Mice at Four Miles Up
Thirteen mummified leaf-eared mice (Phyllotis vaccarum) have been pulled from the summits of three Andean volcanoes, nearly 4 miles above sea level, in country that looks more like Mars than anywhere else on the planet. The finds came from Salín, nearly 19,800 feet (6,029 m), Púlar, over 20,400 feet (6,233 m), and Copiapó, nearly 19,900 feet (6,052 m), all in the Puna de Atacama on the Chile-Argentina border. A research team led by evolutionary biologist Jay Storz of the University of Nebraska-Lincoln, working with colleagues at the University of Montana, published the discovery in Current Biology in 2023. Live mice of the same species turned up on those same peaks, and on nearby Volcán Llullaillaco, at 22,000 feet (6,739 m). That is the highest elevation any mammal has ever been documented living at, and it beats every previous small-mammal altitude record by a wide margin. This is not “mice.” It is one named Andean species, at an altitude where the air holds roughly a third of the oxygen it does at sea level.
The Freeze That Kept Them Whole
Radiocarbon dating, which reads the steady decay of carbon-14 in once-living tissue to estimate how long ago something died, gave the mummies an age nobody expected. Eight of the mice on Salín and one on Copiapó died within the last few decades, likely after 1955. The four on Púlar are older but still recent: at most 350 years, putting their deaths up to a century after the Inca empire had already collapsed. None of this reaches back to any ancient ritual era. What kept the bodies from rotting is simpler: cold, dry, oxygen-starved air acts like a natural freeze-dryer at that altitude, slowing decomposition to almost nothing and leaving both tissue and DNA intact enough to sequence, decades or centuries later.
An Established Neighborhood, Not a Few Stragglers
Genomic comparison across lowland, midland, and summit mice from the Atacama turned up no separate “mountain” lineage. Genetically, the mice on the volcano tops are the same population as relatives living thousands of feet lower; nothing unusual split off to make that climb possible. What did stand out inside the group of thirteen: two pairs of the Salín mummies were closely related, likely siblings or a parent and offspring, and the male-to-female ratio across all the mummies came out roughly even. Put those two things together, family pairs and a balanced sex ratio, and you no longer have a handful of lost travelers. You have a breeding colony, living and reproducing near the summit, generation after generation.
The Inca Myth Runs Out of Road
Archaeologists who first found mummified mice on these ritual peaks in the 1970s and 1980s assumed the animals had been carried up by Inca pilgrims, tucked into firewood or supplies, or included as small companions in Capacocha sacrifice rites. The radiocarbon dates take that idea apart on their own: a large share of these mummies died a century or more after the Inca empire fell, well past any period of ritual traffic to these summits. The genomic evidence finishes it. Mice transported by people would be expected to show up as a distinct, isolated lineage, carried in and stranded. They don’t. They share DNA freely with mice living far below. Nobody carried them anywhere. They walked.
What Still Isn’t Known
The harder question has no answer yet: why would a mouse climb into one of the most hostile environments on Earth in the first place? Storz has called it a genuine mystery, and described the landscape around these summits as Martian even by the standards of the surrounding desert, hard to believe anything could survive there at all. Predator avoidance, escaping foxes and birds of prey that hunt at lower elevations, is one proposed explanation, but it remains unconfirmed. Nor is it clear how a rodent weighing about 2 ounces tolerates that combination of cold, near-zero oxygen, and almost no vegetation for extended stretches. Researchers are now raising captive leaf-eared mice under simulated high-altitude conditions in the lab, trying to work out the physiology piece by piece.
The Day a Mouse Came Out of a Rock Alive
In early 2020, Storz climbed Llullaillaco with mountaineer Mario Pérez Mamani. At the 22,000-foot summit, they found a mouse alive, wedged into the rock, and pulled it out by hand. It was the first mammal ever documented living at that altitude, anywhere. Before that morning, every mouse anyone had found on these peaks had been a mummy, assumed to be someone else’s cargo. This one wasn’t cargo. It was breathing, and it had gotten there entirely on its own.
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