Some families collect travel magnets or beer steins. My family collects “signs of spring,” evidence that winter has come to an end. One of my favorite signs is the call of Spring Peeper frogs. In March or April their loud peeps are a promise of warmer weather. This raises the question: How do frogs, with their fragile skin, survive freezing weather?
It turns out there are several methods. Aquatic frogs hibernate at the bottom of a pond. Some terrestrial frogs in warmer climates dig below the frost line to hibernate. But others, like the Spring Peeper (Pseudacris crucifer) and the Wood Frog (Rana sylvatica) simply freeze in place. Literally.
In the fall Peepers and other “freeze tolerant” frogs nestle into leaf mold. As ice crystals touch their skin the skin begins to freeze. This triggers the frog’s liver to make glucose (sugars).
As the frog gets colder the ice on its skin draws water from inside the cells of the internal organs. If this were to continue the cells would become completely dehydrated, in which case they would collapse and die (taking the frog with them).
Fortunately for frogs, ice formation triggers the release of glucose into the bloodstream where it flows into the cells that were previously filled with water. The cells become filled with a thick syrup of glucose which prevents both freezing and dehydration within the cells. So, the water only freezes in the frog’s body cavity and between the skin and muscle. In other words, the water freezes safely in the areas between the cells rather than inside them.
As the frog continues to freeze its lungs stop pumping, its heart stops beating and blood ceases to flow. The frog is essentially dead. Only it isn’t. It’s frozen. Wood Frogs (the most widely studied) are able to survive with up to 65-70% of their body frozen solid.
Compare this to humans. When we get frost bite ice sucks water out of our cells. Without glucose to give the cells structure they simply collapse and die. Once dead they can’t be revived. If enough cells die you can too (so don’t forget your hat!)
With these frogs, their glucose filled cells are severely dehydrated, but not enough to collapse completely, allowing the cells (and the frog) to survive. It isn’t quite as simple as this, since urea (a waste product in the frog’s urine) and several specialized proteins, also act as an antifreeze outside the cells, but these processes are less well understood.
When the weather warms the frog begins defrosting. Over the course of a day, it slowly recovers its heartbeat and begins to breath. Eventually it’s muscles thaw out and it can move normally.
Frogs are able to go through this freeze – thaw cycle throughout the winter without any ill effects, sometimes staying frozen for a month or more between thaws. Some scientists speculate that the thawing part of the cycle allows for the production of more glucose and so is necessary for survival.
Eventually the weather warms enough for the frogs to thaw completely and hop away. They begin their mating calls, that wonderful cacophony of peeps and croaks that reminds the rest of us that spring has arrived.
Thanks for reading,
March 27, 2018
great video below:
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