Thirteen mouse mummies were found atop volcanoes in Chile and Argentina about 20,000 feet above sea level.
The discovery has baffled scientists who did not think mammals could live at such high elevations.
They are now researching if the mice had physiological adaptations that allowed them to survive.
The discovery of 13 mummified mice atop three volcanoes in Chile and Argentina has scientists scratching their heads.
It's unclear why the tiny mammals would climb to such high, inhospitable altitudes.
The mummies were discovered at about 20,000 feet above sea level, where temperatures don't exceed freezing, the air contains less than half of the oxygen animals get at sea level, and gale-force winds pummel a barren landscape of rock, ice, and snow.
"The summits and upper reaches of [these] volcanoes are incredibly hostile environments, especially for small, warm-blooded animals like mice," Jay Storz, a biologist at the University of Nebraska—Lincoln, who led a study of the mouse mummies, told Insider in an email.
The mummies are leaf-eared mice, which are known to live at high elevations. But warm-blooded life at 20,000 feet is unheard of.
"Such extreme elevations were previously assumed to be completely uninhabitable by mammals," the study said.
The mummies suggest these mice have been reaching outlandish heights for centuries or longer, according to Storz, since this species "has inhabited the high Andes for many hundreds of thousands of years," Storz said. "So they could have been frequenting the volcano summits for quite some time."
They seem to still be living there in small numbers. The research team found a living mouse at those high altitudes, as well as evidence of active burrows, Storz said.
"There is still a lot left to discover," Storz said.
The live mouse and the summit mummies "represent the highest altitude physical records of mammals in the world."
The study was published Monday in the journal Current Biology.
Could these mice have special adaptations?
Storz stumbled upon the first mouse mummy by chance. Soon he and his colleagues were finding more and more freeze-dried mouse cadavers in the crevices of volcanic rock.
The mummies were mostly a few decades old, but some dated back several centuries.
Genomic analysis of the mummies showed that they were equal numbers of male and female mice and that some were closely related. That suggests the mice actually established communities at the tops of the volcanoes, rather than a few individuals randomly climbing up there.
Storz told Insider that the only advantage he can think of for the mountaineering mice is that there are no predators on top of the volcanoes.
In addition to the mice's motives, it's a mystery how they manage to survive.
"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 said in a statement, comparing the summit environment to Mars.
The scientists are now looking for signs of physiological adaptations that may allow these mice to survive at high elevations with low oxygen levels.
They collected mice from different elevations and are now experimentally exposing them to low oxygen to see whether the extreme-altitude mice show any unique physiology.
Storz's team is also continuing to search the volcano tops for signs of mice, living or mummified.
"Whether it's the top of the highest mountains or the darkest depths of the ocean, it is always exciting to discover animals living in places where nobody expects anything to be living," Storz said.
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