Hibernation seems the perfect way to deal with extreme winter weather. When temperatures dip 10 degrees below zero with minus-25-degree wind chills, as they have this winter, I admire wildlife that are adapted to extreme winter weather.
Protected by thick layers of fat and/or a cozy den below the frost line, hibernators are oblivious to subzero temperatures, wind, snow and ice. They just sleep through winter’s worst.
Hibernation, however, is a broad, imprecise term. Though many mammals sleep through at least portions of winter, only a few truly hibernate. Simply put, hibernators stop trying to stay warm when days get shorter and temperatures drop.
Their body temperature drops to reduce the energy required to stay alive. As tissue cools, it uses less energy, so the animal’s fat reserves last longer.
Going dormant
Hibernators go dormant in the fall and do not stir until they emerge in the spring. Furthermore, body temperature, heart rate and respiration rate plunge.
Here in the east the list of true hibernators is short: ground hogs and two species of jumping mice. When ground hogs head underground for the winter, they weigh about 30 percent more than they did in early summer. As ground hogs prepare to hibernate, they plug the entrance to the burrow to keep out potential predators and maintain a stable underground environment.
Then they curl up in the sleeping chamber, and their body temperature slowly drops about 57 degrees to 44 degrees F. Their breathing slows, and their heart rate drops from about 100 beats per minute to about four beats per minute.
In March, ground hogs wake up weighing about 50 percent less than their fall weight. Then it’s time to emerge, eat, and find a mate. When ground hogs emerge in the spring, they mate almost immediately even though they are not in optimal physical condition.
The strategy is to give birth and get young ground hogs out of the den and independent as quickly as possible so they have enough time to gain the weight required to survive their first hibernation.
Meadow and woodland jumping mice behave similarly. In a chamber about the size of a grapefruit up to three feet beneath the ground, jumping mice tuck their nose between their hind legs and wrap their long tail around the body.
Because they are small (they weigh less than one ounce), surviving winter can be a challenge. Some years more than half die in the hibernaculum. Black bears and chipmunks also sleep through much of winter, but they don’t qualify as true hibernators.
Bears fatten up in the fall and sleep deeply, but their respiration, heart rate, and body temperature do not fall significantly. And during warm spells bears sometimes wake and even wander around near the den.
Giving birth
Furthermore, sows give birth to cubs in late January. The cubs nurse for about two months and are ready to leave the den with their mother in late March or early April.
If disturbed during a deep winter sleep, female bears can rouse themselves in just a few minutes. Every bear biologist I’ve ever talked to tells of at least one close call while working with a “hibernating” bear.
Chipmunks, the familiar striped ground squirrel found in backyards, escape winter’s fury underground, but they don’t rely on a layer of fat to survive. During their fall foraging frenzy they collect and cache as much as a bushel of seeds and nuts in their burrows.
They wake periodically during winter and eat the stored food, and on warm winter days they may actually emerge and forage in the daylight.
Squirrels, raccoons, skunks, opossums and foxes remain active all winter long, but during extremely cold weather, they sometimes curl up in a hollow log, den tree or burrow for days.
Some of these mammals conserve body heat during winter weather by sleeping in groups. Flying squirrels frequently huddle in groups of ten or more, and raccoons sleep in pairs or groups of three during very cold weather.
(Send questions and comments to Dr. Scott Shalaway, RD 5, Cameron, WV 26033 or via e-mail at his website, http://scottshalaway.googlepages.com.)
