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Cryobiology: Frozen Wood Frogs and Adaptations for Survival

Updated on March 20, 2017
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Linda Crampton is a science teacher with an honors degree in biology. She loves to study nature and write about animals and plants.

A wood frog photographed in Missouri
A wood frog photographed in Missouri | Source

An Amazing Animal

Cryobiology is the study of biological material that is at below normal temperatures. One example of this material is the body of the wood frog in winter. This amazing animal survives months of hibernation with much of its body frozen and without a beating heart. In most other animals, when the heart stops beating the animal is dead. This is not true for wood frogs, however. Despite the almost complete shut-down of their bodies, the frogs aren't harmed by freezing and become active again when the warmer temperatures of spring arrive.

The wood frog is a fascinating organism to study in its own right. In addition, the adaptations that enable it to survive freezing may be helpful in understanding or even in dealing with human medical problems. These problems include the safe freezing and thawing of organs for cryopreservation and transplants, a high glucose level in the body, and the safe resumption of blood flow after a heart attack or stroke.

A tan-coloured wood frog
A tan-coloured wood frog | Source

The Wood Frog

There are two scientific names for the wood frog—Lithobates sylvaticus and Rana sylvatica. It's a small animal that is around 1.4 to 3.25 inches in length. The frog is brown, orange-red, or tan in colour. There is a dark line in front of each eye and a dark blotch behind it. There may be dark, horizontal bars across the hind legs, a dark patch on the upper inside corner of each leg, and dark patches or speckles on other parts of the body.

The animal's range covers the majority of Canada and extends into Alaska and down into the northeastern United States. There is also a small area in the central United States where wood frogs can be found. The animals live mainly in woods, as their name suggests, but they also inhabit grasslands and the tundra. The adults eat insects and other small invertebrates. The tadpoles eat only plants. The male's call is notable because it resembles a duck's quack.

Wood Frog Call

Cryobiology is the study of proteins, cells, tissues, organs, and organisms that are at an unusually low temperature.

Hibernation

In the northern part of its range, the wood frog experiences very low winter temperatures. Most frogs in this situation bury themselves in the mud at the bottom of a lake, pond, or other body of water. This stops the animals from freezing during hibernation. As winter approaches, the wood frog buries itself in a shallow burrow on land, however.

The leaf litter that covers the frog and the snow that falls on top provide a little insulation from the cold winter temperatures, but not much. In fact, there is so little insulation that the animal soon freezes. The heart stops beating, the lungs and other organs stop working, and a large proportion of the water in the body freezes. The frozen liquid includes the blood.

Wood frogs have evolved ways to freeze solid for up to eight months each year. They’ve accomplished what would seem to be a biological miracle.

— National Park Service

Dangers of Freezing Living Tissue

Freezing of living tissue is normally a dangerous process due to the ice crystals that form as the water in the cells freezes. The crystals can rupture materials and cause rearrangement of cell structures, which can lead to permanent damage. They can also cause water loss and dehydration of cells. If blood vessels are ruptured, cells in the body will no longer receive oxygen and nutrients. The wood frog has overcome these problems, however.

A grey wood frog in Quebec
A grey wood frog in Quebec | Source

Researchers have found that wood frogs can survive when sixty-five to seventy percent of their body is frozen. They can also safely undergo multiple freeze-thaw cycles in a winter.

Preventing Cells From Freezing in the Winter

The frog's liver produces a large amount of glucose as winter approaches. This is transported by the blood and enters the animal's cells, where it acts as an antifreeze. When substances dissolve in water, they lower its freezing temperature. The high glucose concentration in cells prevents their interior from freezing as the temperature drops. The glucose concentration in the cells of the hibernating frog may be a hundred times higher than normal during the winter.

An increased concentration of a waste substance called urea also helps to prevent freezing in the cells. Urea is normally excreted in urine. The high glucose and urea levels don't appear to hurt the frog.

