Which Arctic mammal has a body temperature that can fall to about −2.9°C during hibernation?

The Arctic ground squirrel can lower its body temperature to about −2.9°C during hibernation.

This extreme physiological adaptation makes it the only known mammal whose body temperature can fall below the freezing point of water during natural hibernation. The animal survives because its body fluids contain protective compounds and because it periodically arouses from torpor to restore normal temperature.

Arctic ground squirrels spend much of the year underground in burrows, where they hibernate through the long cold season. They emerge during the short Arctic summer to feed, mate, and build fat reserves. Their ability is different from ordinary winter sleep: hibernation involves profound, regulated changes in metabolism, heart rate, breathing, and body temperature.