How an Andean Mouse Conquers 22,000 Feet Without Boosting Its Blood

A rodent barely bigger than a human thumb spends its whole life above 6,739 meters on an Andean volcano, breathing air that holds only 44 percent of the oxygen found at sea level. A new genetic study finally explains how the Andean leaf-eared mouse pulls this off, and the explanation upends decades of assumptions about what survival at extreme altitude actually requires.

Researchers expected hemoglobin, the oxygen-carrying molecule that shaped how Tibetan highlanders and Andean humans adapted to thin air, to explain the mouse’s success too. It barely touched it. Instead, the animal rewired its metabolism and, in a twist nobody predicted, evolved the ability to detoxify toxic plant compounds in its diet.

A Mystery That Sat Unsolved for 114 Years

British zoologist Oldfield Thomas first collected the species, Phyllotis vaccarum, on Andean summits above 6,700 meters back in 1912, more than a kilometer past the altitude where biologists assumed mammalian life simply stopped.

“It was completely unexpected. People did not think mammals could survive at these altitudes, but they’re there,” said Graham Scott, a professor of animal physiology at McMaster University in Hamilton, Ontario, and one of the researchers behind the new study.

The find sat unexplained for over a century. To finally crack it, an international team collected more than 160 mice from a range of elevations, including specimens pulled from five separate Atacama volcanoes: Copiapó, Llullaillaco, Ojos del Salado, Púlar and Salín. Fourteen of those animals came from above 6,000 meters, including the record holder trapped alive on Llullaillaco’s 6,739 meter summit, on the Chile-Argentina border, where each breath carries less than half the oxygen available at sea level.

The team then combined adaptation across an extreme elevational gradient genomic analysis with muscle physiology tests, metabolic measurements and population genetics to figure out what actually separates a summit mouse from its sea-level cousins. Researchers studying the same volcano summits have also documented mummified mice found even higher than any living population survives, remnants preserved by cold so extreme that summer temperatures on Llullaillaco still drop well below freezing.

Built Like Marathon Runners, Not Sprinters

Physiological tests showed the highland mice generate unusual amounts of body heat while burning very little oxygen to do it. Their skeletal muscles are packed with far more mitochondria, the structures that convert fuel into usable cellular energy, than muscles from lowland relatives or a related lower-altitude species, Darwin’s leaf-eared mouse.

They’re more like a marathon runner than a sprinter. Their muscle cells are packed with mitochondria that allow them to sustain heat-producing activity for longer periods.

Scott made that comparison to describe the muscle biology his team found. Rather than leaning mainly on carbohydrates, the highland mice also shifted toward burning fat, a fuel source that keeps both shivering muscle and heat-generating brown fat tissue running steadily. Together, those changes let the animals hold a stable body temperature in conditions that hover near freezing almost year round.

The Hemoglobin Trick That Wasn’t

Jay Storz, a University of Nebraska-Lincoln evolutionary biologist and member of the research team, has spent much of his career making the opposite case in a different rodent entirely. In 2009, Storz and colleagues sequenced 75 wild deer mice from Colorado, thirty-eight from lowland grassland near the Kansas border and thirty-seven from the 14,345-foot summit of Mount Evans, and traced the highland animals’ oxygen-binding advantage to four separate hemoglobin gene mutations. That work helped cement hemoglobin as biology’s default explanation for how small mammals cope with thin air.

The new Andean leaf-eared mouse genome barely uses that playbook. Genetic changes turned up in genes tied to energy production, blood vessel regulation and long-term hypoxia tolerance, but not in hemoglobin itself.

Humans took the hemoglobin route in two opposite directions. Tibetan highlanders carry a Denisovan-derived fragment of the EPAS1 gene, sometimes nicknamed the superathlete gene, which blunts the usual hemoglobin surge instead of boosting it. Andean human highlanders went the other way, building thicker blood through excess red blood cell production, a condition called chronic mountain sickness that Peruvian physician Carlos Monge first described in 1925 among residents of Cerro de Pasco. That excess hemoglobin route carries a cost the mouse appears to have skipped entirely.

Researchers also found little evidence of large structural rewrites in the mouse genome. Evolution appears to have fine-tuned genes that already existed rather than reshaping the blueprint from scratch.

