A wide range of creatures have adapted to counter the adverse effects of extreme winter temperatures by undergoing physiological and biochemical changes.
Creatures that live in cold climates have evolved various strategies to allow them to survive bitter winter temperatures. Some migrate or hibernate, some burrow below the frost line or seek deep water, while others insulate themselves with extra fur, fat, or feathers.
However, some animals—primarily cold-blooded ones—have evolved metabolic adaptations that allow them to endure exposure to temperatures that would normally freeze their bodily fluids. They prepare themselves internally by undergoing a number of physiological and biochemical changes.
Adaptations typically fall within one of two basic strategies—that of freeze tolerance, or freeze avoidance.
Freeze avoidance mechanisms work by preventing or minimizing the formation of ice crystals within the body cavity. Freeze-avoiding animals actively produce antifreeze compounds – including glycerol, antifreeze proteins (AFPs), and sugars. These cryoprotectants enhance an animal's ability to ‘supercool,’ allowing body fluids to remain unfrozen at temperatures below their freezing point, thus protecting biological tissue from physical damage caused by ice crystals.
Though less common in warm-blooded animals, freeze avoidance mechanisms are not unknown in mammals. According to the BBC, Arctic ground squirrels' body temperatures drop as low as -2.9 degrees Celsius (26.8 degrees Fahrenheit), the lowest measured in a mammal. Despite their core temperature dropping to almost three degrees below the freezing point of fresh water, ground squirrel blood remains liquid, most likely through supercooling mechanisms.
Freeze tolerance adaptations, on the other hand, include the ability to survive ice crystals within the body cavity. Animals that use this mechanism develop strategies to regulate and endure ice formation in their bodies using high accumulations of sugars. The high concentration of sugars protects the inside of cells while allowing ice to form in body fluids outside of cells.
Scientific American reports that freeze tolerant insects promote ice crystal growth outside their cells by producing proteins in their extracellular fluid that initiate ice crystal formation and limit the insects’ ability to supercool. Producing ice crystal growth outside their cells reduces the likelihood that the cells will freeze and burst.
The ability of Wood frogs (Rana sylvatica) to repeatedly freeze and thaw has been widely studied, and these frogsicles are often used as examples of extreme freeze tolerance.
Research into extreme cold adaptations is important to humans for a variety of reasons. If we can understand, and harness, the ability to freeze and unfreeze organs and tissues, it could improve the preservation of organs for tissue-matching, and perhaps increase the success of organ transplants.
Not quite as important to humanity, but still worth noting, is the fact that fish antifreeze proteins are commonly used to prevent ice recrystallization in ice cream.