High up in the Andes Mountains, the world is a barren mix of rock, snow and ice. Temperatures never rise above freezing. The air is so thin, it’s hard to breathe. At 20,000 feet above sea level, researchers had concluded the environment was simply too hostile to host much life.
But then, around ten years ago, a group of mountaineers summiting the 22,000-foot-high Volcán Llullaillaco, a volcano on the border of Chile and Argentina, spotted something odd: a small rodent skittering across the snow. Scientists summited the peak once more in 2020 to confirm the find, and they managed to capture a living yellow-rumped leaf-eared mouse. The creature quickly made headlines as the world’s highest-dwelling mammal, eking out a life more than 2,000 feet above the highest-altitude plants in the area.
“For miles around, it’s this really austere, Martian-like landscape, a hostile environment, and then the summits of these volcanoes are even more hostile,” Jay Storz, an evolutionary biologist at the University of Nebraska–Lincoln who led the 2020 research, tells Meghan Bartels of Scientific American. “When you experience these environments firsthand on the summits of these volcanoes, it’s just mind-boggling that [mammals] could be living up there.”
Since then, Stolz and his colleagues expanded their search for high-altitude mice across 21 peaks in the Andes. In a new study published Monday in Current Biology, they report uncovering the mummified remains of 13 leaf-eared mice on the summits of three different volcanoes. The findings suggest the mouse captured three years ago was not a one-off fluke, but rather a sign that rodent communities have routinely made homes in these harsh conditions.
“This is truly surprising and challenges our previous assumptions about the adaptability of species to extreme environments,” Emmanuel Fabián Ruperto, a behavioral ecologist at the Argentine Institute for Dryland Research who was not involved in the study, tells Anil Oza of Nature News. “Food availability at such altitudes [is] virtually non-existent… so, what do these animals feed on?”
The study raises a multitude of other questions, too, including how and why mice arrived atop volcanoes to begin with. Radiocarbon dating suggested the mice on two of the volcanoes, Salín and Copiapó, were just a few decades old, while the samples from Volcán Púlar were at most 350 years old, per the study.
Archaeologists had previously suggested the mice were brought to the volcanoes—intentionally or unintentionally—by the Incas, who were known to make pilgrimages to these sites for human and animal sacrifices. But the new study indicates the Incas were visiting these regions hundreds of years earlier than the discovered mice lived.
Because the animals were essentially freeze-dried, researchers were able to extract their DNA and compare it to the genetic material of other leaf-eared mice living in the lowland and midland Atacama region. They wondered whether the high-dwelling rodents were a distinct subpopulation.
“Our genomic data indicate no: that the mice from the summits, and those from the flanks or the base of the volcanoes in the surrounding desert terrain, are all one big happy family,” Storz says in a statement. This further suggests that the animals had traveled there of their own accord. Still, why would they seek out an environment so inhospitable that NASA has used it as a training ground in its search for life on Mars?
“It just boggles the mind that any kind of animal, let alone a warm-blooded mammal, could be surviving and functioning in that environment,” Stolz says in the statement, adding that the team plans to continue searching for answers and digging into the physiology of these tiny, two-ounce mice. “How in God’s name is anything living up there?”