Even though the animal's cells are not frozen, they are either inactive or have extremely low activity. Active cells need oxygen and other nutrients from the blood and must send their waste substances into the blood. The blood doesn't flow when a wood frog is frozen, however.

Freeze tolerant animals typically confine ice growth to extracellular spaces of their bodies while using protective mechanisms to keep the water inside their cells from freezing (extracellular freeze tolerance plus intracellular freeze avoidance).

— Janet M. Storey (Carleton University) and Nature North

Frozen But Still Alive

Extracellular Water

Although the water in the frog's cells doesn't freeze, at least some of the water outside the cells does. This includes water on the skin, between the skin and muscle, surrounding the organs in the abdominal cavity, and in the lens of the eye. As a result, a hibernating frog looks as though it's frozen and feels like a solid block. Researchers have discovered that much of the extracellular water is moved to places where its freezing is least likely to damage cells.

Scientists have found that the frog seems to encourage the start of ice formation outside its cells. Its skin is highly permeable to water and its body contains ice-nucleating agents. These act as a seed for ice growth in the water that has collected in the extracellular spaces. The nucleating agents include certain minerals and bacteria that the frog has ingested as well as specific proteins in its body.

Reproduction After Thawing

Thawing Safely in the Spring

Although researchers partially understand the processes that occur in a wood frog's body as it freezes, the signals that stop the heart from beating and the lungs from working are still mysterious. Some aspects of the thawing process are still puzzling as well.

It takes about a day for the wood frog to both thaw and return to normal activity and a bit longer before it's ready to reproduce. The thawing process starts from the inside of the animal's body and moves outwards, causing the frog to gradually come out of suspended animation. The signals that stimulate the heart to start beating again and the lungs to start working is unknown.

The frog appears to be in good condition once it's thawed. There is some evidence that body repair processes become more active than usual during and immediately after thawing, however.

In the northern part of its range, the wood frog has a major advantage over other frogs. In the spring, the land and the frog's body thaw before the icy covering of lakes, ponds, and rivers. Wood frogs are therefore able to breed before most other frog species. They lay their eggs primarily in temporary meltwater ponds, also known as vernal pools. The eggs are also laid in permanent bodies of water, however, especially in the warmer part of the animal's range.

The heart resumes beating even before ice in the body has completely melted, and pulmonary respiration and blood circulation are restored soon thereafter.

— Miami University

Wood Frog Development: Egg, Tadpole, and Adult

Similarities and Differences Between Frogs and Humans

A frog is a vertebrate, like humans. Although the animal looks very different from a human externally, there are many similarities in the internal organs of a frog and a human. Both follow the basic vertebrate plan for internal anatomy. Their bodies also have many chemicals and chemical reactions in common.

One difference between the two organisms is that humans are endothermic (warm blooded) and frogs are ectothermic (cold blooded). An endothermic organism maintains the same internal temperature whatever the environmental temperature, except in special circumstances, due to processes that occur in the body. The temperature of ectothermic organisms is generally the same as that of the environment. Some ectotherms modify their temperature by their behaviour, however, such as by sunbathing when they're cold and entering a shelter of some kind when they're hot. The term "cold blooded" is not always accurate for them.

A wood frog tadpole
A wood frog tadpole | Source

A study of wood frogs may help scientists improve the cryopreservation of tissues and organs, glucose management in diabetes, and the prevention of reperfusion injuries.

Cryopreservation

Understanding how the frog's body responds to temperatures below and then above freezing may help us improve the cryopreservation (preservation at low temperatures) of human cells, tissues, and organs. These need to be preserved in excellent condition so that they can be transplanted into the patients that need them.

Improving the preservation of organs would be especially helpful. At the moment, these are cooled but not frozen, which limits their availability to patients who need them. The organs eventually die unless they are frozen. Freezing and thawing are much more successful for small items such as eggs, sperm, and embryos than for large items such as organs. Frozen organs are damaged by cracking during the thawing process.

