The Wood Frog

At less than three inches in length, the small brown Wood Frog (Rana sylvatica) seems insignificant. Camouflaged to blend in with its surroundings—the leafy forest floor—it is one of the most common frogs, with a habitat range from southern Ohio all the way north into Canada and most of Alaska. It is not in danger of extinction. Its population has not recently gone through a decline due to unknown environmental conditions. It is not brightly-colored with venomous skin. It is not even one of the more popular so-called “edible” frogs, considered delicacies by cultures that do not understand God’s food laws.

So what makes this tiny amphibian so remarkable?

In the wintertime, when the temperature drops, it burrows under leaves and other debris on the woodland floor and freezes—literally. Its heart stops; its brain and other organs cease to function; and it becomes cold and hard to the touch. Most animals, however, when confronted with the same temperature drop, would do the same. The remarkable part is not this little animal’s ability to freeze, but its ability to revive after being frozen, and to return to living a life just like it did the previous year. After staying frozen for sometimes months at a time, when temperatures rise, the Wood Frog will gradually thaw from the inside out over a period of several hours, and go about its business!

Although a few other frogs are freeze-tolerant, the Wood Frog is found the farthest north of all amphibians—and is the only amphibian with populations ranging north of the Arctic Circle!

Just how cold can a frozen frog be? Laboratory studies show that Wood Frogs are capable of withstanding temperatures of 21.2°F (-6°C). Burrowing under layers of debris and snow usually enables them to insulate themselves enough not to experience colder temperatures, which can be lethal. During this period of time, as much as 65% to 70% of the water in their bodies will freeze into solid ice. What makes these frogs different from most other animals and humans is the way their bodies respond to the dropping temperatures and prepare for the freezing process.

Before we explain the marvel of how these frogs survive, we must explain why most living things do not survive freezing. Have you ever frozen fruit, and then thawed it to eat? It is never the same consistency or texture—it is usually mushy and soggy. That is because the water stored inside of the fruit freezes; and when water freezes, it actually increases in size, taking up more space. When water freezes inside tiny cells in plants, animals or humans, its increase in size ruptures the cell walls, destroying the cell. Frostbite is a good example of this—there is too much cellular damage in the afflicted areas for them to remain alive and repair themselves to return to functioning normally.

The Wood Frog, in contrast, has a certain process its body goes through before freezing, which allows it (and the cells inside of it) to survive unharmed. When the frog’s body senses that icy weather is on the way, it starts sending excess of a certain chemical (actually a form of glucose, or blood sugar) into the frog’s system. This chemical ensures that the cells in the body dehydrate to a certain point, and that the water collects outside of the cells, yet still within the body. The water outside of the cells then freezes solid, while the organs gradually cease to function. The frog becomes, to all outside observers, a frogsicle! It remains completely inanimate, with all heart, brain and respiratory functions ceasing. Judging from a medical perspective, the frog is legally dead.

With the first sign of springtime, temperatures warm, and something is triggered within the frog’s body that signals it to begin thawing. In a ten-hour process, the frog goes from completely frozen to active and responsive. Scientists still do not understand how this process works, and are puzzled by the fact that the frog thaws from the inside out, unlike any other example of frozen matter. Jon Costanzo, a professor of zoology who has done extensive work with the Wood Frog, is amazed that the frog is “cryo-preserving its organs — all of them, simultaneously…And we haven’t been able to do that with one [human organ].”

What internally triggers the frog’s body to resume life is yet unknown, and remains another testament to a true wonder of God’s amazing creation!