When temperatures plummet below zero, animals have a few options. They can avoid the cold temperatures all together by finding a nice burrow or staying underwater where it hasn’t frozen, they can be lucky enough to be an endotherm (be able to produce their own body heat), they can die, or some can just let themselves freeze. This remarkable adaptation has been studied over the past 35 years using wood frogs (Rana sylvatica). Wood frogs are found all the way from the Arctic circle into the Appalachian Mountains. Throughout its range, wood frogs overwinter on the forest floor under a thin layer of forest debris (Costanzo, 2019).
Photos: Bethany Williams
Here you can compare what an unfrozen wood frog looks like compared to a frozen frog. Despite this frozen frog having no heartbeat whatsoever, it is alive and will emerge in the spring to mate.
So how do wood frogs do this? They must survive not only freezing, but a long period of not eating, dehydration, and long-term exposure to severe cold. For wood frogs in Alaska, hibernation can last up to 8 months. Therefore, wood frogs have an arsenal of adaptions that they use to survive overwinter (Costanzo, 2019).
To prepare for winter, wood frogs accumulate a large store of glycogen (the storage form of glucose). This stored glucose is an important source of energy during the long period of dormancy when wood frogs are not eating. For the freeze-tolerant wood frog, glucose is especially important as a cryoprotectant, a molecule that protects against freezing—more on those later. Unlike other amphibians which depend on their fat stores during dormancy, wood frogs convert all their fat to glycogen in preparation for winter. Additionally, wood frogs accrue urea as they prepare for winter, which would normally be excreted in urine. Urea helps by depressing the metabolism of the frog and is another important cryoprotectant (Costanzo, 2019).
Freezing injury occurs because as ice forms, water is lost to the ice crystals. This can lead to cell dehydration, membrane failure, and oxidative damage to the cell. Freezing for a long period of time can also deplete energy stores, lead to an accumulation of metabolic wastes that becomes toxic, and cause damage to structures when crystals form. This is where cryoprotectants come in. Once freezing begins, wood frogs mobilize their glucose stores and rapidly distribute them throughout their tissues. Urea, another important cryoprotectant, is already accumulated and distributed to the tissues prior to freezing. Another protective response to freezing is to redistribute tissue water from sensitive organs to places where damage from ice would be minimal like the body cavity. This also means that the concentration of urea and glucose in the cells is higher because there is less water. Both glucose and urea limit ice formation, minimize how much cells actually shrink, and can have other benefits such as regulating metabolism and protecting cell membranes (Costanzo, 2019).
Watch the video below to check out how all of this happens.
Video from PBS Nova ScienceNow
Costanzo JP (2019) Overwintering adaptations and extreme freeze tolerance in a subarctic population of the wood frog, Rana sylvatica. J Comp Physiol B 189: 1-15
Ritsko A (2005). Frozen frogs, PBS Nova Science Now