The idea that an animal could be frozen solid, stop all life functions, and then return to normal activity is a biological paradox. This phenomenon, known as cryobiosis, represents a temporary suspension of metabolism in response to sub-zero temperatures. Organisms capable of this feat enter a state where the body’s machinery halts, with no discernible heartbeat, breathing, or brain activity. The central challenge is managing the lethal threat posed by ice crystal formation, which can puncture cell membranes and draw water out of cells, causing fatal dehydration.
The Biological Strategy: Freeze Tolerance vs. Avoidance
Animals facing extreme cold have evolved two different survival strategies. The first is freeze avoidance, where the organism prevents ice from forming within its body, even when external temperatures drop below the freezing point of its internal fluids. This is achieved through supercooling, where body fluids remain liquid below zero degrees Celsius.
Many insects and fish utilize potent “antifreeze” compounds, such as specialized proteins and high concentrations of glycerol, to depress the freezing point of their body water. These compounds chemically interfere with the formation and growth of ice crystals. This strategy is fragile because a single external ice crystal contacting the animal’s skin (inoculative freezing) can instantly trigger catastrophic ice formation.
The second strategy, freeze tolerance, allows animals to appear to “come back to life.” Freeze-tolerant organisms permit ice to form, but they rigidly control where and how much is allowed. This strategy is an adaptation to environments where contact with environmental ice is inevitable, making freeze avoidance impossible.
How Animals Survive Internal Ice Formation
For freeze-tolerant animals, survival requires restricting ice formation to the extracellular spaces (outside the cell walls). When freezing begins, specialized biological agents called ice nucleators trigger ice formation just below the freezing point of the body fluids. This controlled start limits the rate of ice growth, preventing rapid, uncontrolled crystallization.
As ice forms outside the cells, it draws pure water out, a process called osmotic dehydration. This causes the cell to shrink, concentrating the remaining water and solutes inside. The resulting high solute concentration prevents the water inside the cell from freezing, which is critical because intracellular ice is almost always fatal.
To manage this dehydration, the animal rapidly produces and distributes large amounts of low-molecular-weight substances, known as cryoprotectants. These are often sugars, like glucose, or sugar alcohols, like glycerol, which act as a shield for the cells. Cryoprotectants permeate the cell membrane, stabilizing proteins and membranes against the stresses of cell shrinkage. This deep metabolic depression allows the organism to endure for weeks or months until a thaw occurs, even with up to 65% of the total body water frozen and circulation stopped.
Case Studies: Vertebrate Survivors
The wood frog (Rana sylvatica) is the primary example of a freeze-tolerant vertebrate, found across the northern United States and Canada. This small amphibian hibernates just beneath the leaf litter, fully exposed to sub-zero temperatures. When freezing begins, the frog’s liver mobilizes massive amounts of glycogen, converting it into glucose for distribution as a cryoprotectant.
The concentration of glucose in the wood frog’s blood can increase over 100-fold during freezing, protecting the brain and other vital organs. The wood frog can survive with 60% to 70% of its total body water converted to extracellular ice. Its heart and respiratory functions stop entirely, and the frog becomes rigid and solid, yet its cells remain protected by the concentrated glucose.
Another freeze-tolerant vertebrate is the hatchling painted turtle (Chrysemys picta). These small turtles overwinter in underground nest cavities and tolerate temperatures down to approximately -4 degrees Celsius. Like the wood frog, they utilize glucose and other cryoprotectants to prevent cellular damage during the freezing of internal body fluids. Their freeze tolerance is a temporary adaptation allowing them to survive their first winter before reaching the safety of water.
Case Studies: Extremophiles and Invertebrates
Many invertebrates and microscopic organisms exhibit extreme cold tolerance, often through mechanisms different from the frog’s liquid-based cryoprotection. Freeze-tolerant insects, such as the larvae of the Alaskan upis beetle, utilize glycerol as their primary cryoprotectant. These larvae survive temperatures as low as -60 degrees Celsius and can remain in metabolic suspension for years within frozen wood.
The microscopic tardigrade, or water bear, is a famous extremophile, though its survival is often tied to desiccation. While tardigrades can enter cryobiosis, their most resilient state is anhydrobiosis, or life without water, triggered by drying out.
By shriveling into a durable “tun” and losing up to 97% of their body water, they eliminate the material that forms lethal ice crystals. This desiccated state allows tardigrades to survive exposure to near-absolute zero temperatures and the vacuum of space. It is the removal of water, rather than chemical protection, that grants them this extreme cold tolerance. Nematode worms and rotifers also use similar cryptobiotic strategies to survive both freezing and desiccation.