The Arctic ground squirrel supercools its blood below freezing each winter

Each winter the Arctic ground squirrel does something that should, by the ordinary rules of biology, kill it. During hibernation the animal lets the core temperature of its body drop below the freezing point of water, colder than any other mammal on record, and yet its tissues do not turn to ice and it wakes unharmed in spring. The feat depends on a delicate physical trick called supercooling, in which a liquid stays liquid even below the temperature at which it would normally freeze.

The behavior is one of the most extreme examples of cold endurance known in the animal kingdom. It has drawn sustained scientific attention not only for what it reveals about survival in the Arctic but for the clues it may hold about protecting living tissue at low temperatures.

The coldest body temperature measured in a mammal

Studies of hibernating Arctic ground squirrels have recorded core body temperatures as low as about minus 2.9 degrees Celsius, several degrees below the freezing point of water. That is the lowest body temperature ever documented in a mammal. The reference profile of the Arctic ground squirrel notes that the animal can adopt and then spontaneously recover from these sub-freezing temperatures without its body fluids crystallizing.

The foundational measurement traces to research published in the journal Science in 1989, which established that these squirrels genuinely drop below zero rather than merely approaching it. That finding turned a surprising observation into a documented physiological fact and set off decades of further study.

How supercooling keeps blood from freezing

Supercooling is a real physical phenomenon that anyone can observe under the right conditions. If a liquid is very clean and left undisturbed, it can be chilled below its normal freezing point and still remain liquid, because ice needs a starting point, a speck of dust, a rough surface, or a seed crystal, to begin forming. Without such a trigger, the freezing simply does not start.

The Arctic ground squirrel appears to manage this state inside its own body. During deep torpor it allows its fluids to cool below freezing while preventing the nucleation that would let ice form. Nothing seeds the crystals, so nothing crystallizes, and the animal’s tissues stay intact in a condition that would ordinarily be fatal.

What happens during deep torpor

Hibernation in these squirrels is not a light sleep but a profound shutdown known as torpor. The heartbeat slows to a few beats per minute, breathing becomes shallow and infrequent, and metabolism falls to a tiny fraction of its normal rate. In this state the animal barely consumes energy, which is how it survives months without eating in one of the harshest environments on the planet, as described by the U.S. National Park Service.

Crucially, the squirrels do not stay uniformly cold the entire winter. They periodically rewarm their bodies for short intervals, brief, costly arousals that punctuate the long stretches of near-frozen torpor, before cooling back down again.

Surviving months of Arctic winter underground

Arctic ground squirrels are obligate hibernators, meaning they must hibernate to survive, and they spend roughly seven to eight months of the year in this state within burrows dug below the surface. There, insulated from the worst of the surface cold but still exposed to sub-freezing ground temperatures, they wait out the long polar winter. When spring returns, they rouse fully, resume feeding, and prepare to breed in a compressed active season.

The strategy is an answer to an unforgiving calendar. With food available for only a few months, the animal spends most of the year in a suspended state, trading activity for the energy savings that make year-round residence in the Arctic possible.

Why researchers study this extreme adaptation

Beyond its natural marvel, the Arctic ground squirrel’s tolerance for cold interests scientists working on the resilience of the brain and other tissues. The animal endures dramatic drops in temperature and blood flow and then recovers without apparent damage, a resilience that researchers hope to understand and perhaps one day apply to medicine, from preserving organs to protecting the brain during periods of low blood supply. A small rodent that freezes its year in half offers an unusually direct window into how a warm-blooded body can flirt with freezing and live to see the thaw.

The puzzle of the periodic rewarming

One of the stranger aspects of this hibernation is that the squirrels do not simply stay cold and still until spring. Every couple of weeks they briefly rewarm their bodies to near-normal temperatures, an energetically expensive process that burns through a meaningful share of their stored fat, before cooling back down into deep torpor. Why an animal so carefully conserving energy would repeatedly spend it this way is not fully understood, but researchers suspect the warm intervals let essential biological processes, such as sleep and cellular maintenance, catch up after long stretches of near-frozen shutdown. These arousals may be the price the body pays to survive months in a state that leaves most ordinary functions suspended. Understanding them is part of why the Arctic ground squirrel remains such a valuable subject for the study of extreme physiology.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview