ROBIN CHENOWETH / writer
 

December 17, 2008 / The Columbus Dispatch

     Sunday marks the winter solstice, the day the northern hemisphere is tilted furthest from the sun. Late December days are the year’s shortest in terms of daylight hours, when nature appears to be standing still. The chirps and buzzes of autumn have given way to the hushed tones of winter. Yet beneath the ground and in hollows of trees creatures lie, dormant but alive.

    Hibernation is an adaptation, nature’s defense against the harshness of winter. Freezing temperatures and lack of food would kill most creatures: Exposed to extreme cold, body temperature drops, organs shut down and the heart stops.

    Some animals escape the danger by migrating south; others, including humans, find heated shelter. But hibernating animals survive in a kind of suspended animation: In their dens, they might appear limp or even dead.  Yet their bodies still function, just more efficiently.

    Their body temperatures, heart and breathing rates drop to almost imperceptible levels, lowering the animals’ metabolic  rate — how quickly they use energy.

    Here is a sampling of critters that truly hibernate and those that go into torpor, a slowed state like hibernation — except that their body temperatures fall less or they remain active at times during the winter:


LADY BEETLES

    Some insects, such as the monarch butterfly, migrate. But the majority tough out winter, huddled beneath leaves, in the crannies of trees or underground.

    These insects might be in the form of eggs, larvae, pupae or adults.

    Lady beetles, or ladybugs, go through diapause, or insect hibernation, as adults. In September, they begin building fat stores by eating fruit, said Joe Kovach, an Ohio State University entomologist.

    "When the day length starts to get shorter, it triggers a lot of things," Kovach said. "The food quality suffers."

    As autumn progresses, and as food becomes scarcer, lady beetles begin to gather in groups at the bases of trees, under bushes or in the cracks of buildings. Certain varieties, such as the multicolored Asian lady beetle, crawl into attics or wall cavities.

    Within the ladybugs, a remarkable process takes place. Hormones create a type of antifreeze that circulates through the insects' bodies.

    "Their temperatures are just above their surroundings," Kovach said, but even when it drops well below freezing "their cells don't fracture."

    Not that the insects are invincible.

    "It's not really how cold it is; it's how fast it gets cold. If it goes from 70 to below freezing in a day, (the insects) are going 'aaaaah!' " he said.

    Lady beetles can hibernate for several months.

    Occasionally, the February sun warms up those Asian lady beetles hiding in houses. They come out of diapause and move around, creating a midwinter, multispeckled invasion.


GROUND SQUIRREL

    When the temperature careens near freezing, and wind whips snow across the landscape, how does a wild creature survive? The 13-lined ground squirrel, perhaps the most intense hibernator in Ohio, stays underground.

    "They go into tunnels in September or October, and don't come out until April or so," said Jim McCormac, a botanist and wildlife expert at the Ohio Department of Natural Resources.

    Ground squirrels make extensive burrow systems, McCormac said. A main tunnel often branches into several others, with chambers where ground squirrels can curl up. At 3 1 2 feet beneath the surface, these "bedrooms" are below the frost line (the layer of soil that freezes).

    Inside its burrow, the squirrel's body virtually shuts down. Its heart rate slows from about 280 beats per minute to five beats per minute. Its temperature drops to about 37 degrees Fahrenheit. It takes only a few breaths every couple of minutes. Its body burns "brown" fat - heat-generating fat stored in autumn after the animal gorges on seeds and insects.

    Every week or so, a 13-lined ground squirrel wakes up, repositions itself and possibly urinates, said Matt Andrews, a biologist at the University of Minnesota at Duluth. It doesn't eat and soon returns to its dormant state.


BEARS

   Most people consider bears classic hibernators. A closer look at Brutus and Buckeye - twin Alaskan grizzlies at the Columbus Zoo and Aquarium - dispels the misconception. Even with temperatures in the 20s and snow falling, the 950-pound brothers still wrestle one another, jostling for a bone in their outdoor enclosure.

    The 4-year-olds came to the zoo as cubs after their mother was killed in the wild.

    "This is really the first winter we've actually seen them go inside and lie down for a little bit (during the day)," said Jeremy Carpenter, assistant curator.

    The bears might sleep an hour or two longer than in summer, he said, but the two always come outdoors afterward to romp. That's probably because, unlike grizzlies in the wild, they have a steady source of food.

    Even wild bears never truly hibernate, Carpenter said. Instead they go into torpor.

    Like hibernators, bears fatten up in autumn. They use powerful claws to dig a burrow beneath a fallen tree, or they crawl into a hollow log or rock crevice. In torpor, a bear's temperature can drop from 100 to 88 degrees Fahrenheit, and their heart rate and breathing slow. But if roused, a bear will awaken and might leave its den to search for food.

    One reason bears don't go into a deep hibernation is that females give birth during torpor and must care for their fragile cubs, which can weigh as little as 1 to 3 pounds at birth.

