Full Breakdown
Andean Leaf-Eared Mouse Thrives Above the Traditional Altitude Limit
7/18/2026, 1:57:01 AM
Discovery and New Study
In 1912 British zoologist Oldfield Thomas recorded Andean leaf-eared mice (*Phyllotis vaccarum*) on volcanic summits exceeding 6,700 m—well above the 5,500 m ceiling once thought impassable for mammals. A recent investigation, published in *Science*, captured more than 160 individuals from sea level to the summit of Volcán Llullaillaco (6,739 m). Researchers combined whole-genome sequencing, physiological tests, muscle analyses, metabolic measurements, and population genetics to pinpoint adaptations that enable year-round survival in this “death zone.”
Physiological and Genetic Adaptations
Mice from the highest sites generate heat efficiently while consuming minimal oxygen. Their skeletal muscles contain unusually high mitochondrial densities, allowing sustained aerobic metabolism. Rather than relying on carbohydrate bursts, they preferentially oxidize fats, supporting both shivering muscles and brown-fat thermogenesis. Unlike many high-altitude species, these rodents show little change in hemoglobin oxygen affinity; instead, selection has fine-tuned genes linked to energy production, vascular regulation, and chronic hypoxia tolerance. Genomic scans also revealed strong selection on detoxification pathways, enabling the breakdown of toxic plant compounds found on barren volcanic slopes.
Official Statements & Responses
“It was completely unexpected. People did not think mammals could survive at these altitudes, but they’re there,” said Graham Scott, professor of animal physiology, McMaster University.
“They’re more like a marathon runner than a sprinter. Their muscle cells are packed with mitochondria that allow them to sustain heat-producing activity for longer periods,” Scott added.
“Evolution is a complex process. When animals encounter really challenging environments, there are a lot of different things they need to cope with, not just the obvious ones,” noted Grant McClelland, professor of biology, McMaster University.
“We were initially focused on the most obvious environmental challenges, things like low oxygen and cold, but there were important factors we didn’t expect, including how these animals deal with what they’re eating,” Scott explained.
Verbatim Quotes
- “It was completely unexpected. People did not think mammals could survive at these altitudes, but they’re there,” — Graham Scott, Professor, McMaster University
- “They’re more like a marathon runner than a sprinter. Their muscle cells are packed with mitochondria that allow them to sustain heat-producing activity for longer periods,” — Graham Scott, Professor, McMaster University
- “Evolution is a complex process. When animals encounter really challenging environments, there are a lot of different things they need to cope with, not just the obvious ones,” — Grant McClelland, Professor, McMaster University
- “we were initially focused on the most obvious environmental challenges, things like low oxygen and cold, but there were important factors we didn’t expect, including how these animals deal with what they’re eating,” — Graham Scott, Professor, McMaster University
Remaining Knowledge Gaps
The study acknowledges uncertainty about the mice’s year-round diet and the precise functional impact of each detoxification gene. Future fieldwork aims to catalog seasonal food sources and experimentally verify how these genetic changes translate into toxin resistance, potentially informing biomedical research on hypoxia and metabolic disorders.
