Here is a detailed explanation of the remarkable discovery that Arctic ground squirrels can survive body temperatures below freezing through the mechanism of supercooling.
Introduction: The Physiological Impossibility
For most mammals, including humans, maintaining a stable internal body temperature is non-negotiable. If our core temperature drops even a few degrees, hypothermia sets in, leading to organ failure and death. If the body’s fluids actually freeze, ice crystals form inside cells, shredding their delicate membranes and causing irreversible damage.
However, the Arctic ground squirrel (Urocitellus parryii) defies these biological rules. Native to the tundra of Alaska, Northern Canada, and Siberia, this small rodent possesses a physiological adaptation almost unique among mammals: the ability to drop its core body temperature below the freezing point of water—down to -2.9°C (26.8°F)—without turning into a block of ice.
The Mechanism: Supercooling
The phenomenon that allows the squirrel to survive sub-zero temperatures is known as supercooling.
In physics, supercooling is the process of lowering the temperature of a liquid below its freezing point without it becoming a solid. Water usually freezes at 0°C because impurities in the water (dust, bacteria, or proteins) act as "nucleators." These nucleators provide a surface for ice crystals to latch onto and grow.
The Arctic ground squirrel achieves supercooling through an intense biological purification process:
- Removing Nucleators: The squirrel’s body actively purges its blood and fluids of potential ice nucleators. This likely involves filtering out specific proteins or food particles that could trigger crystallization.
- The Absence of Ice: Because the blood lacks these triggers, the fluids remain liquid even though they are colder than the freezing point. The squirrel is in a precarious, metastable state. Its blood is flowing, its heart is beating (albeit incredibly slowly), but it is literally colder than ice.
- Head Warmth: While the abdominal temperature drops to nearly -3°C, the squirrel maintains its brain and neck slightly warmer—usually just above 0°C. This suggests a vital mechanism to protect the central nervous system from the most extreme cold.
The Cycle of Torpor and Arousal
This supercooled state occurs during hibernation, which lasts for 7 to 8 months of the year (roughly September to April). However, the squirrel does not stay frozen for the entire winter. It undergoes a cyclical process:
- Torpor (2–3 weeks): The squirrel enters a state of suspended animation. Its metabolic rate crashes to 2% of normal. Its heart rate slows from 200–400 beats per minute to roughly 3–4 beats per minute. This is when the body temperature plummets to -2.9°C.
- Interbout Arousal (12–15 hours): Every few weeks, the squirrel begins to shiver violently. Using stored brown fat (a high-energy tissue), it generates massive amounts of heat, warming its body back up to normal mammal temperatures (approx. 36-37°C). It stays warm for less than a day—perhaps to sleep (paradoxically, they cannot experience REM sleep in torpor), repair cellular damage, or boost their immune system—before descending back into the freezing torpor.
Why Do They Do It? The Evolutionary Advantage
Surviving in the Arctic requires extreme energy conservation. The ground there is permafrost—permanently frozen soil.
Most hibernating animals dig burrows below the frost line to stay relatively warm (around 1°C to 4°C). However, in the Arctic, the permafrost prevents squirrels from digging deep enough to escape the freezing soil temperatures. Their burrows can reach ambient temperatures of -15°C to -20°C.
If the squirrel tried to maintain a "normal" hibernation body temperature of 1°C or 2°C against a surrounding temperature of -20°C, it would burn through its fat reserves too quickly trying to generate heat. By allowing their body temperature to drop to -3°C, the temperature gradient between their body and the air is smaller, drastically reducing the energy required to survive the winter.
Scientific Significance and Potential Applications
The discovery of supercooling in Arctic ground squirrels, largely championed by researchers at the University of Alaska Fairbanks, has profound implications for medicine:
- Cryopreservation: Currently, preserving human organs for transplant is a race against time. We cannot freeze organs because ice crystals destroy the tissue. Understanding how these squirrels supercool (remain sub-zero without ice) could lead to breakthroughs in banking human organs for long periods.
- Stroke and Ischemia Treatment: During torpor, blood flow to the squirrel's brain is barely existent, yet they suffer no brain damage. Upon waking, blood rushes back into the brain—an event that causes "reperfusion injury" in humans (common after strokes). Arctic ground squirrels seem immune to this injury. Unlocking this chemical pathway could lead to treatments preventing brain damage in stroke and heart attack victims.
- Alzheimer's Research: During hibernation, the neuronal connections (synapses) in the squirrel’s brain wither away, and proteins associated with Alzheimer’s (tau proteins) accumulate. Astonishingly, during the warming "arousal" phase, the squirrels rapidly regenerate these connections and clear the proteins, essentially curing themselves of neurodegeneration multiple times a winter.
Summary
The Arctic ground squirrel is an evolutionary marvel. By effectively "cleansing" its blood to prevent ice formation, it survives in a supercooled state that would kill almost any other mammal. It turns the lethal cold of the Arctic into a survival strategy, lowering its metabolic demands to match the harsh environment, holding secrets that could one day revolutionize human medicine.