For some animals, enduring winter temperatures means more than just finding shelter or growing a thick coat; it involves a remarkable biological feat: surviving being frozen solid. This phenomenon, known as freeze tolerance, allows certain species to halt their vital signs, with ice forming within their body tissues, and then revive completely when temperatures rise. It is a striking adaptation that challenges conventional understanding of life, as most organisms would sustain lethal cellular damage from ice crystal formation. These animals enter a state of suspended animation, where metabolic activity effectively ceases, preserving their cells and organs through periods when conditions would otherwise be unsurvivable.
Masters of Freezing Tolerance
Several species have evolved the ability to tolerate freezing, spending weeks or months with a significant portion of their body water converted into ice. The wood frog (Rana sylvatica), found across North America, is a prime example, capable of surviving with up to 65-70% of its total body water frozen, including its brain and eyes. These frogs burrow under leaf litter on the forest floor, where temperatures can drop well below freezing, and then thaw in the spring to resume normal activity. Alaskan wood frogs can endure temperatures as low as -14.6°C.
Hatchlings of the painted turtle (Chrysemys picta) also exhibit this rare adaptation, being the only reptile known to tolerate natural freezing of extracellular body fluids. They spend winter in shallow terrestrial nests, enduring repeated freezing and thawing cycles at temperatures as low as -6°C to -8°C, with over 50% of their body water as extracellular ice.
Many insects also demonstrate remarkable freeze tolerance, including the woolly bear caterpillar (Pyrrharctia isabella). This caterpillar can survive extremely cold temperatures, reportedly as low as -70°C in some Arctic populations, by freezing solid. It can undergo multiple freeze-thaw cycles throughout the winter, resuming activity during warmer spells. Certain darkling beetles found in Alaska similarly protect their watery cells from freezing, even at temperatures around -24°C.
Biological Mechanisms of Survival
Animals that survive freezing employ physiological and biochemical adaptations to prevent lethal damage. A primary mechanism involves the production of cryoprotectants, which are natural antifreeze compounds like glucose and glycerol. For instance, wood frogs rapidly produce large amounts of glucose from liver glycogen, circulating it to protect cells from freezing and dehydration. Glucose prevents the formation of damaging intracellular ice crystals and minimizing cell shrinkage as water moves to extracellular ice.
Painted turtle hatchlings similarly use both glucose and glycerol as cryoprotectants, which help regulate cell volume and stabilize cellular structures during freezing. Woolly bear caterpillars primarily produce glycerol, which functions as an organic antifreeze, preventing the freezing of their inner cells even as the rest of their body solidifies.
These animals also control ice formation, typically allowing ice to form only in extracellular spaces. This controlled extracellular freezing draws water out of the cells, concentrating their contents and preventing the more destructive formation of ice crystals inside the cells, which would be lethal. Some species utilize specific ice-nucleating proteins to initiate ice formation at higher sub-zero temperatures, ensuring a slow and controlled freezing process. During this frozen state, animals suppress their metabolic rate to near zero, conserving energy and allowing them to survive for extended periods.
Other Strategies for Surviving Cold
While some animals tolerate freezing, many others employ different strategies to survive cold temperatures without allowing their tissues to freeze solid. One method is supercooling, where an animal’s body fluids remain liquid even at temperatures below their normal freezing point. This is achieved by removing ice-nucleating agents, preventing the formation of ice crystals. For example, the Arctic ground squirrel can supercool its body temperature to as low as -2.9°C during hibernation, keeping its blood liquid.
Another strategy involves the production of antifreeze proteins (AFPs) or glycoproteins (AFGPs). These proteins do not prevent freezing entirely, but instead bind to ice crystals and inhibit their growth, effectively lowering the freezing point of body fluids. Many fish in polar waters, such as Antarctic notothenioids and certain cod species, utilize AFPs to prevent ice crystal growth in their blood, allowing them to swim in waters colder than their blood’s freezing point.
Hibernation and torpor represent another distinct approach, where animals significantly reduce their metabolic rate and body temperature, but do not allow their tissues to freeze. Animals like bears enter torpor, while smaller mammals and insects may undergo deeper hibernation. These strategies differ from true freeze tolerance because the animal’s body fluids remain largely unfrozen, maintaining a state of reduced activity rather than suspended animation with ice formation within tissues.