NORMAN, Okla. – Mammals inhabit a variety of environments on Earth, but few are able to withstand the hostile conditions of low oxygen and freezing temperatures characteristic of extreme elevations. Thanks to the recent discovery of a resilient mouse species living at elevations exceeding 6,000 meters above sea level, University of Oklahoma professor Naim M. Bautista, Ph.D., is working to uncover how these rodents have adapted their physiology to cope with such conditions.
He is one of the leading co-authors of a study published this month in Science that explores how high-altitude Andean leaf-eared mice have evolved to survive these elements. This species has been known to exist from sea level at the northern Chilean coast through the Andes Mountains into Argentina. But they are now the highest-dwelling mammals currently known to researchers, due to their appearance at extreme elevations with such low levels of oxygen that well-trained, acclimated mountaineers can only tolerate for a one-day summit attempt.
Bautista, an assistant professor of ecological physiology at OU's School of Biological Sciences, joined an international team – which includes his friend and colleague, University of Nebraska–Lincoln professor Jay Storz, Ph.D. – that traveled to the Andes starting in 2020 to collect these mice for examination. He said the study examined both high-altitude and sea-level members of the mouse species and simulated the effects of different elevations on both groups.
"We targeted physiological measurements related to thermogenic capacity: the capacity of a body to produce heat," Bautista said. "We wanted to understand how these animals are able to live there, how they resist low oxygen conditions and cold stress, how they breathe, how their hemoglobin is working and how much saturation of oxygen they have in their blood. We wanted to learn everything about these animals."
Among the researchers' findings, mice collected from extreme elevations displayed two apparent local adaptations separating them from their lower-elevation counterparts. One is the ability to generate higher amounts of heat by shivering through their skeletal muscles, an important quality for living in such cold environments. The second was the discovery of an unexpected environmental adaptation: "These mice have genes that allow them to process plant-derived dietary toxins," Bautista said. This means that much of the elevation-related selection of these mice is due to previously unrecognized aspects of their feeding ecology.
Another major finding is that populations of Andean leaf-eared mice from different altitudes exhibit very little genetic differentiation. "Instead of being like several genetically differentiated subpopulations of the same species, the whole species just behaves as one big population," Bautista said.
This lack of genetic differentiation makes it hard for local, population-specific trait differences to evolve, because the gene variants with local benefits must compete with the other gene variants. But since highland mice have evolved different physiological traits than lowlanders, natural selection pressure favoring high-altitude traits seems to be strong enough to counteract the homogenizing effect of gene flow from low-altitude populations.
While their findings have now been published, Bautista and the team are continuing to explore the Andes. They plan to travel to the eastern, Argentinian side of the mountain region to study further populations of Andean leaf-eared mice and other small mammals. They aim to see if the same observations they've made are part of a larger pattern affecting other species.
For Bautista, the research is also helping shed more light on a discovery that remains fascinating.
"From a physiological point of view, the smaller you are, the easier it is for you to lose heat," Bautista said. "Thinking about an extreme high-altitude environment where there is no oxygen, where it is freezing cold and there is virtually no food, why are these animals up there?"
About the research
This study was part of a large collaboration between scientists from the University of Oklahoma, the University of Nebraska, the University of Montana, and three universities in Chile: la Universidad Austral de Chile, la Universidad de Chile, y la Universidad San Sebastián.
