Mice thrive at 6700 meters up—higher than any mammals were thought able to live

Survey turns up several mouse species living on Andes peaks, breaking records for the highest dwelling mammals

Few places are as inhospitable as the top of Llullaillaco, a 6700-meter volcano on the border between Chile and Argentina. Winds howl nonstop and no plants live there; daytime temperatures never get above freezing and plummet even more come nightfall. Oxygen levels are just 40% of those at sea level, too low for mammals to live there —or so biologists thought until 3 years ago when a research team captured a live leaf-eared mouse at its summit. Now new work shows this animal was not a fluke. The team has found other leaf-eared mice on additional volcano tops, and genomic studies of these summit dwellers and their lower elevation relatives confirm the rodents make their homes nearly 7000 meters above sea level, making them the highest dwelling vertebrate found so far. (Some birds soar higher but appear not to dwell at those elevations.) The team has also come across five other mouse species living above 5000 meters on various mountains in the Central Andes.  

The genomic results and other evidence reported today in Current Biology  “lay to rest any doubt that mammals live at these really extreme altitudes,” says Grant Mcclelland,  a comparative physiologist at McMaster University who was not involved with the work. “It expands our understanding of the environmental limits of animals, especially mammals.”

The cold temperatures and low oxygen associated with high altitudes have long been thought to set a limit on the heights where cold-blooded and even warm-blooded animals can permanently live. “Mammals in general are not very good at dealing with low oxygen environments,” says Catherine Ivy, a comparative physiologist at the University of Western Ontario. They require oxygen to convert food into energy and the colder the environment, the more energy they need. So, whereas a 1000-kilogram hairy yak can thrive at 5000 meters, small animals living at those heights shed heat faster and were expected to have trouble generating enough energy to keep warm, says Sahas Barve, an evolutionary ecologist at  Archbold Biological Station. (The previous elevation record holder for mammals was pikas, a rabbit relative, found nearly 6200 meters up on Mount Everest a century ago).  

Hints that some rodents could live at even higher altitudes emerged 50 years ago, when archaeologists studying Inca religious sites in Andes mountain summits came across mice naturally mummified by the dry cold. But researchers assumed the rodents were not local and had instead hitchhiked a ride with Inca people visiting the remote sites. Then in 2013, two mountaineers summitting Llullaillaco filmed a mouse scurrying across the snow. That and other mouse sightings by climbers prompted Jay Storz, an evolutionary biologist at the University of Nebraska—Lincoln who specializes in high-altitude adaptations, to investigate. He; Guillermo D'Elía, a systematist and mammalogist at the Austral University of Chile; and other South American colleagues went on their first 3-week expedition in 2020. D’Elía had to leave the trip early, so Storz sent him a text message about catching the record-high, leaf-eared mouse. “I couldn’t believe it,” D’Elía recalls.  

On that expedition and four subsequent forays up 21 peaks in the Central Andes, they and colleagues found the mummified remains of 13 leaf-eared mice above 6000 meters and photographed or caught dozens of live members of the species (Phyllotis vaccarum), as well as other mouse species at above 5000 meters. Overall, the group trapped almost 500 mice—about half were P. vaccarum—representing 18 species. For five species, the collected specimens established new elevation records, the team reported on 23 August in bioRxiv.

For the humans, the extremely cold nights and low oxygen levels made sleep and concentration difficult and slowed their progress. Even so, “every new place we have a new mix of species,” Storz says. “It’s a little bit like the deepest depths of the ocean.”

The new Current Biology study focuses on genomes of these mice. Jeffrey Good, an evolutionary genomicist at the University of Montana, and colleagues sequenced P. vaccarum from different elevations in depth. The mice have not diverged much genetically, they found. “I’m surprised that one species goes from sea level to 6739 meters,” D’Elía says. Biologists often find that along such a large elevation gradient, an animal specializes into subspecies and even new species. Storz thinks the leaf-eared mouse originally evolved in up high and is gradually expanding into lower elevations.

Still, the DNA analyses did reveal some differences within the species. Leaf-eared mice living above 6000 meters had genomes more similar to one another than to their lower-dwelling kin, the team reports. “It’s really cool to see that they were closely related,” Ivy says. “It helps solidify that these mice were actually breeding and residing at these altitudes.” 

A view from the summit of Volcán Salín (6029 meters) where researchers found mummified leaf-eared mice.JAY STORZ

Other lines of evidence back this conclusion. The team found several burrows above 6000 meters, and soil samples taken along paths thought to be traveled by mice revealed the DNA of rodent-specific microbes there, but not off those paths. And radiocarbon dating of the naturally mummified mice showed they lived no more than 350 years ago—long after the Incas stopped visiting the summit.

These mice “likely inhabit these locations permanently rather than being temporary visitors,” agrees Matthew Webster, an evolutionary geneticist at Uppsala University.

Because the mice live at least 650 meters above the volcanoes’ vegetation line—and sometimes thousands of meters above—what they eat is a mystery. A preliminary analysis of the DNA in the animal’s stomachs suggests lichens are big part of their diet, Storz says.

It’s also not clear how the mice stay warm. Early data suggest the Andes mice do not have the same genetic alterations that help deer mice living 4350 meters high in the Rocky Mountains cope with the cold, Storz says. He and colleagues have established a lab colony of the Llullaillaco rodents and other high-altitude mice in Chile to study their unique physiologies.

Whatever their adaptations, “I might add leaf-eared mice to my list of ‘extremophiles’ normally reserved for microorganisms,” Mcclelland says. Their existence shows it’s important “to not underestimate the capacity for species to invade and adapt to seemingly inhospitable environments.”

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