Winter’s Big Sleep

Animals have developed complex adaptations to keep them alive during the winter, and though they might be sleeping, not everyone is hibernating

By Molly Priddy

Autumn in Northwest Montana means winter is likely just a few snowstorms and cold fronts away. As the leaves fall, the fun of summer is replaced by a frenetic energy to prepare for life in the cold season.

For the valley’s human residents, this might mean new snow tires or chopping piles of wood. But for many of the Flathead’s wilder residents, winter is the season of snooze.

Instead of trying to eke out an existence in the snow, many wild animals prefer to sleep. The level of sleep, from mildly groggy to full-on hibernation, varies by species.

Some of the most famous hibernators – bears – aren’t actually hibernators at all, according to Erik Wenum, bear and mountain lion specialist with Montana Fish, Wildlife and Parks.

“Bears in particular, they really truly just go to sleep for extended periods of time, they’re not true hibernators,” Wenum said. “To say they hibernate, physiologically, that’s very incorrect.”

When a warm-blooded animal truly hibernates, their physiology slows down, and the inactivity is achieved primarily through lowering the animal’s body temperature.

For example, as true hibernators, ground squirrels can lower their body temperatures to 28.4 degrees for days but warm back up to a balmy 98.6 degrees every once in a while to allow their brains to warm up enough to sleep and heal. When their internal temperature drops, so does their heart rate and breathing; a hibernating ground squirrel may only breathe three times a minute.

Bears, on the other hand, only dip their internal temperatures to the low- to mid-90s when they go to sleep for the winter, but they slow their metabolism by about 25 percent and do not eat, drink, defecate, or urinate while sleeping.

And while they may not be truly hibernating, a bear’s ability to keep from urinating and defecating is a scientific marvel, Wenum said, because they manage to avoid kidney damage.

If a human were to go for even three days without urinating, they would suffer damage due to toxic build up. In order for bears to avoid this, Wenum said their bodies strip down muscle and fat, and their bodies consume raw proteins, which are then recycled and converted back to muscle mass.

“We understand what they do; what we can’t figure out is the how they do it,” Wenum said.

If biologists were able to understand this process, if it is a genetic trait, then perhaps that same gene could be located within the human genome, Wenum said. The implications from there would include the end of kidney dialysis and incredible potential for deep space travel.

This process also allows the bears to wake up with a maintained physical fitness without serious muscle atrophy.

Preparing for the winter sleep puts the bears into hyperphagia, a pre-snooze period during which they’ll eat 20,000 to 25,000 calories a day. If a female bear successfully puts on enough fat, she could become pregnant, Wenum said.

Bears typically mate in June, he said, but the fertilized egg will float around a bear’s uterus until the fall, when her body determines she’s mature and fat enough to support cubs. If she qualifies, hormonal changes take place and the fertilized egg implants in the uterine wall.

“They might not be pregnant until the middle of November, then they give birth the first week of January,” Wenum said. “Bears are easily roused; they give birth, they clean them up, eat the placenta, and go back to sleep.”

Bears with new cubs are the last to leave their winter dens, and the cubs don’t meet the new world until about May.

To keep from defecating during their sleep, bears eat a last meal of grasses to form a rectal plug. Wenum said their first meals are usually grass as well, from which they strip the sugars, but, like humans, can’t digest the rest of the plant matter. This means mother bears with new cubs have to pack on enough fat to feed their babies until mid summer.

“It’s pretty stunning, some of the things that they do,” Wenum said.

In Glacier National Park, rangers host snowshoe tours to educate the public about winter ecology. Laura Law, the park’s education specialist, said the seasonal adaptations for surviving winter cover all of the park’s living things, including the plants and insects.

Her office also tracks the winter habits of the local ground squirrels, noting the days when they disappear and when they reappear, as well as their non-hibernating cousins, the tree squirrels, which prepare for the season by burying caches of food when the getting is still good.

“We like to talk about trees, how they actually don’t die,” Law said. “They also have amazing defenses for themselves in ways that they can keep themselves from freezing, like adding more sugar to lower their freezing points.”

There is still plenty of research to be done on winter adaptations – Wenum just recently finished a research project in which he tagged the ears of abandoned or orphaned cubs to see if they knew how to construct a den without their mother to teach them. (The answer is yes. “Bears have been bears for a really long time,” Wenum said.)

Law said the park endeavors to educate the public about what is known about hibernation and other winter defenses. The snowshoe tours, which will begin again in the first half of January, show that nature might be colder during the winter, but no less alive.

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