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Author: Isabel Zhang

From: Chicago, IL, US


Frogsicles, frozen frogs
Source: Smithsonian Channel

Don’t worry, this isn’t about popsicles made out of frogs. Well, it kinda is.

If you were to ever go hiking in the forests of Alaska during the winter, you might see a frozen frog buried in the snow. Frozen rock solid and unmoving– it sure looks dead. You might feel bad for it, but not surprised that it’s dead due to the sub-zero temperatures.

The good thing is, you don’t have to feel bad– because the frog isn’t actually dead forever! At the moment, there’s no muscle movement, heartbeat, or breathing, and it just looks like a lumpy popsicle that’s frog-shaped. Come spring though, and it will be alive and well. It’s a perfect example of the living-dead. But what exactly is this frog?

Wood Frogs, and Why They Freeze

The frog that we’ve been talking about is the wood frog (Lithobates sylvaticus), which lives throughout the forests of Alaska and the Northeastern United States. Wood frogs have a distinct black marking across their eyes that looks like a mask. And they have a unique way of surviving the freezing winter that we’ve been discovering in more recent years.

Like mammals, wood frogs hibernate in the winter– they just hibernate much differently. Since they’re cold blooded and can’t keep themselves warm enough to stay alive by burning fat, they embrace the cold and allow themselves to freeze. As ice surrounds their bodies, their internal organs become encased in ice, and ice forms between layers of skin and muscle [1].

But being frozen actually allows the frog’s body to be preserved. The low body temperature decreases the frog’s metabolism, preventing enzymes from degrading cells and tissues [2]. Essentially, it prevents the frog from decaying, and the frog appears to be in suspended animation.

However, freezing comes with dangers, including the formation of sharp ice crystals. So how exactly are wood frogs able to stay alive while being frozen? To answer this, let’s dive into the frog’s freezing process.

The Freezing Process

As the beginning of winter draws near, the wood frog buries itself under the leafy litter on the forest floor. When temperatures are low enough, the fluid around the frog’s cells freezes, decreasing the amount of extracellular free water (water outside of cells) [3]. This in turn increases the extracellular solute concentration, and therefore osmotically draws out water from cells. Now that the cells are dehydrated, sharp ice crystals won’t have the chance to form and pierce cell membrane from inside the cells.

However, if the extracellular ice was to continue forming, cells would face a new danger: losing too much water. Not to worry though– the wood frog has some ingenious antifreeze mechanisms that are at work even as the frog freezes!

Nature’s Antifreeze

One of the wood frog’s antifreeze mechanisms is the activation of antifreeze proteins (AFP). These special proteins bind to the surfaces of extracellular ice crystals and limit the formation of new ice crystals [3]. As a result, the extracellular solute concentration stops decreasing, and cells stop losing water.

The other mechanism is a natural antifreeze made of glucose and urea. It may sound gross, but it plays a huge role in the wood frog’s survival! As winter approaches, the frog’s liver produces and stores large amounts of glycogen. Once the freezing starts, the liver uses a process called glycogenolysis to convert the glycogen into glucose [4]. In addition, the wood frog holds onto water in its abdomen instead of urinating as normal [4].

After the glucose has been produced, the liver releases the glucose into the bloodstream, and the abdomen releases the urea. This mixture displaces water molecules and prevents hydrogen bonds from forming between them that would create an ice structure. As a result, it lowers the freezing point of the remaining water in the bloodstream, making it harder for water to freeze and preventing cellular water loss [4]. Talk about an effective antifreeze that’s all-natural!


The wood frog stays in its frozen state for the rest of the winter. When temperatures rise at the arrival of spring, signs of life start to appear as the frog thaws from the inside outward. First the heart, then the brain, and finally the legs are back in working order [1]. Thawed completely, the wood frog then sets off in search of food and a mate– as if nothing happened.


In a study investigating free-living wood frogs in Interior Alaska, the frogs remained frozen for about 193 days at an average temperature of -6.3°C, or about 20.5℉! The wood frog’s ability to survive harsh winters by freezing and thawing without damage is truly amazing– in fact, it’s inspired many researchers to develop the next big breakthroughs. For example, researchers are applying the wood frog’s biological mechanisms to try to develop a method that extends the preservation life of human organ transplants on ice [1]. If they’re successful, more people would be saved: we would have enough time to safely transport organs from the donor to the recipient, as well as make sure that the organ matches the recipient.

The wood frog reveals to us how life-changing it can be to apply biology to innovation. All we have to do is take some time to learn about the miraculous life that exists around us. And now you know: if you ever see a frozen frog that has a black marking across its eyes, it’s a frogsicle that knows what it’s doing.


About the Author: Isabel Zhang

Isabel is a senior in high school, and is interested in biology and engineering. In her free time, she loves to bake, sketch, and hang out with her family.



  1. Biological Miracle. (2015, April 14). Retrieved August 8, 2020, from

  2. Michel, S. G., LaMuraglia Ii, G. M., Madariaga, M. L., & Anderson, L. M. (2015). Innovative cold storage of donor organs using the Paragonix Sherpa Pak ™ devices. Retrieved August 8, 2020, from

  3. Boisselet, A. (2017, April 08). Frog's natural antifreeze to survive freezing. Retrieved August 8, 2020, from

  4. Costanzo, J. P., do Amaral, M. C., Rosendale, A. J., & Lee, R. E., Jr (2013). Hibernation physiology, freezing adaptation and extreme freeze tolerance in a northern population of the wood frog. The Journal of experimental biology, 216(Pt 18), 3461–3473.

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