At the summit of Llullaillaco, a volcano in the Andes that rises 6739 metres above sea level, lives a mouse. It is the highest dwelling mammal in the world 鈥� and how it survives in an environment so hostile that it has been compared to Mars has left scientists baffled.
Life isn鈥檛 easy at the top of Llullaillaco. Average temperatures are -15掳C and the air pressure is so low that there is less than half as much oxygen in each lungful of air as at sea level.
Humans can鈥檛 survive for long at the peak. In 1999, archaeologists uncovered the frozen and perfectly preserved mummies of three Inca children who had been drugged and left to die on the summit 500 years ago, as part of a ritual sacrifice. But the yellow-rumped leaf-eared mouse (Phyllotis xanthopygus) apparently thrives here.
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Mountaineers have reported seeing mice near the top of Llullaillaco, so last month an international team of biologists journeyed to the Andes to investigate. The team, led by Jay Storz at the University of Nebraska and Guillermo D鈥橢l铆a at the Austral University of Chile, spent weeks studying mice at a range of elevations before Storz and a colleague journeyed to the peak of Llullaillaco, where they spotted and trapped a mouse.
Puzzling diet
鈥淚 felt like I was staggering around up there,鈥� says Storz, describing how difficult it was to move at such high altitude. 鈥淏ut the mouse didn鈥檛 seem too impaired.鈥�
What makes the fact that mice are so active on the volcano鈥檚 peak even more astonishing is that they are so small. 鈥淭hey lose heat so much more easily because they have a higher surface area to volume ratio,鈥� says Graham Scott at McMaster University in Canada. 鈥淭hey鈥檙e having to generate lots of body heat to keep warm, but they鈥檙e doing it even though there is very little oxygen available.鈥�
This suggests the mice must have a voracious appetite to gain the energy they need to survive. That puzzles biologists because there are no green plants growing anywhere near the summit, making exactly what the mice eat a mystery.
鈥淭he summit is more than 2000 metres above the limits of green plants,鈥� says Storz. He thinks the mice might survive at least in part on insects, but more clues will come in the months ahead when the researchers analyse the mouse鈥檚 gut contents.
Astrobiologists draw parallels between the conditions on Llullaillaco and those on other worlds including Mars in terms of temperature, aridity and exposure to ultraviolet light. 鈥淚t鈥檚 not the moon, but Llullaillaco is definitely not a place productive enough and hospitable enough that we would expect a mammal to live there,鈥� says Scott.
Grant McClelland, also at McMaster University, points out that some birds can briefly survive at even greater altitudes: geese migrating over the Himalayas reportedly reach heights above 7200 metres. But the mice on Llullaillaco may spend their entire lives above 6700 metres. 鈥淚t certainly changes our idea of the limits of where mammals can live,鈥� he says.
Reference: bioRxiv,
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