The North American Wood Frog lives accross Canada, Alaska and the US states around the great lakes. Its habitat is dense forest. Its call is often confused with a duck's quack.

The North American Wood Frog (Rana sylvatica) survives a near-complete freezing every single year. It's heartbeat and other bodily systems stop and about 65% of its bodies' water turns to ice. It spends as much as 3 months frozen solid in this way, as surrounding temperatures remain well below zero.

As temperates slowly drop, the frogs respond by flooding their systems with glucose and burying themselves amongst the vegetation of the forest floor. Their organs will then contain about 100 times their normal concentration of glucose. This measure prevents the cells from dehydrating too far.

At -7°C, its cells are somewhat dehydrated, and blood vessels have expanded to accomodate the forming ice crystals. If it gets much colder the vessels rupture and the cells are damaged; the frog will not recover.

Cell dehydration occurs as fluid is drawn out of the cells to replace moisture that is lost to freezing outside the cells. The level of dehydration is critical. Research has shown that the wood frog can survive a loss of 60% of the water in its cells. Beyond that point, the cells cannot regain their normal shape and are effectivly destroyed. The extra glucose that saturates the cells in winter helps them to hang on to water.

On thawing, the frogs breathe very deeply and shake out their limbs. This is because their frozen blood was concentrated in the center of their bodies- none remained in the legs at all. When fully recovered they can hop away has though nothing as happened. Typically, their first port of call is a swamp or pool where they can breed!

Wood frogs are inspiring research into preserving organs for transplant. Rat livers have been successfully thawed intact from -3°C in the lab by saturating them with a cyroprotectant similar to glucose.


References:
  • http://www.exploratorium.edu/frogs/woodfrog/index.html
  • http://library.thinkquest.org/11034/woodfrog.htm
  • http://tqd.advanced.org/11034/woodfrog.htm
  • http://allaboutfrogs.org/weird/strange/freeze.html
  • http://www.npwrc.usgs.gov/narcam/idguide/wood1.htm
  • http://www.pbs.org/saf/transcripts/transcript704.htm

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