Wood Frogs (Rana sylvatica)
Last spring, after hiking daily through the snow to the vernal pool, my sister and I finally experienced the thrill of witnessing that single day in the early spring when the hundreds of wood frogs, hibernating in the forest floor, nestled below the leaf and snow, come out of their frozen state and hop across wet leaves and patches of snow, back to the vernal pool to mate. I carefully picked one up and inspected it closely. It’s skin was loose and pale, kind of translucent, and the frog itself was almost lethargic. All the while, we were listening to their mating calls, hundreds of male frogs, all at once, sounding like a bunch of old ladies in the audience of a comedy show, laughing and cackling uncontrollably.
Time stood still.
The Wood Frog, along with the Gray Tree Frog, the Spring Peeper, and the Chorus Frog, is one of the most common frogs in North America.
They actually survive the cold winter months by being frozen. They can sustain temperatures up to -8 degrees C before the cold becomes lethal.
Up to 65% of the total body water in wood frogs could actually be ice! The ice formation in the frog’s body cavity extends to the spaces between the cells, but not the cells themselves. Should their body temperatures extend below the lethal low limit, and ice forms in the cells themselves, the cells can then cut, like knives, and do multitudes of damage. They can slash membranes, puncture cell organelles, and break cells apart. Under most circumstances, the wood frog’s hibernacula under leaves and snow will protect him from the lower temperatures that can result in death.
This is an extremely interesting, very complicated process. In anticipation of winter, many insects will store alcohol glycerol and glucose (antifreeze) during the fall months, before going through the same freezing process as the frogs. Unlike the insects, however, the frogs wait until the ice first forms on their skin. This triggers the “fight or flight” response of their central nervous systems to release adrenaline into their bloodstream. Adrenaline has many effects on the body, but basically it prepares the body to meet whatever challenge is occurring. This effect is modified in wood frogs to activate the conversion of glycogen in the liver to glucose, which takes on the role of “antifreeze“. Nothing like waiting until the last minute!!
But it doesn’t stop there! Simultaneously, the opposite phenomenon is occurring between the cells. Special proteins promote ice crystal formations, which in turn create pockets of concentrated fluid that osmotically withdraw water from the cells to make them resistant to freezing.
Amazingly, this all happens very quickly! Within 15 hours, the frog, with exception of its actual cells, is frozen solid. There is no heart beat. The blood no longer flows through that little body. And breathing ceases. Sounds like being dead to me. My question here is, “Is this an example of true hibernation?”
For more detailed information on how this happens, go to Monarch's blog and watch this amazing video:
This whole process is barely conceivable to me.