February 27, 2008
By Susan Benson,CNHM Director of Education
So you think you get cold in the winter? Consider the wood frog. This little amphibian actually freezes solid during the winter months. While frozen, these frogs have no heartbeat. They do not breathe. Their blood does not circulate. Their nerves barely register electrical impulses. Yet their vital organs are not damaged, even after being frozen solid for weeks on end. Come spring, the little frogs simply thaw out and hop away to woodland ponds to mate and carry on with life. Spring peepers, gray tree frogs and upland chorus frogs also may freeze and thaw several times during the course of a winter. How do they do it? The answer is antifreeze. When ice begins to form on a wood frog’s skin, the frog’s liver releases a high level of blood sugar, enough to fill the blood vessels in all vital organs. The blood of frozen wood frogs may have a glucose level 100 times as concentrated as that in average human blood.
This highly concentrated sugar solution resists freezing, and although nearly two thirds of the frog’s body water may turn to solid ice, that ice is located in non-vital areas such as the bladder, stomach cavity, beneath the skin, between organs and muscle fibers and between cells. Ice does not form within the living cells and therefore does not damage organs. Come spring, the sugar is quickly removed from the frog’s blood by the liver, and the frog returns to what we’d call “normal.” Frogs are not the only animals that survive the winter as living ice cubes.
Garter snakes, normally the last snake species to hibernate in autumn and the first to emerge in spring, can tolerate a night or two of being frozen if they’re caught away from their hibernation hideaway. Adult box turtles can survive freezing and newly hatched painted turtles freeze solid and thaw out repeatedly when in their nests during their first winter. Not all animal adaptation to winter is so extreme. Many birds that spend winters up north grow a winter coat of heavy-duty feathers to help insulate them from the cold. Goldfinch feather weight is about 50 percent higher in winter than summer. Even with feathers fluffed, however, most birds need to generate extra heat to survive, and they do this just as humans do: by shivering. Chickadees, for example, shiver more or less continuously throughout winter except when they’re flying. This winter, when you may be feeling chilly and tempted to exclaim “I’m freezing!” remember that there are animals out there that actually are freezing!
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.