Alaska Wood Frogs
JB: This is Earth and Sky—on northern Alaska’s only amphibian—the wood frog.
DB: Last month, Alaska’s wood frogs got ready for winter by burrowing into decayed leaves on the forest floor. As the temperature drops, two–thirds of the water in the frogs’ bodies will freeze. For seven months, the frogs won’t draw a breath—their hearts won’t beat. They’ll seem more like frog–shaped Popsicles than living creatures.
JB: The frogs’ bodies contain a chemical that protects their frozen tissues. As soon as freezing begins, the animal’s liver converts a stored compound called glycogen into glucose, a sugar. Glucose circulates in the bloodstream and enters the frogs’ cells, where it lowers the freezing point of water—much as salt lowers the freezing point of seawater. The glucose also seems to organize what little water remains in the cell so that the water coats delicate membranes and protects them from damage. And glucose helps to minimize the effects of dehydration—the most damaging aspect of the freezing process.
DB: Scientists are studying these frogs in hopes that someday, human organs can be preserved by freezing, for future transplants. Scientists also want to know how wood frogs tolerate high levels of glucose in their bloodstream. People with diabetes may someday benefit from this research on frozen frogs.
JB: Thanks today to the U.S. Forest Service—caring for your national forests and grasslands. We’re Block and Byrd for Earth and Sky.
Our thanks to the following individual who assisted in the preparation of this script:
Dr. Brian Barnes
Associate Professor of Zoophysiology
Institute of Arctic Biology
University of Alaska, Fairbanks
Fairbanks, AK
Dr. Jon P. Costanzo
Adj. Assoc. Professor
Laboratory for Ecophysiological Cryobiology
Department of Zoology
Miami University, Oxford, OH
http://miavx1.muohio.edu/~cryobiocwis/
The following articles were used in preparing this script:
Rozell, Ned. “Looking for a few good wood frogs.” Alaska Science Forum, Article # 1297, August 7, 1996.
Rozell, Ned. “Wood Frogs: The Farthest North Amphibian Cannibals. Alaska Science Forum, Article #1236, May 25, 1995.
Abstracts consulted:
Storey, K.B., D.D. Mosser, D. N. Douglas, J.E. Grundy, and J. M. Storey. “Biochemistry below 0 degrees C: Nature’s frozen vertebrates.” Brazilian Journal of Medical and Biological Research 29 (3):283–307; 1996.
King, Patricia A., Rosholt, Mary N., and Kenneth B. Storey. “Seasonal changes in plasma membrane glucose transporters enhance cryoprotectant distribution in the freeze–tolerant wood frog.” Canadian Journal of Zoology 73 (1): 1 – 9; 1995.
Storey, Kenneth B. “Organic solutes in freezing tolerance.” Comparative Biochemistry and Physiology A 117 (3) 319–326: 1997.
Web site to visit:
To see a picture of a frozen frog, go to http://www.ualberta.ca/~smri/frog.htm
Other references:
Hodge, R.P. Amphibians and Reptiles in Alaska, the Yukon, and Northwest Territories, Alaska Northwest Publishing Co.
Author’s notes:
Wood frogs are just one of many creatures that use so–called “cryoprotectant” chemicals to surviving freezing temperatures; others include insects such as the goldenrod gadfly, other frogs such as spring peepers, and painted turtles. In fact, scientists have counted eleven species of amphibians and reptiles that can tolerate the freezing of up to 70 percent of their body water and can survive being cooled down to minus 6 degrees Centigrade. Different creatures use different kinds of cryoprotectants to survive freezing temperatures–wood frogs use glucose but other animals may use polyhydric alcohols or protein molecules.
Alaska isn’t the only place where wood frogs freeze in winter. These frogs are found from Georgia northward throughout the United States. And wherever winter temperatures drop below zero, the frogs freeze. What’s unusual about Alaskan wood frogs is that they tolerate colder temperatures and freeze for longer periods of time than wood frogs in other locations. For example, Jon Costanzo studies wood frogs in Ohio, where even in midwinter frogs stay frozen for just a few days at most, as air temperatures constantly go up and down. But Brian Barnes says Alaskan wood frogs freeze in November and stay frozen right through April. Also, even though 25 degrees Fahrenheit seemed to be the limit for wood frog survival both in lab studies and in studies of wood frogs in temperate locations, Brian Barnes has found that Alaskan wood frogs can tolerate temperatures down to minus 12 degrees Centigrade.
One important distinction about wood frogs is that, compared to other cold–tolerant amphibians, they seem to “wait till the last second,” according to Jon Costanzo, before starting to circulate their cryoprotectant compounds.
