Can A Frog Freeze And Survive

[Karri Weston]

This is the winter world in which the wood frog must survive. Remember, frogs are cold blooded, so their body temperature is about the same as the surrounding air. How do these delicate little creatures endure the intense, protracted, iron-cold subarctic winter?

Wood frogs have a different strategy. They hibernate by nestling down into the leafy litter on the forest floor. The leaves, duff and overlying snow give some insulation from extreme cold, but the frogs are not protected from subfreezing temperatures as they would be if they chose the underwater strategy.

For most other animals, survival depends on protecting themselves from any condition that could freeze their flesh. Why is freezing so dangerous? Several things can happen: If ice crystals form inside an animal they can puncture blood vessels. When blood freezes, there is no mechanism to deliver oxygen and nutrients to organs, so extreme metabolic damage occurs. And ice severely injures cells by drawing out water and causing dehydration, scrambling the interior structure of cells and fracturing the cell walls. The result is pervasive and deadly internal damage.

Hibernating wood frogs can tolerate blood sugar levels 100 times higher than normal without the damage suffered by human diabetics when their blood sugar is only 2 to 10 times above normal. Understanding how frogs can do this might provide valuable knowledge to help in the management of high blood sugar in people with diabetes.

Also, the wood frog's ability to withstand freezing may help researchers discover how human organs used for transplants could be frozen and thawed without damage. This would increase the allowable time between removing an organ from a donor and implanting it within the recipient, which could make many more transplants possible.

Researchers are also interested in how the wood frog's body can stop blood circulation and start it again many months later without blood clots or other injuries. Understanding the mechanism which allows this could be valuable for treating people after their blood flow is temporarily halted by heart attack or stroke.

There are 14 species of frogs and toads in Minnesota, and eight of them can be found in Three Rivers Park District. While all of them stay in Minnesota for the winter, they have very different approaches to surviving the cold.

American toads cannot freeze and survive, so they need to stay below the frost line all winter. They tend to stay within a couple of inches of the frost line and will move up and down throughout the winter as the frost line changes.

Their unusual overwintering strategy was first documented right here in Three Rivers Park District at Elm Creek Park Reserve by Bill Schmid from the University of Minnesota in the 1960s. Before that discovery, scientists did not know where these frogs went in the winter.

Dr. Schmid was helping a student collect land snail shells in the winter and found what he thought was a rock shaped like a treefrog. After a while, he realized it was a frozen frog and started doing research on how and why it happened.

His research found that some frogs accumulate glycerol their bodily fluids during the fall when the temperature begins to cool. This protects the cells from rupturing when the frogs partially freeze.

The last group of frogs in Three Rivers, which includes the northern leopard frog and green frog, spends winters under water in lakes, ponds and rivers. These frogs tend to sit on the bottom, sometimes in large groups. They will also move from time to time.

If there is clear ice on a lake you may be able to see frogs swimming under the ice. They need water that has enough oxygen for them to survive. These are normally the lakes and rivers that have fish. These frogs can absorb the oxygen through their skin and mucous membranes.

John Moriarty is the Senior Manager of Wildlife at Three Rivers Park District and has been with the Park District for 15 years. He has been involved in many of the wildlife restoration efforts and initiated the snake and butterfly efforts. John has led several projects to increase prairie habitat in the Park District. John likes exploring natural areas and looking for all types of plants and animals, but especially turtles.

Where do spiny softshell turtles go after nesting on the beach at French Regional Park? How far do softshell, painted and snapping turtles travel in the water? Are they active in winter or affected by water quality? Find out what we learned during the Medicine Lake urban turtle project.

Researchers are still studying this and the mechanisms that protect the frog, aside from glucose. Urea, a waste that frogs get rid of in their urine, was recently shown to help them survive freezing. And proteins may bind to the inside and outside of the cells to keep them from shrinking too much, suggests Kenneth Storey, a professor of biochemistry at Carleton University in Ottawa, Canada, who also studies freeze-tolerance.

For as long as seven months, up to 60 percent of their bodies freeze solid. They stop breathing. Their heart stops beating. This semi-frozen state allows them to survive temperatures that that dip below zero, explains Brian Barnes, researcher and director of Arctic Biology at the University of Fairbanks, Alaska. Come spring, they thaw out and come back to life.

