The mouse living atop a 6,739-meter volcano — cold and thin air weren't its only challenges

At 6,739 meters above sea level, the volcanic summit is nearly devoid of trees and grass. Temperatures stay below freezing for virtually the entire year.

A single breath delivers only 44 percent of the oxygen available at sea level — enough for a trained climber to endure a day-long ascent, but far too little for permanent human habitation. Yet a remarkable mouse has made that summit its home.

In the July 2026 issue of Science, a joint research team — including Schuyler Liphardt of the University of Montana and Naim Bautista of the University of Nebraska — published findings on the survival strategies of the Andean leaf-eared mouse (Phyllotis vaccarum), the world's highest-dwelling mammal.

The study was co-led by Jay Storz, a professor at the University of Nebraska, along with professors Jeffrey Good and Zachary Cheviron of the University of Montana and professor Guillermo D'Elía of the Universidad Austral de Chile.

The research team collected 167 mice across 33 sites in northern Chile between 2020 and 2023, including five high-altitude expeditions. The sites spanned a vertical range of more than 6,700 meters — from coastal sand flats in the Atacama Desert to the summits of five volcanoes exceeding 6,000 meters.

The team brought summit-caught and coastal-caught mice into the laboratory and raised them under identical conditions for more than two weeks. The goal was to eliminate differences shaped by environment and isolate only innate ones.

The researchers then simulated conditions equivalent to 7,000 meters above sea level inside a chamber and lowered the temperature to minus 5 degrees Celsius.

When an animal's body cools, it shivers to generate heat — a process that requires oxygen. When oxygen is scarce, the capacity to produce heat diminishes accordingly.

Under those conditions, the coastal mice showed a sharp drop in performance. The summit mice, by contrast, held up far better — despite belonging to the same species. While the coastal mice were becoming immobilized by the cold, the summit mice kept moving.

The difference came down to the calf muscles — the muscles used for shivering. The calf muscles of summit mice burned oxygen at a higher rate and were also more efficient at metabolizing fat.

That finding upended a long-standing hypothesis in the field. Researchers had long assumed that animals at high altitude fare better burning carbohydrates than fat, since carbohydrates require less oxygen.

Yet the summit mice were burning fat even at altitude — suggesting they use oxygen efficiently enough that conserving it is not a constraint.

The research team conducted a full genome-wide analysis of the mice to identify which genes had changed most significantly in relation to the altitude at which the animals lived.

As expected, genes involved in fat metabolism and oxygen use emerged — but so did genes responsible for breaking down toxins, a function with an entirely different purpose. In humans, equivalent genes are involved in metabolizing alcohol and drugs.

These toxin-processing genes had not changed only in the summit mice. The coastal mice showed the same alterations.

The research team attributed this to the mice's diet. True to their name, leaf-eared mice eat leaves — and the plants growing in coastal deserts are entirely different from those found near volcanic summits. Plants produce their own toxins as a defense against being eaten, and as the mice move to different elevations, the toxins they encounter change as well.

The research team concluded that these mice had not merely endured cold and low oxygen — they had also developed distinct physiological adaptations to digest the toxic plants they eat every day. Because the toxins differ by elevation, summit mice and lowland mice evolved along separate paths.

Invoking Darwin, the team said "the struggle for existence is almost exclusively a struggle against the conditions of nature," but added that what these mice face is not only cold and thin air — what they eat matters just as much.

DOI: 10.1126/science.aec8347

Schuyler Liphardt et al., "Adaptation across an extreme elevational gradient in Andean leaf-eared mice, the world's highest-dwelling mammal." Science 393, eaec8347 (2026).