Glucose only starts to circulate once the frog has actually started to freeze. Other frogs, and also cold–tolerant insects and reptiles, start to make cryoprotectants well before they reach the freezing point.
Wood frogs ARE the only amphibian found in northern Alaska and Yukon territory. Six species of amphibians live in southeast Alaska. There are three species of newts and salamanders (rough–skinned newt Taricha granulosa, northwestern salamander Ambystoma gracile, and long–toed salamander Ambystoma macrodactylum) and three species of toads and frogs (western toad Bufo boreas, spotted frog Rana pretiosa, and wood frog Rana sylvatica). Western toad is the most common of these.
The wood frog’s practice of digging a “hibernaculum” or winter resting chamber in leaf litter on the forest floor is somewhat unusual among frogs. In temperate climates, many frog species burrow into mud on the bottom of a pond. Although the pond may become ice–covered, temperatures at the bottom of the pond remain just about freezing all winter. In the hibernaculum, temperatures can get much colder. A protective blanket of snow does add some insulation.
A common assumption is that freezing damages living tissues because jagged ice crystals form and rupture delicate cell membranes. In fact the biggest problem with freezing is dehydration. Here’s how it works. The water in the spaces BETWEEN cells starts to freeze first. That leaves the remaining intracellular water with a HIGH concentration of solutes (dissolved compounds). Now, remember your high–school definition of osmosis? Water wants to move from an area of high concentration of water to an area of a low concentration of water. So, as freezing proceeds in the spaces between the cells, water starts to leak OUT of the cells into the extracellular spaces. The frog’s organs and tissues start to dehydrate. (In a frozen frog, Brian Barnes says, the liver goes from looking large, plump, and moist to looking like a little strip of beef jerky. Even the eyeballs of a frozen frog look shrunken. Meanwhile, ice forms in the body cavity; if you dissected a frozen frog, says Dr. Costanzo, you’d see what looks like a little snow–cone of ice in its coelomic cavity. One study estimates that as much as 65 percent of the frog’s total body water becomes extra–cellular ice. Brian Barnes notes that if you bang a frozen frog against a countertop, it feels and sounds like an ice cube.)
Anyway, as this extracellular freezing begins, wood frogs respond by breaking down glycogen, a storage compound in the liver, into glucose. Glucose circulates in the blood (sometimes reaching a concentration 22 times higher than normal) and gets taken up by the cells. With glucose going into the cells, now there’s a high concentration of solute INSIDE the cells as well as outside, so osmotic balance is restored, and water stops leaking out. That’s what’s meant in the script when it says the glucose helps to prevent dehydration of cells.
As for the other protective actions of glucose: one is that the glucose simply takes up space–it replaces some of the volume of water that was lost.
And glucose also seems to somehow coat the cell organelles—to interact with cell proteins and membranes—in a way that protects them during freezing–although scientists don’t know exactly how this works. They think the glucose somehow “organizes” the water to keep a coating over the essential membranes.
Scientists note that core organs, such as the heart and liver, freeze last and thaw first. That means vital body functions such as circulation and metabolism are maintained for the longest possible time.
The freezing process needs to take place fairly slowly: You can’t take a wood frog from the forest, stick it in your freezer, and expect it to survive. (Barnes said, since wood frogs are so widely distributed, you might want to warn listeners about this, lest they try freezing frogs at home.) Dr. Jon Costanzo takes frogs from the forest into the lab when air temps are plus 4 degrees C and cools them to minus 2.5 degrees C over a 24–hour period, and they can survive this rate of freezing.
Although the study of frozen frogs may seem esoteric, it has some practical applications. For one thing, around the world many frog species seem to be experiencing population declines, and scientists aren’t sure exactly why this is going on. Costanzo is interested in creating a “bank” of frozen frog eggs and sperm as a hedge against possible future extinctions. This kind of effort already exists for many endangered species, but mostly for the “charismatic megafauna,”–the big cute and cuddly zoo animals.
Also, right now, although some kinds of human tissue such as eggs and sperm can be successfully frozen and thawed, human organs can NOT be stored frozen–they are damaged by freezing. So organ donation poses some logistical difficulties: as organs become available, they must be transplanted immediately to a compatible recipient. If scientists can learn how frogs can organize the freezing process within their organs to prevent damage, we may be able to apply these principles to storing human organs.
Finally, a frog that is breaking down massive amounts of glycogen into glucose is essentially experiencing what a human with the disease diabetes experiences. Yet for frogs, this condition is not harmful. So, studying freezing frogs may help researchers better understand diabetes and how to control it.
