Alaskan Wood Frogs Survive Winter by Getting Frozen; The Secret Lies in Their Antifreeze-Like Blood

The northern forests of Canada and Alaska have some of the most dramatic temperature ranges on Earth. In these regions, summer days stretch to 24 hours with temperatures that can reach up to 90 degrees Fahrenheit (32 degrees Celsius). On the other hand, the winter season brings some of the coldest temperatures on the planet, around -50 degrees Fahrenheit (-46 degrees Celsius).

This is the winter world in which animals need to survive. Amphibians like frogs had to endure the intense, protracted, iron-cold subarctic winter through hibernation.

Surviving Alaskan Winters

Alaska is home to a species called wood frog (Lithobates sylvaticus), an amphibian less than three inches (7.62 centimeters) long with paper-thin skin. It is known for being the only frog to live north of the Arctic Circle.

Most frogs survive northern winters by hibernating deep under water, in lakes, streams, and ponds. During this period, they are cold and dormant, but their body temperature does not fall below freezing point.

Wood frogs have a different strategy to survive the harsh winter. These animals hibernate by nestling down into the leafy litter on the forest floor. Together with duff and overlying snow, the leaves provide some insulation from extreme cold. This means that the wood frogs are not protected from subfreezing temperatures as they would be if they chose the underwater strategy.

In a study led by Don Larson of the University of Alaska Fairbanks, a group of scientists investigated how these frogs alter their physiology to endure the long and extremely cold winter months. They discovered that wood frogs freeze up to 60% of their bodies during this season.

During this critical time, the wood frogs stop their heart beat. This amazing technique enables the frogs to become active very early in spring, since the land thaws and warms more quickly than the ice-covered lakes. The newly active wood frogs can mate and lay eggs in small ponds or in melt water pools that dry up by midsummer. On the other hand, frogs that hibernate underwater take longer to become active, so they must breed later. This is because they need permanent water that does not dry out.

Secret of Frozen Frogs

For most animals, they need to protect themselves from any condition that would freeze their flesh. Freezing is so dangerous because it can affect the animal's body in several ways.

First, ice crystals that form inside the body can puncture blood vessels. Second, frozen blood has no mechanism to deliver oxygen and nutrients to organs, leading to extreme metabolic damage. Lastly, ice can injure cells by drawing out water and causing dehydration from fractured cell walls, a form of pervasive and deadly internal damage.

Yet wood frogs have evolved ways to freeze out solid for up to eight months a year. They have accomplished what would seem to be a biological miracle.

When ice crystals start to form on a wood frog's skin, an adrenaline response is triggered, much like the fight or flight response in mammals. The frog responds by flooding its bloodstream with blood sugar which acts as an antifreeze that protects the cells.

In an average human, it is normal to have a blood sugar level of 90 micrograms per 100 milliliters of blood. In a freezing wood frog, the blood sugar level is 450 times higher. This amount would kill a human many times over, but this is the key to frog survival because their metabolism has essentially shut down.