Hibernation: winter slumber

The preservation crew found a little brown bat hibernating in the dome of the Jefferson Memorial two winters ago.

“Animals are hibernating all around, and people would never know it,” says Gopaul Noojibail, resource manager for the National Park Service, National Capital Region.

The staff, which has not found any hibernators before or since, brought the bat to Mr. Noojibail in a box.

“We warmed it up so it woke up completely and let it go. That’s what we could do,” he says. “What we hoped is it found another space quickly and went back to sleep.”

Hibernators, like the little brown bat, enter into a state of dormancy to survive the winter, selecting shelters safe from predators. They respond to changes in temperature, day length and quality or lack of food, a response that is not learned but is innate, says John Carnio, general curator for the Maryland Zoo in Baltimore.

The animals’ food supplies are reduced as the day length shortens and temperatures drop, leaving a cold, frozen ground inefficient for plant growth. Without hibernation, some animals would expend more energy searching for food than they would gain from the food they eat, if any, and others would not be able to maintain their body temperatures in the cold.

“The point is they’re trying to avoid a period of food shortage by allowing biochemical processes to go much slower,” says Randall Packer, professor of biology at George Washington University (GWU), about hibernators. He holds a doctorate degree in zoology. “They can get enough energy from fat stored in the body to live all winter long until growth occurs in the spring.”

Besides hibernating, animals adapt to winter by migrating, as do some bird species, or by adjusting to staying out in the cold conditions, like foxes, shrews, gray squirrels and wild turkeys.

Animals that do hibernate enter into different levels of dormancy. Deep hibernators, such as bats, chipmunks, woodchucks, turtles, toads and snakes that inhabit the metropolitan area enter into a state of inactivity for several days, weeks or months. Their body temperature drops; their circulation becomes limited; and their metabolism, heartbeat and breathing slows.

“If you picked them up, you would swear they were dead,” Mr. Carnio says.

Torpors, animals who hibernate for a short period of time overnight or for a few days, undergo a slowdown in their bodily processes that is not as extreme as the deep hibernators. Their body temperature drops to 59 degrees Fahrenheit (15 degrees Celsius), while the temperature of deep hibernators dips to as low as 41 degrees Fahrenheit (5 degrees Celsius), near freezing, Mr. Packer says.

Torpors — including bears, deer mice, skunks and raccoons that live in the Washington area — are able to arouse themselves during a stretch of warm weather in order to eat and remove waste materials.

Deep hibernators do not and remain in dormancy. However, if their body temperature dips too low, such as to 30 degrees Fahrenheit (-1.1 degrees Celsius), they will awaken and shiver until their bodies return to above freezing temperatures. They repress this shivering instinct when they enter into hibernation, Mr. Packer says.

Animals have an average body temperature of 100.4 to 104 degrees Fahrenheit (38 to 40 degrees Celsius) and humans, 98.6 degrees Fahrenheit (37 degrees Celsius), Mr. Packer says. Their body temperature remains the same when they sleep or are awake.

“Basically, they are shutting down their systems,” Mr. Noojibail says. “It’s not sleep. It’s much deeper than sleep. … There’s a lot less going on internally.”

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