How Arctic Ground Squirrels Survive Freezing Hibernation Temperatures

The Arctic ground squirrel holds the lowest core body temperature ever recorded in a mammal, dropping to around minus 2.9 degrees Celsius during hibernation without turning its blood to ice, according to research published by Brian Barnes of the University of Alaska Fairbanks in a 1989 paper in Science.

How Arctic Ground Squirrels Achieve Supercooling

That subfreezing measurement makes the species unique among warm-blooded animals. Water does not automatically freeze the moment it drops below zero degrees Celsius; freezing needs a trigger—such as a speck of grit or an existing ice crystal—to start the process of nucleation. According to the University of Alaska Fairbanks, the Arctic ground squirrel avoids this by supercooling, allowing its body fluids to drop below their freezing point without any crystals forming.

This physiological strategy differs sharply from the one used by wood frogs, which tolerate ice formation in parts of their bodies. The squirrel actively avoids freezing altogether, keeping its fluids liquid even at temperatures where ice should form.

The Metabolic Cost of Interbout Arousals

This subfreezing state does not last the entire winter. Every two to three weeks, the squirrel shivers itself back up to a normal mammalian body temperature of roughly 37 degrees Celsius for a short period before cooling down again, according to UAF research. These events are known as interbout arousals.

These transitions demand enormous amounts of energy. Research from the UAF lineage shows that repeated rewarming can account for the large majority of the animal’s total energy expenditure across a full torpor and arousal cycle. While maintaining a cold body state is cheap, the periodic returns to warmth burn through the fat reserves the squirrel spent all summer building.

Potential Implications for Human Medicine

Laboratories continue to study the Arctic ground squirrel because of how its tissues behave around these extreme temperatures. During deep torpor, the squirrel’s brain loses many neural connections, and its organs operate under low oxygen and reduced blood flow that would damage most mammalian tissue, as reported by Scientific American.

On rewarming, its brain reconnected. This biological capacity has drawn interest from researchers studying stroke and the handling of tissue under low oxygen. However, this remains basic animal physiology under study, and any direct medical application is currently a hope rather than an established result.

Frequently Asked Questions

What is the lowest core body temperature ever measured in a mammal?

According to Brian Barnes’s 1989 study, the Arctic ground squirrel reaches around minus 2.9 degrees Celsius, the lowest core body temperature ever recorded in a mammal.

What does supercooling mean for the squirrel?

Supercooling is a metastable state where body fluids chill below their normal freezing point without forming ice crystals, allowing the animal to remain subfreezing without freezing solid.

Why do Arctic ground squirrels wake up during hibernation?

Every two to three weeks, squirrels undergo interbout arousals, shivering to warm back up to roughly 37 degrees Celsius. While these warm periods consume most of the animal’s winter energy reserves, the exact reason for these costly arousals remains an open question in the field.