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 – and how it survives in an environment so hostile that it has been compared to Mars has left scientists baffled.

Life isn’t easy at the top of Llullaillaco. Average temperatures are -15°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. …