There was a tiny hero who appeared on TV in the 1990s and won the love of Korean children. It was Mighty Mouse, who wore a red cape like Superman, flew through the sky, and defeated villains. 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. This mouse, the mammal that lives at the highest place on Earth, was found to endure by eating toxic plants in oxygen-poor, subzero cold.
A research team led by Jay Storz, a professor in the School of Biological Sciences at the University of Nebraska in the United States, said on the 10th in the international journal Science that "in the highlands of the Andes in South America at elevations over 6,700 meters, the leaf-eared mouse Phyllotis vaccarum has evolved to simultaneously handle not only low oxygen and cold but also the toxicity of its prey."
In 2020, Professor Storz captured a live leaf-eared mouse at the 6,739-meter elevation zone of the Llullaillaco volcano, which straddles the Chile-Argentina border. No mammal has ever been found inhabiting such a high elevation. Until then, the animal holding the high-altitude survival record was pikas, a lagomorph discovered a century ago at the 6,130-meter elevation zone of Mount Everest.
◇ Forced oxygen supply through hyperventilation, maintaining heat production capacity
The place where this small mouse, also called the Andean leaf-eared mouse, lives is called Earth's Mars by the scientific community. The oxygen concentration is only 44% of that at sea level, and the temperature is always below freezing. That is why NASA tests Mars exploration equipment here. Scientists thought mammals could not survive at elevations over 6,000 meters. The leaf-eared mouse's survival ability is literally Mighty Mouse level.
The researchers 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. The experiments found that leaf-eared mice living at high elevations did not show a significant drop in heat production even under conditions where the oxygen concentration was lowered to the equivalent of 4,300 meters and 7,000 meters. The leaf-eared mouse endures subzero cold by generating heat through shivering in the hind leg muscles.
When oxygen is scarce, it is difficult to burn fuel in the body's engine. People or animals living at high altitudes have hemoglobin in the blood with high oxygen affinity. That means they have excellent ability to supply oxygen to the body's engine. But the hemoglobin of the leaf-eared mouse did not differ from other animals in oxygen affinity.
Instead, the mouse hyperventilated the mitochondria that produce energy in the hind leg muscles. That means it forcibly increased oxygen supply. When you take in a lot of oxygen through hyperventilation, that much more carbon dioxide leaves the body. Then the blood suddenly turns alkaline, which is harmful to the body.
The mouse chose to lower the activity of carbonic anhydrase (CA) in red blood cells. This enzyme is a catalyst that converts carbon dioxide into a gaseous form that is easier to expel from the body. The leaf-eared mouse effectively suppressed exhaust emissions to forcibly supply oxygen to the engine.
◇ Burning logs instead of kindling, and enduring toxic plants
The leaf-eared mouse also used different fuel. In places with little oxygen, organisms mainly use carbohydrates as an energy source. Carbohydrates produce more energy per the same amount of oxygen, making them economical in terms of oxygen use. In other words, it is like using thin kindling that catches fire easily. Thanks to forced oxygen supply, the leaf-eared mouse was found to be able to burn fat, which has a higher energy density. In other words, it is like burning whole logs instead of axe-split kindling.
Then where does the leaf-eared mouse get its fuel? In high mountains there is nothing to eat but grass. The problem is that plants the leaf-eared mouse eats, such as those in the amaryllis family and the mallow family, harbor potent toxins to protect themselves from predators. The leaf-eared mouse strongly evolved specific enzyme genes in the liver to detoxify the toxic substances in grasses. That does not mean all the problems are solved.
Denise Dearing, a professor in the School of Biological Sciences at the University of Utah, summarized the significance of this study under the title "To eat or to breathe?" in a commentary published the same day in Science. She said, "In the leaf-eared mouse, the hypoxia response and the detoxification response are not completely separate issues and can conflict because they share some molecular regulators."
In the mouse's body, the molecular pathway that responds to hypoxia and the pathway that detoxifies toxins both share a protein called ARNT2. If ARNT2 is used to breathe due to a lack of oxygen, the ability to detoxify toxins declines, and conversely, if it focuses on detoxification, adaptation to hypoxia weakens. Yet when it goes to the lowlands where non-toxic grasses grow, other animals already occupy them.
Dearing assessed that the leaf-eared mouse is not simply an animal that endures oxygen scarcity but has adapted under complex evolutionary pressures that require it to handle the toxic substances of food plants that vary with elevation. High-altitude survival is an exquisite balancing act between "breathing" and "eating." In other words, it is like two core apps (applications) splitting a computer's limited RAM capacity by the 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