High up on volcano summits, the Andean leaf-eared mouse is showing how life can thrive in places where humans cannot. This plucky rodent, according to researchers, has evolved an unusual mix of metabolic and genetic adaptations that help it survive across the broadest elevation range known for any mammal.
This mouse, known for its rounded leaf-shaped ears, inhabits a punishing landscape: from the desert coast of northern Chile to the crest of the Andes, the vast mountain range running along the west coast of South America. No other mammal is known to live at such high elevations.
"On the summits of approximately 22,000 feet (6,700 meters) peaks where we have trapped these mice, there is less than half the oxygen that is available at sea level, and temperatures are almost always below freezing," said University of Nebraska evolutionary biologist Jay Storz, leader of the research published in the journal Science.
"With proper acclimatization, a well-trained mountain climber can tolerate the lack of oxygen at such heights during a one-day summit attempt, but such elevations far exceed the limits of long-term human survival," Storz added.
To understand how the species copes, researchers analysed whole-genome data from 167 of these mice collected across the elevation range of the species. The findings indicate that the success of this mouse lies in how its body handles energy and oxygen at the same time.
"When a mammal is challenged by extreme cold, it relies on metabolic heat production to maintain a constant body temperature. This ... requires oxygen as fuel. Since there is less oxygen at high elevation, mammals that live in such environments make metabolic adjustments to use oxygen more economically," Storz said.
The mice collected at extreme elevations differed physiologically from those collected at lower elevations. The highland mice turned out to better maintain thermogenic capacity – the ability to generate body heat – than their lowland counterparts, the researchers said.
Their skeletal muscles also exhibited a greater capacity to produce energy inside mitochondria, the subcellular structures that serve as a cell's power plant, while their heat-producing brown fat was better equipped to burn fats, another adaptation that likely helps them stay warm in the oxygen-poor cold, they said.
The study also pointed to diet as an unexpected dimension of handling extreme altitudes. The researchers found genetic evidence indicating that this species has adapted to different plant toxins encountered across its elevational range, alongside its adaptations to the cold and to the low-oxygen setting.
"The discovery that the mice have evolved the ability to metabolize dietary toxins was unexpected," Storz said. "In the high-elevation environment ... they can't even get a decent meal – the only available food-plants are loaded with toxic secondary compounds. Things are tough all over."
"We identified signals of selection on genes involved in antioxidant defence and the metabolism of dietary toxins. ... We know genes under selection are involved in those biological functions but cannot state adaptation to which particular toxin or plant," added systematist and zoologist Guillermo D'Elía of the Universidad Austral de Chile, another of the study's co-authors.
The findings may shed light on how animals can flourish in low-oxygen environments.
"High-elevation mammals have evolved the ability to survive and function under conditions of chronic oxygen deprivation that mimic disease states in humans," Storz said.
Understanding these mechanisms could help guide the design of treatments for people with cardiorespiratory diseases that impair oxygen delivery even at sea level, Storz added.
"We now have a good basis of the physiological aspects allowing those mice to live at 6,700 meters above sea level. We also have some understanding on the genomic basis of some of these adaptations. But there's much to learn," D'Elía said.
D'Elía noted that it is still unclear what the mice actually eat at such heights, how long they live and whether they reproduce on the summits.
Even the most barren-looking mountain landscapes are far from empty, hosting unique life still waiting to be discovered, according to biologist and study co-author Zachary Cheviron of the University of Montana.
Beyond the unanswered questions concerning these mice, the findings are a reminder, D'Elía said, of how much remains unknown about life on Earth and of the need to protect nature.
