The joyous ''peep'' of the spring peeper is the herald of North American spring. But how do these songsters survive the winter?
The answer appears to be that they, and some other frogs, make sophisticated use of antifreeze.
As biologists continue to investigate tolerance for cold, they are finding that the use of antifreeze is widespread in both the plant and animal kingdoms. It is found in such diverse creatures as overwintering insects and Arctic fish. Many plants, or their seeds, also survive cold in this way. Often the key cryoprotectant, as biologists call it, is glycerol, which is also used in automobile antifreeze.
William D. Schmid of the University of Minnesota has found that frogs use this chemical too. Reporting this in Science, he notes that many frogs, toads, and tree frogs hibernate with only a shallow covering of leaf litter. If there is a thick snow pack on top of that, they are well insulated. But when there is little or no snow, they are exposed to freezing temperatures - perhaps 4 to 10 degrees below zero Celsius. Thus one would expect such animals to have some internal defense against freezing.
Schmid looked for this in the gray tree frog (Hyla versicolor), spring peeper (Hyla crucifer), and wood frog (Rana sylvatica). Experiments showed they can survive a degree of freezing in which an average of 35.6 percent (give or take 1 .1 percent) of their body water turned to ice. This involved slow freezing and thawing, as would happen in nature. For example, Schmid reports that, for specimens kept frozen for periods at -6 degrees C., it took 2 to 4 days of thawing at 4 to 8 degrees for such vital signs as limb movement to return.
Some organisms use antifreeze to lower the temperature at which ice forms in their tissues. Others follow a more sophisticated strategy in which ice is allowed to form outside the body cells themselves. Antifreeze within the cells protects these vital units of organic life.
Some biologists, notably John G. Duman of the University of Notre Dame, have studied such freeze-tolerance in insects. As he reported two years ago, the formation of ice outside body cells would tend to draw water from the cells and dehydrate them. However, glycerol within the cells keeps them moist.
Such a protective system works only if freezing occurs slowly. Too rapid a freeze could cause the cells to freeze first. Thus freeze-tolerant insects have a protein, called a nucleator, which encourages freezing in fluids outside the cell while the antifreeze, glycerol, inhibits freezing of the cell itself.
Now Schmid has found that some frogs use a similar strategy. This came as something of a surprise. He says he had expected these frogs to survive by using antifreeze to prevent freezing. Instead, he found an example among vertebrates of survival after extensive body freezing. They're hardy, those spring peepers.