There was a tiny hero who appeared on TV in the 1990s and won the love of Korean children. Like Superman, it wore a red cape, flew through the sky, and defeated villains: Mighty Mouse. Scientists have found a Mighty Mouse in the Andes. It is a mouse that lives on a volcano at an elevation of 6,739 meters. Living at the highest altitude of any mammal on Earth, this mouse appears to endure oxygen scarcity and subzero cold while eating poisonous plants.
A research team led by Jay Storz, a professor in the School of Biological Sciences at the University of Nebraska, said on the 10th in the journal Science that Phyllotis vaccarum, the leaf-eared mouse of Punta de Vacas, which lives at elevations above 6,700 meters in the Andes of South America, has evolved to simultaneously handle not only low oxygen and cold but also the toxicity of its prey.
In 2020, Professor Storz spotted a living leaf-eared mouse on the Llullaillaco volcano, which spans the Chile-Argentina border, at an elevation of 6,739 meters. No mammal has ever been found living at such a high altitude. Previously, the animal holding the altitude survival record was pikas, lagomorphs found a century ago at 6,200 meters on Mount Everest.
◇ Forcing oxygen supply through hyperventilation, maintaining heat production
The place where this small mouse, also called the Andean leaf-eared mouse, lives is called Earth's Mars in the scientific community. Oxygen concentration is only 44% of that at sea level, and temperatures are constantly below freezing. NASA tests Mars exploration equipment here for that reason. Scientists thought mammals could not survive at elevations above 6,000 meters. The leaf-eared mouse's survival ability is, literally, on the Mighty Mouse level.
The team captured 167 leaf-eared mice from lowlands and highlands for experiments. A closely related species (Phyllotis darwini) that lives only in lowlands was also compared. They decoded the mice's genes and analyzed which enzymes operate under high-altitude conditions. Experiments showed that leaf-eared mice living at high altitudes did not see a significant drop in heat production even when oxygen levels were reduced to the equivalent of 4,300 meters and 7,000 meters. The leaf-eared mouse endures subzero cold by generating heat through shivering in its hind-leg muscles.
When oxygen is scarce, it is hard to burn fuel in the body's engine. People and animals living at high altitudes have hemoglobin in their blood with high oxygen affinity. That means they are good at supplying oxygen to the body's engine. But the leaf-eared mouse's hemoglobin did not differ from other animals in oxygen affinity.
Instead, the mouse made the mitochondria that produce energy in its hind-leg muscles hyperventilate. That means it forcibly increased the oxygen supply. When you take in a lot of oxygen through hyperventilation, that much more carbon dioxide leaves the body. Then the blood suddenly becomes alkaline, which is harmful to the body.
The mouse chose to reduce the activity of carbonic anhydrase (CA) in red blood cells. This enzyme catalyzes the conversion of carbon dioxide into a gaseous form that is easy to expel from the body. To forcibly supply oxygen to the engine, the leaf-eared mouse also suppressed the emission of exhaust gases.
◇ Burning logs instead of kindling, and withstanding poisonous plants
The leaf-eared mouse used a different fuel as well. In places with low oxygen, animals mainly use carbohydrates as an energy source. Carbohydrates produce more energy per unit of oxygen, making them economical in terms of oxygen use. In other words, it is like using thin kindling that burns easily. Thanks to forced oxygen supply, the leaf-eared mouse was able to burn fat, which has high energy density. In other words, it is like burning whole logs instead of split kindling.
Then where does the leaf-eared mouse get its fuel? In highlands, there is nothing to eat but grass. The problem is that the plants it eats, such as those in the amaryllis and mallow families, harbor potent toxins to protect themselves from predators. The leaf-eared mouse has strongly evolved specific enzyme genes in the liver to detoxify the plants' toxic compounds. That does not mean all the problems are solved.
Denise Dearing, a professor in the School of Biological Sciences at the University of Utah, summed up the significance of the study with the title "To eat or to breathe?" in a commentary published in Science the same day. Dearing said, "In leaf-eared mice, the hypoxia response and the detoxification response are not completely separate issues; they can conflict because they share some molecular regulators."
In the mouse's body, the molecular pathway responding to hypoxia and the pathway detoxifying toxins both share a protein called ARNT2. If ARNT2 is used to breathe when oxygen is scarce, detoxification capacity declines; conversely, if the body focuses on detoxification, adaptation to hypoxia weakens. Yet if they move down to lowlands where non-toxic grasses grow, those areas are already occupied by other animals.
Dearing assessed that the leaf-eared mouse has adapted not merely to withstand oxygen scarcity but under a complex evolutionary pressure that also requires processing the toxins of food plants that vary with altitude. High-altitude survival is an exquisite balance between "breathing" and "eating." In other words, it is like two core apps sharing a computer's limited RAM, sliced up every second. Not just anyone becomes Mighty Mouse.
References
Science (2026), DOI: https://doi.org/10.1126/science.aec8347
Science (2026), DOI: https://doi.org/10.1126/science.aei7713
Current Biology (2023), DOI: https://doi.org/10.1016/j.cub.2023.08.081