Wood Frog: Fantastic Beasts and Where to Find them, eh?

Tina-Louise Rossit,
Guest Blogger.

This blog is written by Nature Canada guest blogger Tina-Louise Rossit.

Canada is cold. We know it, tourists know it, and the wildlife know it. But despite the cold, we’ve been doing fine with clothing and heating devices to help us through. Well, same goes for our Canadian wildlife! Over the course of evolution, these animals developed remarkable features to survive our winters, and maybe even better than any man-made technology!

Today’s honourary species is an expert at hibernation. The Wood Frog, Rana sylvaticus, has amazed scientists with their ultimate freezing method. They can freeze their bodies, stop their heartbeat, and remain in this state all winter. Then, springtime hits, and they come back to life. It’s spectacular. Ready to defrost this mystery?

Wood frogs are tiny frogs about 5cm in length, with molted tan-brown skin that camouflages among forest floor leaf litter. They have a distinctive dark brown “eye-mask”, making them an easy species to identify.  A true Canadian amphibian, they’re found in every province and territory.

As their name implies, they like wooden areas. Their only condition is that there is a fresh water source for breeding and laying eggs. When the winter melts away, vernal pools or temporary ponds without fish, are the best places for Wood Frogs to breed in.  Since vernal pools have no big predators, insects and other invertebrates come to breed here too, and it’s perfect for Wood Frogs and their tadpoles, since that’s exactly what they like to eat!

After a good summer’s worth of eating and building up reserves, the usual frog routine would be to find a pond, dig a hole in its bottom, and avoid freezing over winter. Not the Wood Frog! They do not go for the usual routine and instead choose to borrow under a thin layer of leaf litter to allow their bodies to freeze. And this happens rather quickly since temperature rapidly fall to sub-zero temperatures. With a range well into the Arctic Circle, we’re talking real cold conditions!

Ice crystals form along their tiny bodies and as they turn rock solid. For scientists who study living tissues at extreme low temperatures (cryobiologists), they are astounded at the Wood Frog’s capability to preserve themselves for 4-6 months in this icy coma every single year. Obviously, research and studies on these frogs were needed at once! And so, studies conducted were to recreate Wood Frogs’ natural habitat and temperature cycles in labs such as to observe them and run tests on their tissues and blood component levels.

Freezing your cells can cause rupturing and irreversible damage yet Wood Frogs bypass this. As winter arrives, the Wood Frog’s liver begins overproducing glucose (sugar) and urea (a waste product found in urine) to be transported throughout the organs, tissues and cells. The high concentration of both substances is naturally dangerous but does nothing to harm these frogs. Actually, this is their antifreeze recipe! Their cellular activity slows down to absolute minimum but is not stopped completely. Although their blood is functionable, it will not flow at that low activity level.

Once winter is over, their cells, tissues and organs start to function until they reach their usual capacity without any difference. They can hop around and start their springtime day. They gain a vital advantage by doing this; since all animals that are still hibernating under water, it’s the Wood Frogs that get to the vernal pools first and start finding food and breeding.

Wood Frogs represent extreme evolutionary outcomes. There is still much to study from this species since their natural hibernation mechanism can help improve the science of organ preservation for human transplants, glucose management for diabetic patients and reperfusion injuries in heart attacks and strokes. Understanding what Wood Frog’s have been doing for eons can help save human lives in the future!


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Bibliography

http://www.naturenorth.com/spring/creature/woodfrog/wf2.html
http://www.naturenorth.com/winter/frozen/Ffrozen2.html
https://www.units.miamioh.edu/cryolab/projects/woodfrogfreezing.htm
https://owlcation.com/stem/Frozen-Wood-Frogs-and-Adaptations-for-Survival