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13-lined ground squirrel.

To metabolize fat so it can be used as energy for their hearts, hibernating ground squirrels "turn on" a gene in their hearts to produce an enzyme normally used in the pancreas to aid digestion.

Sleeping it off

How animals use hibernation, and other cold-weather survival strategies, to stay alive

By Deane Morrison

Published on January 7, 2005

Editor's note: This is the first article in a series we're calling "SciFri." On Fridays, science writer Deane Morrison will explore an area of science, nature, astronomy, or medicine--with a University connection--to elucidate and entertain you. Hope you enjoy this new column.

If you wish you could hibernate for the winter instead of facing minus-20 windchillls every day, blame our distant ancestors. Really distant. According to Matt Andrews, biology professor at UMD, our reptilian forebears, like reptiles today, would have been able to experience large fluctuations in their body temperatures, a key trait for a hibernating species.

As humans evolved, however, we were selected for the ability to maintain our temperatures. That was easy in the tropics, but for us in the frozen north, the struggle to keep warm occupies us for months on end. We have our houses, furnaces, and polar fleece, but other warm-blooded animals that share our habitat rely on their biology to get them through. Here's a look at some of their strategies.

Appropriately, one of the best winter survivors is the 13-lined ground squirrel, otherwise known as the golden gopher. The ground squirrel, a true hibernator, spends the cold months holed up in burrows two or three feet underground, says Andrews. Before turning in, the animals stuff themselves with seeds and may double their weight. The extra fat provides energy for the long sleep.

NASA is also interested in how bears overwinter because if we want to send humans to Mars, they may have to be in a hibernation-like state.

Even with all that fat, ground squirrels could not survive if they maintained their body temperatures. Ordinarily close to humans' 98.6 degrees F, their temperatures fall to a few degrees above freezing during hibernation. Their heart rates drop from 200-300 beats per minute to 3-10 beats per minute. Ground squirrels burn their stored fat in their hearts to keep them pumping. To digest that fat so it can be used as energy, the ground squirrels "turn on" a gene in their heart during hibernation to produce an enzyme normally used in the pancreas to aid digestion.

But their hibernation isn't just one big sleep.

"They wake up every two weeks or so," says Andrews. "They get up, reposition themselves, and may urinate or drink snowmelt. Hibernation is not continuous-it's punctuated."

At the other extreme are the birds that tough it out through the cold months.

"A high percentage of birds leave town," says Scott Lanyon, director of the Bell Museum of Natural History. "Around 40 species may stick around Minnesota."

The main issue for birds isn't warmth, but food. Birds that can find winter food tend to stay put. Chickadees, woodpeckers and nuthatches eat insects they find in bark, in furled leaves of grasses, or inside galls (scars) on plants such as goldenrod.

But bad weather can be dangerous for wintering birds. "Freezing rain is especially bad for chickadees, woodpeckers, and nuthatches, because it can create a thick icy barrier preventing birds from reaching their food," says Lanyon.

Sparrows and cardinals eat seeds, which they find in seed heads of plants like grasses and Echinacea (cone flowers).

And robins? They always leave because they can't get worms, right?

"Robins eat fruit except in the breeding season," says Lanyon. If that fruit hangs around, so may the robins. Ornamental fruit trees like crabapples can feed the red-breasted birds through the winter if there are enough of them.

Feathers keep birds warm, but a wind that ruffles the feathers can chill the birds. Birds face the wind to keep the feathers in place, but if the wind swirls, it can spell trouble. Tiny birds with a relatively high surface area--chickadees, for example--may roost communally in a tree cavity. Woodpeckers may take shelter in tree holes.

But what's really cool, says Lanyon, is how birds keep from losing all their body heat through their feet. Arteries and veins run side by side in the foot and lower leg. Arteries come from the body, carrying warm blood. As the blood continues toward the foot, it loses heat to the blood in veins coming back from the foot. The net result: Blood that makes it to the foot is almost as cold as the ambient environment, but blood coming back to the body is warm.

Feathers keep birds warm, but a wind that ruffles the feathers can chill the birds. Birds face the wind to keep the feathers in place, avoiding swirling air that can spell trouble.

Bears follow a middle course between birds and true hibernators. Like ground squirrels, bears put on fat--about 50 to 70 pounds of it--between summer and fall. Beginning in late fall, they hole up in their dens and sleep, lowering their heart rate from 80-100 beats per minute to an average of about 15. But their body temperature only drops from about 101 degrees F to around 92 degrees F, says Paul Iaizzo, a professor of surgery and physiology who studies how black bears keep their hearts and muscles healthy during the winter. When their cores start to dip below 91 degrees, they begin to shiver, which uses lots of energy and warms the animals.

A human going without food for as long as an overwintering bear would lose 90 percent of his or her muscle mass, compared to only about 23 percent for bears. That's because humans burn muscle for energy if it isn't being used for movement. As muscle breaks down, nitrogen--an essential component of DNA, RNA, and protein--is excreted as urea molecules. In humans, once the urea is formed, you cant recapture the lost nitrogen. But bears' guts have bacteria that can recycle nitrogen from urea, reclaiming it for use in muscle proteins, including the heart.

(Unlike bears, ground squirrels don't have to worry about losing nitrogen. Below 64 degrees F, protein and DNA synthesis cease, which removes the demand for fresh nitrogen, Andrews says.)

Bears also get a boost from a hormone that seems to trigger the hibernation-like state and to protect vital organs from the damage caused by a lack of oxygen found in tissues undergoing reduced metabolism.

Iaizzo is studying whether the hormone or similar molecules can be used to prevent such damage in people if tissues are exposed to low-oxygen conditions, during surgeries, for example. NASA is also interested in how bears overwinter because if we want to send humans to Mars, they will have to be in a near hibernation-like state.

Plenty of questions remain. One puzzle is how bears denned up for the winter manage to give birth and suckle young without a water supply.

"It's not well understood how they reabsorb water," Iaizzo says. "We are beginning to study bear bladders with ultrasound to see if reabsorption occurs there."

Different as they are, the strategies of bears, ground squirrels, and birds don't imply that these animals have different genes from each other for dealing with cold weather, says Andrews. We probably have much the same genes; it's how we use them that counts.

"I think it's differential expression of genes that makes the difference," he says. In other words, what animals turn what genes on and when. At a certain time, the fat-burning enzyme in ground squirrels gets turned on in the heart, not the pancreas, to allow them to hibernate. If we could turn that enzyme on in our hearts, we would be a step closer--though still far away--from being able to hibernate ourselves. In fact, there is a hibernating primate: the fat-tailed lemur of Madagascar, which stores fat in its tail.

Lots of us seem to do that already. Maybe shivering through the winter isn't such a bad idea after all.