Winter World: The ingenuity of animal survival by Bernd Heinrich, Ecco Press/HarperCollins, $24.95, ISBN 0060197447 Reviewed by Jonathan Beard
鈥淭HE dead of winter鈥� is a common metaphor, but it doesn鈥檛 apply to the bitterly cold, snow-covered woods of Vermont and Maine where biologist Bernd Heinrich lives and works. The skies, trees, leaf litter and frozen soil all teem with life.
Some of these organisms, it is true, are merely biding their time until spring. At the bottom of each frozen pond lie turtles, buried in the mud at 0 掳C. But what is impressive is not spending 6 months at freezing point, it is that turtles are air-breathing reptiles. Heinrich shows how winter is, in effect, a 6-month-long dive, with the turtles coming up for air in May. It may have been this adaptation, he suggests, that enabled them, unlike dinosaurs, to survive the 鈥済lobal winter鈥� that followed an asteroid impact 64 million years ago.
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We are more familiar with mammal hibernation and think of bears dozing, warm in their cosy dens. But Arctic ground squirrels are not cosy: these animals carefully regulate their body temperature to 鈭�1 or 鈭�2 掳C for 8 months of the year, and supercool their tissues to prevent them from freezing.
And their brains? Researchers who have given the squirrels EEGs found no sign of activity. But from time to time during hibernation, the squirrels start shivering to warm themselves, and their brains then start to display signs of rapid-eye-movement (REM) sleep, or dream sleep. Astonishingly, they rouse themselves to sleep. Sleep, apparently, is something a hibernating squirrel can鈥檛 do without.
Another of the creatures Heinrich features is the kinglet. While most small birds desert New England鈥檚 woods and migrate south, the kinglets, little balls of feathers weighing barely 6 grams, fly and feed all winter. They flit from tree to tree from dawn to dusk, plucking tiny frozen caterpillars off pine needles. Even a few hours without feeding means death. Unlike many animals, which devote time and energy to nest building to preserve heat at night, kinglets roost wherever they find themselves at nightfall. They pay a price for staying in Maine 鈥� 87 per cent mortality each winter.
Why they have chosen the perils of the cold over the rigours of migration, Heinrich does not know. He is leaving some mysteries for next winter.