Which squirrel species holds the record for the lowest body temperature during hibernation?

The Arctic ground squirrel holds the record for the lowest recorded body temperature during mammalian hibernation. Its core temperature can fall to about −3 °C, below the freezing point of water, while its heart rate drops to roughly one beat per minute.

The species, Urocitellus parryii, lives across Arctic and subarctic tundra in North America and northeastern Asia. It spends the brief summer feeding on grasses, berries, seeds, roots, and other tundra foods, building fat reserves before entering a long winter hibernation. Adult females may hibernate from early August to late April; males generally enter later and emerge earlier.

The squirrel’s body avoids lethal freezing despite subzero temperatures. Researchers think it removes or suppresses ice-nucleating particles that would otherwise trigger crystal formation in body fluids, allowing some tissues to remain supercooled. During hibernation, the animal periodically arouses, briefly raising its temperature before returning to torpor.

This record concerns ground squirrels, not squirrels that glide or live in trees. The Arctic species is especially remarkable because its temperature falls below 0 °C without its blood turning into damaging ice.