Freeze and Not Die Trying: the Curious Strategy of Alaskan Frogs to Survive the Winter

A fundamental ingredient circulates in the blood of this little frog so that it can remain frozen during the eight months of intense cold.

Ranas, Alaska
The forest frog has black spots on its eyes, reminiscent of a mask. Image: Wikimedia Commons- Kristof Zyskowski and Yulia Bereshpolova

When winter arrives and you have to face the cold, many species appeal to the natural strategy of reducing their metabolism to the minimum expression, suspending brain activity and reducing breathing. They enter a kind of long and deep sleep, which is known as hibernation.

Thus, in places like Alaska, where winter can take thermal marks below -42 °C, bears, hedgehogs and even other frogs retire to remote places to spend the hardest months.

Most frogs hibernate in the depths of lakes, streams and ponds, where they remain asleep but with a body temperature that does not drop from the freezing point.

Freezing can be very dangerous. The ice crystals that form inside the body can pierce blood vessels and damage the walls of cells. In addition, frozen blood cannot transport oxygen and nutrients to the organs, which causes fatal metabolic damage.

But there is a species that shines for its ingenious play. The forest frog, an amphibian of just 7.5 centimeters, with extremely thin skin, and a peculiar black mark on the eyes that looks like a mask. It is the only frog that lives north of the Arctic Circle.

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This frog is not hidden in remote places, but rests quietly among the frozen litter of the forests. And there he alters his physiology until he freezes. It can stay like this for up to 8 months.

The ingredient hidden in the blood

How do they do it? Scientists at the University of Alaska Fairbanks discovered that, when ice crystals begin to form on the frog's skin, its system triggers a torrent of adrenaline and fills the blood with sugar.

About 90 micrograms per 100 milliliters of blood, an amount 450 times greater than what a human being could endure.

Sugar acts as an antifreeze, which protects the cell walls. Thus, the interior of the cells does not freeze, but ice does form between the cells. For several months, his body remains frozen like a block of ice, including his eyes.

When the first signs of spring appear and temperatures rise, the heart of the frogs begins to beat again, and thawing begins. Thanks to the fact that they did not hide deep in a lake covered with ice, the frogs wake up before the other species, and are ready to mate and lay their eggs in the nearest pond.

During the breeding season, which begins in March, the females lay between 1000 and 3000 eggs, which hatch nine to thirty days later.

The new frogs reach maturity in one or two years, depending on the sex and the population. The life of each of them does not usually exceed three years. Therefore, each frog will only spend two or at most three winters in a freezing state.