What is the only Arctic mammal known to hibernate with its body temperature below 0°C?

The Arctic ground squirrel is the only Arctic mammal known to hibernate with its body temperature below 0°C.

During hibernation, this squirrel allows its core temperature to fall to around −3°C, while brief periodic arousals raise it again. This remarkable adaptation helps it survive the exceptionally cold winters of northern Alaska, Canada, and Siberia, when food is unavailable and snow covers the ground.

The species spends much of the short summer feeding on plants, seeds, insects, and carrion. It stores little food underground compared with some other squirrels, instead relying heavily on body fat accumulated before hibernation. Researchers have studied its extreme cooling because it may help explain how mammals protect cells during very low body temperatures.

The Arctic ground squirrel is sometimes confused with a marmot or prairie dog, but it is a distinct species adapted to tundra conditions. It is also an important prey animal for Arctic foxes, golden eagles, and other predators.