Amphibians are cold-blooded, but they would still die if they froze. Over the years, scientists have finally figured out how most species are able to get through the winter. They use several different strategies, which don’t always work, but they allow enough individuals to survive the cold to be able to reproduce the species.
Most of our lake-oriented frogs, such as the spotted leopard frogs, enter the water and spend the winter under the ice, where they snuggle down in the bottom mud. Their metabolism slows down greatly, but they are still capable of some activity, however slow, beneath the ice. The way that they get oxygen from the water is by breathing through their skin. This occurs over any exposed surface of skin. Although frogs take to the mud, they don’t bury themselves completely.
Another strategy, adopted by toads, is to burrow deep below the frost line in the ground. Their metabolism greatly slows down, but they are able to avoid actual freezing by their deep burrowing.
A much greater problem is faced by those frogs that spend the winter in locations where the temperatures fall well below freezing. These animals face the same problem as insects, and they have found some very innovative solutions. Like insects, frogs must avoid having ice crystals forming in their cells because the crystals damage the cells and essentially kill them.
Tree frogs and wood frogs, common inhabitants of our woodlands near a lake, stay on land throughout the winter. They burrow down into crevices or into the ground, but even so, they may be exposed to temperatures well below freezing – as cold as -4o F. One strategy that they use to avoid becoming solid ice cubes is by supercooling (water remaining liquid below its normal freezing point), which can be done almost instantaneously and works if the ambient temperature drops to a few degrees below freezing.
Their main defense is to produce large amounts of sugar (glucose) in their liver. To do so, in the fall they use up some of their muscle tissue to help provide the raw materials for glucose formation. The liver then ships the newly made glucose out to the rest of the body, where it acts as an antifreeze. Specifically, the glucose gets into the cells and prevents much of the water inside the cells from leaving so that the cells neither dry out nor develop ice crystals when it gets very cold. At least one other frog species mimics insects by forming glycerol, a natural antifreeze, as well as glucose. The kidneys of these frogs also produce high concentrations of urea, which also helps to reduce the freezing point of the water inside the frogs. This follows the same principle as ocean salts lowering the freezing point of salt water to below 320. Glucose is heavily concentrated in the liver and heart, and these organs do not freeze. The liquid around other tissues of the frog’s body actually does freeze solid when it gets cold. Once everything goes into cold storage, so to speak, the frog is able to spend the entire winter without needing to breathe or use any energy. Then in the springtime, when the weather begins to warm, the heart starts to beat and the liver begins to function again before any of the other tissues start to thaw out. Gradual thawing of the internal ice proceeds from inside to outside until the frog is fully functional. Interestingly, if thawing occurred from outside in, the outer tissues would die because the non-beating heart wouldn’t be able to supply these tissues with the oxygen that they need.
