Mammals inhabit a wide variety of environments on Earth, but only a few are capable of surviving in the harsh climate of high-altitude regions, characterized by low oxygen levels and subzero temperatures. Thanks to the recent discovery of a species of mice capable of surviving at elevations above 6,000 meters above sea level, University of Oklahoma professor Naim M. Bautista is trying to determine how these rodents adapted their physiology to such conditions.

He is one of the co-authors of a study published in the journal Science. This species is known to inhabit the northern coast of Chile up to the Andes in Argentina. At present, these are the highest-altitude mammals known to researchers, as they live at elevations where oxygen levels are so low that even well-trained and acclimatized mountaineers can remain there for only one day.

In 2020, scientists traveled to the Andes to collect these mice for study. The research examined both high-altitude representatives of the species and mice living at sea level, and also modeled the effects of different elevations above sea level on both groups, according to Nauchnaya Rossiya.

“We focused on physiological measurements related to the body’s ability to produce heat,” Bautista said. “We wanted to understand how these animals can exist there, how they tolerate low oxygen levels and cold stress, how they breathe, how their hemoglobin functions, and how oxygen-saturated their blood is. We wanted to learn everything about these animals.”

Among the discoveries made by the researchers was the finding that mice collected at high elevations have two clear local adaptations that distinguish them from their relatives living at lower elevations. One is the ability to generate more heat through shivering of skeletal muscles, which is an important trait for life in such cold conditions. The second is an unexpected environmental adaptation: “These mice have genes that allow them to process plant-derived toxins,” Bautista says. This means that selection associated with altitude is largely driven by previously unstudied aspects of their dietary ecology.

Another important discovery is that populations of Andean leaf-eared mice living at different elevations show very weak genetic differentiation. “Instead of being several genetically differentiated subpopulations of one species, the entire species behaves like one large population,” Bautista says.

Because of the lack of genetic differentiation, it is difficult for population-specific trait differences to evolve, since gene variants that provide local advantages must compete with other gene variants. But because mountain mice have developed physiological traits that distinguish them from lowland mice, the pressure of natural selection in favor of high-altitude traits is strong enough to counteract the homogenizing effect of gene flow from lowland populations.

Although the study’s results have already been published, Bautista and his team continue to study the Andes. They plan to travel to the eastern, Argentine side of the mountain region to investigate other populations of leaf-eared mice and small mammals. The scientists want to find out whether the observations made are part of a broader pattern affecting other species.

Bautista adds that the study raises an intriguing question: “From a physiological point of view, the smaller the organism, the easier it is for it to lose heat. If you think about high-altitude regions, where there is little oxygen, it is very cold, and there is practically no food, why do these animals live there?”