Toxic Plants, an Unexpected Twist

The volcanic slopes the mice call home look almost empty of life. Yet the animals get by on sparse alpine vegetation, lichens and whatever else clings to the rock, and many of those plants carry defensive chemicals that are toxic to most mammals.

“We were initially focused on the most obvious environmental challenges, things like low oxygen and cold, but there were important factors we didn’t expect, including how these animals deal with what they’re eating,” Scott said of the genome analysis.

The team found signs of natural selection acting on genes involved in breaking down plant toxins, a detoxification upgrade that likely lets the mice process compounds that would poison many other mammals.

• Energy production – denser mitochondria and higher aerobic capacity in muscle tissue
• Fuel switching – a shift toward burning fat instead of carbohydrates for heat
• Circulatory changes – genetic adjustments tied to blood vessel regulation under chronic low oxygen
• Detoxification – selection on genes that break down toxic plant compounds

Those four categories, taken together, are what the researchers describe as the suite of adaptations behind the mouse’s survival, and the detox piece is the one nobody on the team expected to find.

Same Genes, Wildly Different Zip Codes

High-altitude and low-altitude Phyllotis mice still interbreed constantly, moving genes back and forth across elevations that differ by kilometers. Despite that steady mixing, the adaptations needed for life near the summit have persisted, which tells researchers that natural selection keeps re-favoring them generation after generation.

The same research network has documented this kind of gene flow before. A related 2025 paper, listed among a study on Phyllotis hybridization across species boundaries, found similar mixing patterns in the broader leaf-eared mouse group, reinforcing the picture of a species that refuses to split cleanly along elevation lines even while adapting sharply to it.

Could a Volcano Mouse Help Cancer Research?

Possibly, though only indirectly for now. The mouse’s newly discovered hypoxia-tolerance genes work through some of the same biological pathways that let tumors survive in oxygen-starved tissue, and scientists already study a related human gene for clues about cancer risk, though no treatment has emerged from either line of work yet.

“Evolution is a complex process. When animals encounter really challenging environments, there are a lot of different things they need to cope with, not just the obvious ones,” said Grant McClelland, a professor of biology at McMaster University and one of the study’s researchers.

The study has real limits. Researchers still do not know exactly what the mice eat across a full year, or precisely how each detoxification gene contributes to survival. Future field expeditions will look more closely at the animals’ diet and test whether these genetic changes let the mice directly exploit toxin-laden food sources most mammals would avoid.

Frequently Asked Questions

What Is the Highest Altitude Ever Recorded for a Wild Mammal?

A live Andean leaf-eared mouse was captured at 6,739 meters on the summit of Volcán Llullaillaco, the highest confirmed elevation ever documented for a wild mammal. That is roughly 1,400 meters higher than La Rinconada, Peru, a mining town often described as the highest permanent human settlement on Earth.

What Do Andean Leaf-Eared Mice Eat on a Barren Volcano?

A gut content analysis of a summit mouse captured on Llullaillaco found an opportunistic but purely herbivorous diet built around lichens. That result favors the lichen theory over a competing idea, sometimes called the arthropod fallout hypothesis, that the mice survive mainly on wind-blown insects trapped in snowdrifts.

Are Andean Leaf-Eared Mice Related to Common House Mice?

Not closely. Phyllotis vaccarum belongs to a South American rodent genus distinct from Mus musculus, the common house mouse. Its nearest relatives in this research include Darwin’s leaf-eared mouse, a lower-elevation species researchers used for genetic comparison, along with several other Phyllotis species sampled from Chile and Peru.

How Long Did the Andean Mouse Study Take to Complete?

The project spanned nearly five years, combining repeated mountaineering expeditions across Atacama volcanoes with laboratory work that included whole genome sequencing, hypoxia chamber experiments, muscle analysis and population genetics on more than 160 collected mice.

Could This Mouse Research Help Treat Human Diseases Like Cancer?

It could eventually offer clues, though nothing clinical exists yet. Scientists already study variants of the Tibetan EPAS1 gene, tied to the hypoxia-inducible factor 2 alpha pathway, for a possible link to lung cancer risk in Nepalese patients, since the same pathway that helps bodies cope with low oxygen also shapes how tumors grow in oxygen-starved tissue.