The Wood Frog (Rana sylvatica) is a small amphibian famous for its remarkable ability to survive the harsh winters of North America, including regions north of the Arctic Circle. Unlike most frogs that hibernate underwater or burrow deep below the frost line, the wood frog settles in shallow leaf litter or under moss, exposed to freezing temperatures. This overwintering location requires an extreme physiological adaptation: the capacity to survive having most of its body frozen solid. This survival strategy involves temporarily halting nearly all bodily functions, including waste elimination.
The Duration of Retention
The wood frog retains its urine for the entire duration of its frozen state, a period that can last for many months. In the coldest parts of its range, like Alaska, this retention can extend for up to eight months, covering the whole winter season. The ability to hold its waste is a necessary component of its overwintering physiology. Retention is directly linked to the frog’s metabolic shutdown, which repurposes the waste product for survival.
Urea as a Cryoprotectant
The waste product urea is actively accumulated and concentrated in the frog’s tissues and blood. Urea levels can increase by as much as 25-fold during hibernation, reaching concentrations of up to 90 mmol/L. This concentrated urea functions as a cryoprotectant, a substance that helps prevent freezing damage at the cellular level. It works synergistically with massive amounts of glucose, which the liver rapidly produces from stored glycogen in response to freezing.
The combined presence of high concentrations of urea and glucose helps to lower the freezing point of the intracellular fluid. This limits the amount of water drawn out of the cells when ice forms externally. By maintaining an osmotic balance, these cryoprotectants prevent the cellular shrinkage and collapse that occurs due to severe dehydration. Urea is comparable to, or even better than, glucose in protecting red blood cells and tissues from freeze-thaw damage.
Surviving the Deep Freeze
When the wood frog freezes, the process begins externally, with ice crystals forming on the skin and in the spaces between organs and tissues. As the water outside the cells freezes into pure ice, the remaining extracellular fluid becomes highly concentrated, drawing water out of the cells through osmosis. Up to 65 to 70% of the wood frog’s total body water can be converted into extracellular ice without causing death.
The concentrated urea and glucose allow the cells to withstand this extreme physical stress. These protective solutes keep the internal cellular environment from freezing and stabilize cell membranes and proteins. During this deep freeze, the wood frog enters a state of suspended animation as physical processes cease. There is no detectable heartbeat, breathing stops, blood circulation halts, and brain activity is undetectable. This metabolic suppression is facilitated by the retained urea, which acts as a metabolic depressant.
Spring Thaw and Elimination
Once temperatures rise in the spring, the wood frog begins the process of thawing and recovery. The heart often resumes beating even before all the ice in the body has completely melted, quickly restoring blood circulation and pulmonary respiration. The accumulated waste and cryoprotectants must then be dealt with to allow the frog to return to its normal metabolic state.
The kidneys rapidly restore their function to process the highly concentrated load of urea and other metabolic byproducts retained for months. The concentrated waste is then voided in a long-overdue excretion. With the elimination of the waste load, the frog’s metabolic activity quickly ramps up. It is often ready to begin feeding and mating within hours of thawing, ensuring it is one of the first amphibians to reach the vernal pools for breeding each spring.