Wood Frogs Spend Winter as Ice Cubes

wood frogs

Rana sylvatica is a species of wood frog that posses an incredible level of tolerance for freezing conditions. In fact, R. sylvatica doesn’t try to fight off the cold conditions but instead embraces them. These wood frogs survive the winter season by allowing themselves to freeze solid just like ice cubes. They survive not just for hours or days on end frozen but for weeks. The majority of their body water freezes completely and they not only stop breathing but their internal functions cease. In addition to being frozen for weeks their hearts stop beating, not intermittently, but also for weeks at a time.

Every part of this frog’s physical processes including breathing, heart beating, and waste production simply stop and come to almost a complete standstill. And this time spent being a frogsicle can happen multiple times a winter as the temperatures fluctuate from above to below freezing. The cold temperatures freeze the skin, blood and brain and yet these frogs survive the winter, albeit as ice cubes.

Most other creatures migrate to warmer climates to avoid the coldest temperatures. Some others survive the winter weather by hibernating in holes or in caves. The warmer season arrives and those animals that have chosen to hibernate begin to wake up and those who have migrated return to the area. Not so with this little amphibian. This wood frog, living anywhere from Georgia up through the Arctic Circle, simply lets nature take its course and allows the freeze to claim them.

Different areas have frogs that are able to survive at different temperatures. As might be imagined, those more northern wood frogs experience temperatures that are lower than their southern cousins and survive much longer periods of frost and freeze. Those frogs studied in the laboratory did not do as well as those studied in the wild. Frogs studied in the laboratory were found to die off in two months at temperatures around 20 degrees Fahrenheit. The wild frogs, on the other hand, which were studied in their natural habitat, experienced a state of being frozen for an average of 193 days with temperatures dropping as low as zero degrees. While many of the laboratory frogs expired, none of the frogs outside in their natural habitat died.

It appears that the wood frogs utilize the ability of glucose, a simple sugar molecule, to protect their freezing cells from drying out while they are being crystallized. The frogs which stayed outdoors had over ten times the amount of glucose in many of their tissues than did the frogs from the laboratory. It appears that the deciding factor was the cycling of the temperatures. The outdoor frogs had more chances to cycle glucose through their systems than did the laboratory frogs which only had one chance to manufacture glucose as the scientists did not cycle the temperature but only turned it down once. While the glucose helps the initial freezing of the frogs, scientists found something called an antifreeze glycolipid molecule in the frogs’ bodies, as well. These molecules stay outside of the cells and attach to the ice crystals, preventing the ice from penetrating the cell membrane and entering the cell itself.

The end result of all this may be the ability to use this technology and apply it to donated organs. Currently, there are only several hours before a donated organ may be surgically implanted into the living patient before the organ undergoes too much damage and is useless. Freezing organs with current techniques is not an option because the cells dehydrate and render the organs unviable. Scientists are hoping to take the fact that these wood frogs spend the winter as ice cubes and learn how their organs survive in order to apply that knowledge to donated organs.

By Dee Mueller
on twitter @TuesdayDG

National Geographic
Miami University

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