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By Root 1230 0

and explored the physiological underpinnings of freezeing-tolerance in frogs. They confirmed and extended what Schmid had discovered, and we now know that four common North American hibernating frogs—the wood frog, gray tree frog, spring peeper, and chorus frog—all tolerate being frozen. In freezing-tolerant frogs there is extensive ice formation in the body cavity and in the spaces between the cells (up to 65 percent of the total body water in the wood frog may be ice), but in frogs that survive there is no ice crystal formation within the cells themselves. Ice crystals are normally lethal when they form within the cells because they cut like knives, slashing membranes, puncturing cell organelles and breaking cells. The frog’s lethal low temperature limits go only as low as -8°C because at lower temperatures than that ice does form within cells. But lower temperatures are seldom encountered in wood frogs’ hibernacula under leaves and snow. (Where spring peepers, chorus frogs, and gray tree frogs hibernate is not well known. I’ve often heard the peepers piping in the woods in the fall, so they presumably hibernate somewhere in the woods. One colleague told me of finding a tree frog under loose bark in the winter, and another had one hibernate on a houseplant she brought inside in the fall.)

All of the four above-mentioned frogs that hibernate on land can tolerate about half of their body water turning to ice, but that feat is not possible without the aid of chemistry that addresses two main problems. First, one chemical (primarily the alcohol glycerol) protects membranes when freezing does occur, and second, the other (primarily glucose) is mostly but not exclusively involved in an osmotic response that restricts ice-formation to outside the cells. However, unlike in the cold-hardening of many insects that similarly use alcohols and sugars to perfuse their tissues prior to winter, frogs do not accumulate these chemicals in the fall in anticipation of freezing. Instead, frogs wait for the ice to form, and in one day they change themselves to become frost-tolerant. The ability to tolerate freezing is acquired in a modification of the adrenaline-mediated fright response of other vertebrate animals, such as in ourselves.

All vertebrate animals have a fight-or-flight response in which the sensory input of a threat, say a charging lion, causes the hormone adrenaline to be released from the adrenal glands. Adrenaline has wide-ranging effects, but the net effect is to prepare the body to meet the challenge. Heart rate increases, blood glucose concentrations rise, and blood flow is redistributed to the muscles. This adrenaline response has been modified in wood frogs to meet the freezing challenge that is lethal to aquatic frogs, and of course to all of us.

When the first ice crystals begin to form on or in the skin of a wood frog, it sets off an alarm reaction. Skin receptors relay the message of freezing to the central nervous system (CNS), and the CNS activates the adrenal medulla to release adrenaline into the bloodstream. When the adrenaline circulates to the liver, it there activates the enzymes that convert the liver’s stores of glycogen to glucose. As a result, the frog responds with a quick rise in blood glucose. In the wood frog, this response is massive and before the ice reaches the cells they become packed with glucose that acts as an antifreeze. Precisely the opposite occurs outside, between the cells, where special proteins act as ice-nucleating agents to promote ice crystal formation in areas of dilute fluid. As a result, pockets of concentrated fluid are created, and these act to osmotically withdraw water from the cells, making them even more resistant to ice formation. In about fifteen hours, the frog is frozen solid except for the insides of its cells. Its heart stops. No more blood flows. It no longer breathes. By most definitions, it is dead. But it is prepared to again revive at a later date.

Glucose is the normal vertebrate blood sugar that is used by the cells for energy. In the healthy, active animal,