On the summit of Llullaillaco, a 6,739-metre volcano straddling the Chile–Argentina border, conditions are so harsh that researchers compare them to Mars: permanent frost, almost no water, next to no plant life, and air that holds only about 44 percent of the oxygen available at sea level with every breath. Biologists long assumed mammals could go no higher than roughly 5,500 metres. That a small mouse lives up here anyway is forcing a rethink of the limits of animal life.
The Andean leaf-eared mouse (Phyllotis vaccarum) is Earth's highest-dwelling mammal. An international team including researchers from McMaster University in Canada and the University of Montana has now described in the journal Science how the animal withstands these extremes. Rather than a single miracle adaptation, the scientists found a whole bundle of changes – in behaviour, in metabolism, and right down to individual muscle cells.
Breathing like a marathon runner
Unlike many other high-altitude animals, the summit mice do not carry a more efficient version of the oxygen-carrier haemoglobin. Their trick is different: they simply breathe faster. An enzyme called carbonic anhydrase keeps the level of CO2 in their blood stable, preventing their constant hyperventilation from tipping into something harmful. That lets the animals extract enough oxygen to keep their body warm even in the cold.
The heat is generated in unusually active muscle and in brown fat tissue. At the cellular level, the highland mice's muscles look more like those of endurance athletes: densely packed with mitochondria, the cell's power plants. “They're more like a marathon runner than a sprinter,” says study author Graham Scott. The mice also burn fat by preference – an efficient fuel for shivering muscle and heat-producing tissue.
The genetic analysis delivered one more surprise, with little to do with cold or oxygen: signs of an enhanced ability to break down plant toxins from their meagre diet. For the researchers, the mouse is far more than a curiosity. It is a living model of how evolution adjusts many small dials at once to make life possible where no one expected to find it – and a starting point for understanding how bodies cope with a shortage of oxygen.