A more mature tadpole
A more mature tadpole | Source

A High Glucose Level in the Blood and in Neurons

Discovering the details of glucose management in the frog may help doctors deal with diabetes. Insulin transports glucose into most of the cells in our body. The glucose molecules are used as an energy source. In diabetes, blood glucose (also known as blood sugar) rises, either because insulin is no longer being made by the pancreas or because insulin is no longer doing its job.

Neurons, or nerve cells, don't use insulin to absorb glucose, but they do have molecules called glucose transporters that serve a somewhat similar function. In diabetes, a high blood sugar level sometimes causes neurons to have a higher than normal glucose concentration. Intracellular glucose in neurons can be toxic at high concentrations and is thought to contribute to diabetic neuropathy. A high glucose level doesn't seem to be toxic in wood frogs, however, at least heading up to and during hibernation. It would be interesting and possibly useful to know why.

The adult
The adult | Source

Reperfusion Injury

There is another way in which the study of the frogs could help humans. Humans may experience a reperfusion injury, or tissue damage, when blood returns to an area after being absent for a while. The absence of a blood supply may be caused by a heart attack or by a stroke.

The lack of blood flow to a part of the body means that the area lacks oxygen and nutrients and that toxins build up. These factors may damage the area. The area is then susceptible to being further damaged by reactive oxygen species when blood returns. The reason for the appearance of these chemicals is still being investigated.

The wood frog doesn't appear to experience any harm when its blood starts to flow again in the spring, or if it does, the damage is quickly repaired. Understanding how damage from the stoppage and restarting of blood flow is prevented or very significantly reduced could be useful.

An Intriguing Amphibian

The wood frog is an intriguing animal that may have much to teach us. Hopefully, understanding its biology will help us deal with medical problems. Even if this doesn't prove to be true, the frog is a fascinating little creature that is worth studying. Its adaptations for survival in winter are very impressive.

References

© 2017 Linda Crampton

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    • AliciaC profile image
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      Linda Crampton 4 months ago from British Columbia, Canada

      Thank you very much, Eric. It's nice to meet you!

    • EricMBordner profile image

      Eric M Bordner 4 months ago from Florence

      This is a very thorough article and it is really well written. Good Job !

    • AliciaC profile image
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      Linda Crampton 5 months ago from British Columbia, Canada

      Hi, Vellur. I agree—cryobiology is a very interesting topic to study. I hope scientists learn more about the wood frog's survival mechanisms soon.

    • Vellur profile image

      Nithya Venkat 5 months ago from Dubai

      Never knew about the freezing wood frog until I read this, thank you for sharing. It is amazing how the freezing frog survives with much of its body frozen and without a heart beat. Cryobiology is amazing and an interesting branch of study. Thank you for sharing.

    • AliciaC profile image
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      Linda Crampton 5 months ago from British Columbia, Canada

      Thank you, Suhail. I know that some other frogs and some reptiles are freeze tolerant, but I think the wood frog is the champion. It definitely has some impressive abilities.

    • Suhail and my dog profile image

      Suhail Zubaid aka Clark Kent 5 months ago from Mississauga, ON

      Very informative article, Linda.

      I always wondered how the snakes and frogs of Ontario hibernate in winters. This article explains well.

    • AliciaC profile image
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      Linda Crampton 5 months ago from British Columbia, Canada

      It will be very interesting to see if humans can ever do what the wood frog can do! Thanks for commenting, Manatita.

    • manatita44 profile image

      manatita44 5 months ago from london

      Amazing what God has or is allowing the wood frog to do. Perhaps humans will do this in 40 years. Who knows?

      Interesting noises that they make, Linda.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Thank you, Devika. I appreciate your visit.

    • DDE profile image

      Devika Primić 6 months ago from Dubrovnik, Croatia

      Informative and well-presented with photos. I learned about wood frogs from this interesting hub.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Thank you for the comment and for sharing your experience, Flourish. It's an interesting story, even though it's related to a sad situation.