The North American wood frog survives the winter with a heart-stopping strategy. This frog, found from southern Ohio up to the Arctic Circle, puzzles researchers with its ability to literally "freeze and thaw" along with normal winter-spring weather patterns.

Jon Costanzo, a professor at Miami University in Ohio, studies amphibians that live in harsh climates, especially cold ones. His research was featured on NOVA's "scienceNOW," a television magazine series on PBS. The "Frozen Frogs" episode shows Costanzo holding a frozen frog in the laboratory, as well as identifying the frigid amphibians in their natural environment.

"That touch of ice immediately sets off signals inside the frog that pulls water away from the center of its body, so the frog's internal organs are now wrapped in a puddle of water that then turns to solid ice," says Costanzo in "Frozen Frogs."

The frog's heart stops beating, its kidneys stop functioning and its respiration ceases--for months. The frogs endure this suspended animation by producing a type of antifreeze made with glucose, keeping the water in their cells in a liquid state at temperatures below 32 degrees Fahrenheit (0 degrees Celsius).

When warmer spring temperatures trigger the frog to thaw, its heart and brain thaw first, followed by its body, all in perfect synchrony. Costanzo refers to this series of events as "a spontaneous resumption of function." Within 10 hours, the frog is fully functional.

"Currently it's not possible to freeze human organs for transplantation," said Costanzo, "yet the wood frog and several other amphibians and reptiles have solved not only the problem of freezing individual tissues and organs, but also that of simultaneously freezing all organ systems."

According to the Scientific Registry of Transplant Recipients, more than 85,000 patients were on the national waiting list for an organ transplant as of June 2004. At present, a heart can be persevered for only five hours before it must be implanted into a recipient; a kidney might last 72 hours. If organs could be preserved longer, more time would be available for both the recipient and the associated surgical team, leading to improved transplantation success rates.

During the long, cold winter months, have you ever wondered how a cold-blooded creature, such as a frog, can survive? Fortunately, they have developed special behaviors and physical processes to survive the long, cold, and snowy winter.

As mentioned above, frogs and toads are cold-blooded, so their body temperature actually takes on the temperature of the environment around them. During the winter, they go into a state of hibernation, and some frogs can be exposed to temperatures below freezing.

Frogs and toads that spend most of their time out of the water and on land can usually burrow down below the frost line in burrows or cavities that are their hibernating space for the winter. Frogs go as deep as they can or squeeze into cavities, crevices, and logs.

Winter is an extremely harsh time for reptiles and amphibians in colder climates. As cold-blooded animals, exposure to even mildly freezing conditions can be fatal. Many northern species find places to overwinter that protect them from freezing temperatures by burrowing underground. Others seek refuge from the freeze underwater, but need to survive with minimal access to oxygen or even complete oxygen deprivation for prolonged periods; conditions that would kill most mammals in a matter of minutes. Because Mudpuppies mostly live in fast-flowing water that holds oxygen throughout the year they are spared the harshest conditions winter has to offer, but other reptiles and amphibians do manage to survive freezing conditions and prolonged oxygen deprivation. The ways in which they do this are quite fascinating and I would love to share with you what I know about these remarkable survival strategies, starting with how some species survive freezing.

While freezing solid at -10C (14F) almost always causes turtle death, the hatchlings are tolerant of freezing at milder temperatures. The same glucose that helps the turtles supercool also protects their tissues from damage caused by the freezing process. High concentrations of glucose between the cells draws water into the extracellular spaces so when freezing does occur, it occurs outside of the cells and the ice does not rupture the cells as it expands. Consequently, the hatchlings can survive freezing to about -4C (25F). In this state, all but the liver and other vital organs freeze solid and can remain so for several days without causing harm to the turtles. Only a small handful of turtle species can freeze as hatchlings, and of those, only the Box Turtle (Terrapene carolina) can also freeze as an adult (Box Turtles being the only species with freeze-tolerant hatchlings that overwinters on land as an adult). The only other reptiles native to North America with some freezing ability are gartersnakes (Thamnophis sp.).

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