Читать книгу Extreme Nature - Mark Carwardine - Страница 36

NAME	wood frog Rana sylvatica
LOCATION	Canada and Alaska
ABILITY	withstanding being frozen for weeks

© J M Storey/Carleton University

There’s an African midge so well adapted to drought conditions that, as a sideline, it can withstand being artificially frozen to –270°C (–454°F). Lots of other insects can survive freezing, too, but the creatures that can withstand cold for the longest period are probably bacteria in Antarctica.

The most freeze-tolerant higher animal is the wood frog, which can literally become a ‘frog-sicle’, enabling it to live further north than any other amphibian and to hibernate close to snowmelt ponds, presumably to give it a head start and enable it to reproduce quickly before the ponds dry.

When the temperature drops below freezing, the frog’s liver starts converting glycogen to glucose, which acts as an antifreeze. The blood passes the glucose to the vital cells, which are then protected from freezing on the inside, all the way down to –8°C (18°F). But the rest of the frog’s body fluids, up to 65 per cent of them, turn to ice and the organs, deprived of blood, actually stop working. Even the eyeballs and the brain freeze. It is effectively the living dead. (The painted turtle Chrysemys picta can do this, too, but only briefly.) When a thaw comes, the frog’s heart starts beating and pumps blood containing clotting proteins around the body, which stops bleeding from wounds caused by the jagged ice crystals. The frogsicle quickly comes back to life and, just as miraculously, so do the frozen parasitic worms in its body.