The Wood Frog's Winter Survival Strategy
By Catherine Liguori

The wood frog (Rana sylvatica) is one of only six species of amphibians that inhabit Alaska and is the only frog you will find living north of Southeast Alaska. It can be found further north than any of the other six amphibians, well above the arctic circle (Rozell 1995). The reason the wood frog is able to survive in these extreme climates where most cold blooded amphibians would die is that it is “freeze tolerant,” meaning it has the ability to cryogenically freeze during the winter. When the frog is frozen, it's heart stops beating, its blood stops flowing and it stops breathing. Its eyes turn white because the lenses freeze (Storey). When warmer weather arrives the frog thaws out within hours and essentially comes back to life. Depending on winter weather this can happens several times or only once.

In order for the frog to survive the freezing process it needs to control where in its body ice forms and when that begins to happen. Ice crystals can puncture blood vessels, cause cell deformation or breakage, and disrupt the internal structure of a cell. This could be very harmful to the frogs organs. In order to prevent this damage the ice needs to be confined to the extracellular spaces of the body (Storey) .

The frog is able to initiate the freezing process, causing it to start at a higher temperature than it normally would. This is beneficial because it means the rate of internal ice formation is slower and gives the frog more time to make the necessary metabolic adjustments. It also allows more time for cryoprotectants (antifreeze chemicals) to be produced by the liver and absorbed by the cells (Wood Frog Freezing Survival). “Ice nucleating” molecules are molecules that help instigate the formation of ice. Ice itself is an ice nucleator. The frogs skin is thin and permeable so ice on the ground can trigger internal freezing. Other ice nucleators include particles of bacteria that when ingested by the frog speed up the freezing process (Wood Frog Freezing Survival).

When the frog begins to freeze fluid compartments and the abdominal and thoracic cavity begin to freeze first and encase all the internal organs (Storey). The ice forming in extracellular spaces causes dehydration of the cells through osmosis (the liquid water flows to where it is becoming less concentrated outside the cell) so more water is brought to these areas to freeze (Fox 1999). The frog can survive with as much as 65-70 percent of its total body water frozen (Wood Frog Freezing Survival) and its cells can endure as much as 60 percent dehydration without damage (Fox 1999).

As the freezing is taking place the liver produces enzymes that break down glycogen into glucose. Glucose is distributed to the organs through the circulatory system (Roach 2007). A special form of insulin is also produced by the liver that accelerates the absorption of glucose into the cells (Goertzel 2002 ). Glucose acts as an “antifreeze” agent. It bonds to water molecules and lowers the freezing temperature (Fox 1999). Chemicals that lower the freezing temperature in freeze tolerant animals are called cryoprotectants. A Wood frog's blood glucose levels can get to over 100 times its normal levels without causing the frog harm like this would in a human (Wood Frog Freezing Survival). By the time the frog is frozen the glucose is like a thick syrupy substance in its cells (Roach 2007). Glucose inside the cells also helps them to retain their shape.

Another cryoprotectant the frog uses is Urea. The cells accumulate urea throughout the autumn and early winter and it is already in the cells when freezing begins (Wood Frog Freezing Survival).

Other organisms like insects produce different cryoprotectants that aid in freeze tolerance. Some insects for example synthesize ethylene glycol, which is the same chemical used in the antifreeze in our car radiators. Glycerol is another cryoprotectant more commonly found in insects and much less toxic than ethylene glycol. In the winter glycerol can make up 20-25% of these insects body weight (Storey).

Once the frog is frozen its organs must be able to survive without the oxygen or nutrients that are normally essential to an organism and delivered to the organs through the blood. Survival is accomplished by dramatically slowing the metabolism and also because the organs of the wood frog have an enhanced ability to survive without oxygen (Storey).

The frog thaws quickly when the weather warms. As the internal ice melts water flows back into its cells. It thaws basically from the inside out which is important because if the limbs thawed before the vital organs the tissue in the limbs would die (Fox 1999). Its fundamental functions come back first; heart beat, respiration and circulation. Then more and more complex functions begin to return. Within a day the frog is almost back to itself, though higher order behaviors take days to return. The wood frog's damage repair mechanisms are enhanced to repair any injuries that could have been cause to organs by ice crystals. For example the levels of clotting proteins rise in the blood so that any bleeding from the organs can be quickly stopped (Storey). Within days of thawing the frog finds its way to a pond to mate.

During warmer winters when the frog doesn't freeze it needs to find food to sustain itself. As long as there are enough insects, worms, snails and other food sources available the frog will be fine. Dissection of frogs in regions where they freeze shows that they eat other frogs. This is not found in wood frogs who live in warmer regions. It is speculated that this is because food is more scarce in these areas in the fall and early winter, when temperatures are cold but not cold enough yet for the frogs to freeze (Rozell 1995).

Scientists are studying the wood frog with the hope that they can replicate its freezing process. If successful this technique could have applications in organ transplantation. Organs can usually only last hours outside of a living organism without damage. If they could replicate the wood frogs technique organs could be preserved for much longer (Roach 2007).