The ability of certain organisms to survive the winter with a significant portion of their body water turned to solid ice represents one of nature’s most extreme survival strategies. This phenomenon, known as freeze tolerance, is a biological form of cryopreservation that defies the lethal effects of freezing temperatures. For most life forms, ice formation inside the body is fatal, leading to cellular rupture and tissue damage. A select few vertebrates and invertebrates have evolved biochemical mechanisms to manage and control ice formation, allowing them to endure months in a state of suspended animation until warmer weather returns.
Amphibians and Reptiles That Freeze
The most well-studied animal capable of surviving a frozen state is the Wood Frog (Rana sylvatica), an amphibian found across North America, extending into the subarctic. As temperatures fall, the frog burrows into leaf litter, allowing ice crystals to form on its skin and seed the freezing of fluids throughout its body. Once frozen, the frog’s heart ceases to beat, breathing stops, and all blood circulation halts.
The physical transformation is profound: the eyes turn opaque white, and ice forms between the skin and muscle layers, encasing the internal organs. Up to 70% of the wood frog’s total body water can be converted into extracellular ice without causing death. This strategy allows the frog to occupy terrestrial hibernation sites that thaw earlier than the ponds used by other species.
Among reptiles, the hatchling of the Painted Turtle (Chrysemys picta) is the only known reptile to naturally tolerate freezing of its body fluids. These tiny turtles overwinter in shallow nests, surviving temperatures as low as -6°C with approximately half of their body water as ice. This capacity for cryopreservation is confined to the hatchling stage.
The Chemistry of Survival
The internal survival mechanism relies on natural cryoprotectants, which act as a biological antifreeze to protect cells from damage. When ice begins to form outside the cells, the frog’s liver rapidly converts stored glycogen into massive amounts of glucose. This surge of sugar is distributed throughout the bloodstream and tissues in concentrations up to 100 times higher than normal.
Glucose, along with compounds like urea in some species, prevents the formation of lethal ice crystals inside the cells. As water is drawn out to form extracellular ice, the cryoprotectants maintain a high solute concentration within the cell cytoplasm. This osmotic balance minimizes the dangerous shrinkage and dehydration that would otherwise destroy the cell membranes.
Freezing Versus Deep Hibernation
The state of biological freezing is fundamentally different from deep hibernation or torpor seen in many mammals. Deep hibernation, such as that practiced by bears or groundhogs, involves a drastic reduction in metabolic rate and a drop in body temperature. However, the hibernating mammal’s core temperature is actively regulated to remain above the freezing point of water, and vital signs continue throughout the winter.
By contrast, freeze-tolerant animals enter a state where all vital signs have ceased and a substantial percentage of the animal’s body water has turned into solid ice. The wood frog survives a condition that would be lethal to a true hibernator. This tolerance for ice formation allows these species to survive in shallow, exposed environments where temperatures frequently fall below zero.
The Process of Reanimation
The revival process is triggered by rising ambient temperatures, which begin to melt the extracellular ice within the animal’s body. As the ice melts, the concentrated cryoprotectants diffuse out of the cells, and the tissues begin to rehydrate. The heart is typically the first organ to reactivate, restarting its beat as blood flow resumes throughout the body.
Once circulation is re-established, the frog’s nervous system and respiratory function begin to recover. The speed of this reanimation can be remarkable, with the heart starting to pump again within 30 minutes of the thaw. Within a day or two, the animal is fully functional and capable of beginning its breeding season.