Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [70]
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