"The most surprising thing about our discovery is that mammals could be living on the summits of volcanoes in such an inhospitable, Mars-like environment," says senior author Jay Storz, a biologist at University of Nebraska, Lincoln.

"Well-trained mountain climbers can tolerate such extreme elevations during a one-day summit attempt, but the fact that mice are actually living at such elevations demonstrates that we have underestimated the physiological tolerances of small mammals."

Storz and his colleagues discovered the first mouse mummy on the summit of Volcán Salín by chance when they stumbled across the desiccated cadaver at the edge of a rock pile. But, now knowing what to look for, they soon turned up others.

"Once my climbing partner and I started searching through the rest of the rocks, we found seven more mummies on the same summit," Storz recalls.

They then started searching systematically on the summits of all the Andean volcanoes. So far, they've searched 21 volcano summits, including 18 with elevations over 6,000 meters. All told, they've found 13 mummified mice on the summits of multiple volcanoes with an elevation greater than 6,000 meters. In some cases, the mummies were accompanied by skeletal remains of numerous other mice.