    But during a particularly harsh winter, or when food is scarce, bears can go into torpor for up to six months. They sometimes develop a plug at the end of their intestinal tracts so they won’t defecate.  They also can recycle their urine, breaking it down to make proteins that help maintain muscles and organs. When it emerges in spring, a bear can be hundreds of pounds thinner, so it immediately begins searching for food.


TURTLES

    Like all reptiles, turtles are ectothermic. They don't generate heat internally but absorb it from their environments. During Ohio's frigid winters, red-eared sliders and map turtles must seek refuge, so they bury themselves in the mud beneath ponds and rivers. But beforehand, turtles spend weeks preparing for brumation, or reptile hibernation.

    "It's a slow process," said Ron Hatcher, senior reptile keeper at the Columbus Zoo and Aquarium. "As it gets cooler, they stay underwater more because the wind starts kicking up.

    "When the temperatures get down to what they don't like (usually between 55 and 40 degrees), they burrow into the mud."

    Like hibernating mammals, turtles' metabolisms and heart rates slow. They survive underwater because they can absorb oxygen through their cloaca, the opening where they expel waste.

    "It doesn't bother them," Hatcher said. "They don't breathe air, just absorb it through their skin and cloaca. As the temperature warms up, they get the urge to breathe" and come up.


GROUNDHOG

    Many people associate groundhogs with hibernation, thanks to the species' most celebrated member, Punxsutawney Phil. Each Feb. 2, cameras train their lenses on Phil's burrow (really a zoo habitat) to see whether he will emerge and be frightened by his shadow. If he goes back into his hole, tradition says, we're in for six more weeks of winter weather.

    There might be a smidgen of truth to the folklore, said Stan Gehrt, an OSU assistant professor of wildlife ecology.

    True hibernators, groundhogs - also called woodchucks - are triggered by the shortening days to fatten up on clover and leafy plants.

    "They're basically doubling their body weight during the fall and late summer, then they metabolize the fat slowly during winter," Gehrt said.

    A cold snap will drive them into their burrows, where they stay up to six months. There, a groundhog's body temperature hovers above freezing, and its heart and breathing rates drop drastically.

    Like ground squirrels, groundhogs sometimes awaken, often shivering to generate body heat. The males emerge first, almost never before mid-February, and it's to look for a female, not their shadow.

    "It's to the male's advantage, if he can afford to do it, to be out there waiting on the females when they emerge," Gehrt said.


SNAKES

    When it comes to surviving the elements, snakes can be sociable creatures - think of the snake-pit scene in Raiders of the Lost Ark. Although those snakes were estivating, or escaping the heat, cold-climate snakes also congregate, in winter. (But not in the large numbers seen in the movie and without the aggressive tendencies.)

    "Different breeds can be very passive toward each other," said Ron Hatcher, senior reptile keeper at the Columbus Zoo and Aquarium. "If a person went into a (snake) den and didn't move a lot, you'd be fine, too."

    That's because hibernating snakes, like hibernating mammals, are mostly inert. They've slowed their bodily functions, and they're not quick to react. And like turtles, they look for a special place to rest.

    "(Snakes) are very attuned to humidity levels," Hatcher said. "They cannot dry out too much, so they find places that hold a little moisture," such as rock crevices, basements, trash dumps and wood piles.

    Ohio's black rat snake is large and dark, so it retains heat, tolerating cold better and entering hibernating later. Others, such as the eastern plains garter, go into torpor in November.


BATS

    Bats are special among hibernators: They spend almost 90 percent of their lives in some state of dormancy.

    Most bats in Ohio weigh less than a half-ounce. Such small bodies lose a lot of heat, said Merrill Tawse, a zoologist and bat expert at the Gorman Nature Center near Mansfield.

    Even in summer, after hunting for food all night, bats enter a daily torpor.

    "If they were to maintain their high body temperature, even with a full stomach . . . they would use up their energy and starve to death by noon the next day," Tawse said. "So in summer, they enter a daily hibernation . . . turning their body temperature down."

    In winter, a few species migrate, but most Ohio bats hibernate in groups. Northern long-eared, big brown, little brown and Indiana bats leave summer habitats in October to look for underground shelter. Called a hibernaculum, the shelter can be an abandoned mine, cave or sandstone crevice.

    The hibernaculum temperature is critical because bats adjust their heart rates to their surroundings. Too hot, and their hearts beat too fast, burning off too much energy. Too cold, and the bat will awaken and be forced to move to warmer shelter. The ideal temperature is 35 to 37 degrees Fahrenheit.

    Just before a bat enters the hibernaculum, it mates. The female’s body either delays fertilization, or stores away the fertilized egg, “putting on pause” the baby’s development until spring, Tawse said.

    Disrupting bats' hibernation can be fatal to them. Scientists once studied bats in their hibernacula but found fewer bats each time they entered.

    "They (learned) that every time they would go into a cave and wake the bats . . . the waking would cost those bats about 10 days' worth of energy," Tawse said. "So bats were dying off (before spring) just because people weren't aware."


Copyright Columbus Dispatch.

 

Long Winter’s Nap

Some critters wait out the cold ‘all snug in their beds’