    • FlourishAnyway profile image

      FlourishAnyway 6 months ago from USA

      This was very interesting, and I can see so many potential applications for use. The fact that these frog ties can survive at all is astounding. I discovered by accident that freezing destroys tissue when I worked it it's a charity that neutered and spayed cats. One of the cats that was euthanized because of its injuries needed a necropsy that couldn't be immediately performed. The vet told me to store the body for a few weeks but I froze it instead of refrigerating it. Freezing destroys the tissues.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      You have some great ideas, Mel! The second scenario sounds especially interesting. Thanks for the visit.

    • Mel Carriere profile image

      Mel Carriere 6 months ago from San Diego California

      This sounds like an excellent scenario for a science fiction movie - alien frogs arrive from outer space cryogenically frozen, then thaw out and proceed to take over the earth.

      In all seriousness, could the lowly wood frog hold the key for long-distance space travel? Could we freeze astronauts to travel across light years, then thaw them out at the other end? This little critter has fascinating possibilities. Great work opening our eyes to this scientific secret.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Thank you very much for the comment, Genna. It must have been interesting learning about biology from your father.

    • Genna East profile image

      Genna East 6 months ago from Massachusetts, USA

      I've heard of these frogs before from my father; he was a research biologist and fascinated with their methods of survival, and the production of a kind of antifreeze. Intriguing article; and so well written, Linda.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Hi, Dora. I'm interested in seeing whether discoveries about wood frogs can help humans, too. I think that the animals are very intriguing.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Hi, Mary. Like you, I think that nature has much to teach us. Exploring the natural world is both interesting and helpful.

    • MsDora profile image

      Dora Isaac Weithers 6 months ago from The Caribbean

      The wood frog is a marvelous creature. Thanks for the lessons on these extraordinary animals I would not hear of otherwise. It would be interesting to learn more about how the study of these animals can aid the health of human beings.

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      Mary Wickison 6 months ago from Brazil

      There is so much we can learn from nature. I believe all the answers to the problems we have, can be solved naturally.

      This frog is an excellent example of how complex but useful the ability to adapt to the surroundings is. Fascinating animal and interesting article.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Thank you for such a kind comment, Bill. I appreciate it very much.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Hi, Buildreps. Yes, there are certainly some wonderful creations in nature! Thank you for the comment.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Hi, Penny. It is surprising that glucose can be so helpful! The frog has a great protection system.

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      Bill Holland 6 months ago from Olympia, WA

      Linda, your articles are always like watching a show on public broadcasting, educational and very interesting. I always learn from you and for that I thank you.

    • Buildreps profile image

      Buildreps 6 months ago from Europe

      I can imagine why you are fascinated by this little creature. Amazing animal, and also fascinating how nature is able to produce such wonderful creatures. Interesting article as always, Alicia

    • Penny Sebring profile image

      Penny Sebring 6 months ago from Fort Collins

      Fascinating! It's amazing how much a little extra glucose can change so much.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Thank you for such a kind comment, Martie! I appreciate it very much.

    • MartieCoetser profile image

      Martie Coetser 6 months ago from South Africa

      What an amazing amphibian! I am totally gobsmacked. Thank you, Linda, you are the best teacher in my world.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Hi, Louise. It is amazing that the frogs can survive being frozen. It should be very interesting to see what else is discovered about them.

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      Louise Powles 6 months ago from Norfolk, England

      Goodness, I had no idea about wood frogs. It's amazing how they can still live even when frozen. I enjoyed the videos too. It's useful to see them too.

    • AliciaC profile image
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      Linda Crampton 6 months ago from British Columbia, Canada

      Thank you very much for the comment, Heidi. Yes, there are so many cool creatures left to explore. Nature is fascinating!

    • heidithorne profile image

      Heidi Thorne 6 months ago from Chicago Area

      Wow! There are so many cool creatures that we still need to understand. And just imagine if medical science could emulate some of their amazing abilities. Thanks for sharing your research and expertise with us!