FROZEN FROGS ON ICE - Winter World: The Ingenuity of Animal Survival - Bernd Heinrich

Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)


In the fall after the birds have left, I often hear birdlike chirps coming from the woods, especially after it warms up for a few days. I’ve tried numerous times to sneak up carefully and identify a caller, but so far I have discovered little more than that it was hiding on or near the ground. Whenever I’ve come close enough to almost step on the sound source, it went silent and I saw nothing.

By chance I found a passage in the writings of the famous nineteenth-century American naturalist-writer John Burroughs, who succeeded in tracing apparently similar sounds. Walking in the woods in his native New York on the last day of December 1884 when it was so unusually warm that bees were flying outside their hives, he paused in the shade of a hemlock tree where he heard a froglike sound from beneath the wet leaves on the ground in front of him. Determining the exact spot from where the sound came, he lifted up the leaves and found a wood frog. “This, then” Burroughs concluded “was its hibernaculum—[where] it was prepared to pass the winter, with only a coverlid of wet matted leaves between it and zero weather.” (We now know a great deal about how amphibians survive the winter, but we still don’t know why some of the ones that have loud mating choruses only in the spring after the ice goes out, may also sometimes call individually in the fall.)

Amphibians, especially toads, were known to dig into the ground to escape frost. Gardeners, your narrator included, often find them when turning soil in the fall. Some toads go down deep, as John R. Tester and Walter J. Breckenridge, two biologists from the Museum of Natural History at the University of Minnesota, Minneapolis, learned while studying the Manitoba toad (Bufo hemiophrys) in the Waubun Prairie in northwestern Minnesota. The area is known for its extremely cold winters. The toads are associated with water in the numerous potholes, and they travel 75 to 115 feet from water to burrow and hibernate in gopher mounds. In three years, the two researchers collected 7, 483 toads from eight mounds, tagged them with radioactive chips (100 microcuries of tantalum-182), and then periodically traced their whereabouts from above ground with a portable scintillation counter. Their study revealed that the toads dig even while they are in hibernation, going ever deeper throughout the winter. They dig down to four feet or deeper, staying just barely ahead of the ever-deepening frost line. They stop digging and stay at temperatures 1° to 2°C above freezing, which is then also their body temperature.

Burroughs reasoned that surely his wood frog would have, like a toad, buried itself into the ground if it anticipated a severe winter. However, since the frog had remained at the ground surface that would soon freeze solid, Burroughs thought that surely a mild winter was in the offing. Instead, a severe one followed: The ice on the nearby Hudson River was nearly two feet thick, and it was bitter cold even in March when Burroughs went back to reinvestigate the frog in its hibernaculum under the leaves.

Wood frog (Rana sylatica).

The matted leaves were then frozen hard. Burroughs lifted them and found the frog “as fresh and unscathed as in the fall” even as the ground beneath it “was still frozen like a rock.” He wrote: “This incident convinced me of two things, namely, that frogs know no more about the coming weather than we do, and that they do not retreat as deep into the ground to pass the winter as has been supposed.” Undoubtedly Burroughs would have been even more convinced about how little frogs know about the coming winter if he had found one frozen solid. He wrote that the frog he found on the rock-hard frozen ground “winked and bowed its head” when he touched it.

Fast forward now almost a hundred years later, to William D. Schmid, a comparative physiologist at the University of Minnesota in Minneapolis. Schmid had previously studied the tolerances of frogs to dehydration, and like Burroughs he also made a serendipitous discovery of a wood frog shallowly hidden under leaves in the winter. But Schmid’s observation would soon revolutionize accepted ideas, largely because he made a second observation: While handling the frog he noticed that it did not do what handled frogs usually do; which is to wink. Instead, it appeared to be frozen solid.

Having previously learned that different frog species have appropriate adaptations to survive in their unique habitats, Schmid doubted that the frog he found had made a lethal mistake in not burying itself deeply enough or in choosing the wrong spot to spend the winter. He therefore followed up his hunch that the frozen frog might still be alive, and a now-classic study of cold-weather survival in frogs followed. He published it in the prestigious journal of Science under the unassuming title “Survival of Frogs in Low Temperatures.” As far as I know, this is the first documentation of freezing-tolerance as a winter survival strategy of any vertebrate animals. Perhaps Burroughs had not believed his senses when he found his wood frog to “wink” underneath frozen foliage and on top of rock-hard frozen ground; given the conditions he described, the frog that he found should also have been frozen solid. To most animals, freezing means certain death.

Kenneth B. and Janet M. Storey at Carlton University in Ottawa took up the banner of frost-tolerance 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, blood glucose concentrations are normally precisely regulated by two hormones, insulin and glucagon. When we don’t have enough glucose in the blood we become unconscious, and when we have too much we suffer numerous short-and long-term consequences. We normally regulate our blood glucose at near 90 micrograms per 100 milliliters of blood, although during stress these levels rise slightly. When levels of blood glucose reach and are chronically maintained near 200-300 mg/ml blood, we’re diagnosed as ill with a disease called diabetes mellitus. Diabetes can now be treated by administering artificial insulin, the hormone that the pancreas is not producing in sufficient amounts in one version of the disease. (In Type I diabetes the pancreas is destroyed in an autoimmune response. In Type II there are no or few hormone receptors.)

In frogs that have felt ice crystals forming on as little as a toe, the massive blood glucose levels (to 4, 500 mg/ml of blood that are released in a seeming hyper-alarm response), would be high enough to send us into a coma and death several times over. But to the frogs, which survive it, in part because they are then at near 0°C, and metabolically relatively inert, it is their ticket to survival, to tolerate freezing.

Soon after the frog’s heart and breathing stop, its tissues would become starved of oxygen if metabolism continued. However, at high concentrations the glucose acts as antifreeze, a mechanical protectant from ice crystals, and an agent to help draw water from the cells. It also reduces the frog’s already very low aerobic metabolism and thus acts as a metabolic depressant to conserve the cells’ limited energy reserves for the winter. The glucose that enters the cells also becomes a substrate for anaerobic metabolism when the body can no longer supply oxygen.

Frozen bodies that can revive upon thawing out have long been a pipe dream of cryobiologists. The frogs that hibernate in the forest floor do it routinely, coming out of their frozen state at the first flush of spring when it is time to mate. They, like the hibernating bears I’ll discuss later, are biological marvels that challenge the limits of our beliefs of what seems possible.