The Arctic ground squirrel hibernates with its body at about -3 °C, below the freezing point of water.
Why it is true
It is the lowest body temperature ever measured in a mammal. Its blood is supercooled: it cools below zero without forming ice crystals. Every two or three weeks the squirrel warms up for a few hours and then cools down again.
Source: Science (Barnes, 1989)
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.
