Warner Corner - January 2008

Frozen Frogs
Bekah Dalen, Interpretive Naturalist

When we think of animals hibernating in the winter, we usually envision a furry creature snuggled up in a cozy den somewhere awaiting the spring thaw. This is a fairly accurate picture, however, mammals are not the only group of animals that hibernate...and it is not always so cozy.

Think, for a moment, of movies where someone is frozen and then years later brought back to life to save the world or be cured of some disease. It sounds like science fiction, right? For humans, the process of being frozen and then being brought back to life is science fiction, but it has been happening in nature with frogs for thousands of years. Yes indeed, I said frogs. Many northern terrestrial frog species, like the wood frog (Rana sylvatica), have a complex adaptation called a freeze tolerance, which allows them to do what humans can only do in the movies. These frogs are able to freeze in the winter, lie dormant just below the leaf litter on the forest floor, then thaw in the spring to live on another year. So, how do they do it?

As soon as a single ice crystal touches the frog's skin, the process begins. Buried under moist leaf litter, the frog tucks its legs and digits under its body to avoid drying out during the freeze. Instead of waiting until temperatures plummet, it invites in the freeze as soon it drops below 32 degrees. Higher freezing temperatures mean a slower freezing process which allows the frog more time make adjustments. Amazingly, the frog controls where ice begins forming in its body. Using ice nucleators, the frog “seeds” the ice in places where it cannot do as much damage to delicate tissues and organs, like in the abdominal cavity. At the same time, the frog's liver, which is the last to freeze, produces a sugar called glucose which builds up inside of the cells. As ice forms around the cells and draws out the water, a thick syrupy solution is left behind which cannot freeze. This solution is similar to antifreeze which is made out of a sugar alcohol called ethylene glycol. Finally, all systems stop, including the heart and lungs. Metabolism halts almost completely, which is only possible because the frog's organs are somehow able to survive without oxygen or energy. Brain function levels are unknown, but the brain does freeze and has to survive without oxygen along with the rest of the organs. The frog then remains in this suspended state of animation until it is warm enough to once again join the living.

When the spring sun begins to penetrate the leaves on the forest floor, the frog begins to thaw. Instead of warming from the outside in, which seem the most logical, the frog's heart is the first to thaw, then the brain and other organs, and finally the limbs. If the process was reversed and the limbs were the first to thaw, they would die before the heart could begin pumping blood to them. When the thaw is complete, the frog heads to the nearest breeding pond like nothing ever happened. Still don't believe it? Check out this excellent video of a wood frog freezing and thawing by Nova at www.pbs.org/wgbh/nova/sciencenow/video/3209/q05.html.