biologist who organized the February expedition. There's no vegetation, little rain, and apparently nothing to eat; temperatures can get below minus 75 degrees Fahrenheit, and oxygen is so low researchers could only remain on the summit for a few minutes. New Scientist compares it to the surface of Mars.
"Nobody expected mice to be living that high," the biologist says. The yellow-rumped leaf-eared mouse was known to be living in the foothills and mountains of the Andes, sometimes as low as sea level, giving it an "unprecedented" 22,000-foot elevation range, per NatGeo. "No other species does that," says one expert. Llullaillaco is at the edge of the Atacama Desert and offers particularly unforgiving conditions—scientists are now attempting to understand just how exactly the mice survive them. They caught mice from four different species at various elevations, including the one found at the summit. Scientists think they might survive by eating bits of debris blown around by the wind, possibly insects and lichens, but they may also have slower muscle metabolism and a cardiovascular system that has adapted to such extreme conditions. Whatever scientists discover could have implications for human diseases related to oxygen, including heart disease and lung conditions. (Read more discoveries stories.)