Why do frozen frogs come back to life?
― They freeze ice only outside their cells, and sugar stops them freezing too far

In North American forests, wood frogs freeze solid every winter. Their hearts stop. Their breathing stops. And yet, come spring, they thaw out and hop away as if nothing happened. The trick that neither our bodies nor a frozen strawberry can pull off comes down to two things: where the ice forms, and how far the freezing is allowed to go.

Two reasons frogs don't break

Neither of these is special magic. Both simply make clever use of the same property that keeps a salted road from icing over: dissolving something in water makes it harder to freeze.

Life or death depends on where the ice forms

A living body is a collection of tiny cells. Each cell is a bag with a thin membrane, with water both inside and out. Take a look at Figure 1.

On the left is a case like a strawberry: slow freezing that lets ice form even inside the cells. The ice crystals are needle-sharp, and as they grow they pierce the membrane and cell wall from the inside. When it thaws, the contents leak out through the torn bag. That's exactly the juice that pools on the plate.

On the right is the frog's case. The frog's skin and blood are thought to contain substances that trigger ice formation. Because of this, freezing begins from outside the cells first, at a mild cold of around -1 to -2°C. Once the outside water turns to ice, the remaining water becomes concentrated with salts and other solutes. This draws water out of the cells toward the more concentrated solution outside. The cell loses water, shrinks down, and its interior stays unfrozen.

Sugar isn't there to stop freezing — it's there to stop over-freezing

Once freezing starts, the frog's liver rapidly converts its stored starch-like reserves into sugar. Blood sugar levels are reported to rise to tens or even over a hundred times their normal level.

Here's the surprising part: even with all that sugar, the temperature at which freezing starts drops by only about 0.5°C. The frog ends up freezing anyway. So what is the sugar actually doing?

The more the outside water freezes, the more concentrated the remaining water becomes. And that remaining water stops freezing once it reaches "the concentration at which it would just start to freeze" at that temperature. The more solute particles are dissolved to begin with, the sooner that concentration is reached. In other words, adding sugar reduces the fraction of water that turns to ice at any given cold temperature.

Take a look at Figure 2. Turn the dial down in temperature, and without sugar, over 80% of the body's water turns to ice almost immediately. A frog loaded with sugar can hold that down to the 60% range even at around -3°C. Wood frogs are thought to be able to tolerate freezing up to around 65% of their body water. Sugar acts as the "brake" that keeps them from crossing that limit.

Summary

Frogs can come back to life after freezing because they protect their cells by freezing ice only outside them, and because sugar keeps the frozen fraction of their body water below the tolerance limit. They don't stop freezing itself — they manage where, and how much, freezing happens.

It's not that they avoid freezing — they choose how they freeze.
A frog's winter survival is the art of knowing how to get along with ice.

For the puzzle of water that stays liquid even below 0°C, see our article on water that doesn't freeze even at -20°C, and for how ducks' feet withstand the cold in a similar way, see our article on why ducks' feet don't freeze.

※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding, and may vary across studies. Never attempt to catch and freeze wild frogs or conduct similar experiments.