People often think of bears as the classic hibernators, Thao said, but in fact "bears are not true hibernators at all." Hibernation involves extremely low body temperature and heart rates. What bears do sounds closer to what we call sleep, except in longer stretches. Their body temperature and heart rate drop by only about 10 percent. They wake up every now and then.
Female black bears give birth in the winter. A sleep-deprived human mother might envy her ursine counterpart those postpartum months when the mom gets to doze while her cubs nurse. To prepare to provide milk, the mother bear stores fat and other nutrients as cold weather approaches, then loses a third or more of her body weight over the winter while nursing. At birth, the cubs weigh less than a pound, have only a light covering of fur, and can barely crawl. The mother stays somewhat alert, moving as needed to let the cubs nurse without squishing them.
Ground squirrels and chipmunks practice true hibernation, maintaining only essential bodily functions. As they curl up in some out-of-the-way spot, their hearts slow to a few beats per minute, their body temperatures drop almost as low as the surrounding air. Still, they also wake up from time to time to snack on acorns and seeds collected the previous fall. Rabbits and squirrels hibernate just briefly, in extreme cold.
For the mice, voles and other small rodents that stay active in winter (or at least emerge occasionally for food), winter is a relatively safe time of year. A layer of snow keeps their tunnels a comfortable temperature and offers concealment — unless they get nabbed by an owl, wolf, fox or coyote. Those predators have keen hearing (also, in the owl's case, good eyesight) and can detect tiny scurrying paws even through the snow. When canine predators hear a rodent, they pounce. To the uninformed, this movement — hopping into the snow with their front paws — might look like frolicking. But actually, they're busy killing.
On those cold winter mornings when you're fumbling for the alarm clock in the dark, hibernation might sound like a pretty good idea. Considerably less enviable is brumation, a winter strategy employed by many coldblooded animals — turtles, amphibians, salamanders— that sounds almost like cryonic suspension. Their physiological functions slow nearly to a halt and their hearts stop beating. Some frogs actually freeze solid; their individual cells protected by body chemicals. When spring comes, rising temperatures zap their hearts back into action, their bodies thaw and they go about their business. Assuming, that is, that they spent the winter sufficiently sheltered against cold and disturbance. If a wood frog or spring peeper gets stepped on in its frozen state, its body could break. Turtles bury themselves in the mud until late February or March, emerging before spring weather has fully arrived. "I have seen turtles, especially painted turtles, start walking on ice," Thao said. Snakes spend winters in crevices and other protected spots, generally in groups. In spring, they often emerge en masse.
