Fattening Up for Winter: We All Do It!

            With our belts still loosened from our Thanksgiving feast, it’s tempting to blame our recent indulgence on some ancient genetic code.

And it makes sense: Bulk up to survive the dark, cold, lean months of winter.

Whether we’re bulking up, stockpiling food, conserving our energy or racing around to find more food, these are all strategies among wildlife to get through winter in decent shape.

For Bears, Fall is for Fattening Up

            The ultimate bulker-upper is the black bear. Between late-summer and early fall, black bears go through an absolute feeding frenzy, eating five times what they normally eat during the rest of their waking months. They’ll gorge on acorns, beech nuts, apples and other sources … if they’re available. When those natural sources are in low supply, bears increasingly look to backyard bird-feeders for their food.

            For black bears, fall is all about fattening up for their big winter snooze. If natural foods are in good supply, a black bear will put on 30 pounds a week, building up a 5-inch layer of fat, thanks to those fatty and nutritious nut crops. By late-fall, as a bear prepares to den, a hormone called leptin starts to kick in and suppress its appetite. Once the bear is comfortably asleep in its den, it’s obviously eating nothing and drawing off those fat reserves.

Pregnant females will start going to bed around mid-November. It’s hard to predict when adult male bears will retire to their dens; if there’s plenty of food, they’ll stay up and eat as long as they can. Wouldn’t you? Young males will stay up into mid-January if there’s enough to eat and the temperatures are tolerable.

Bears that go into the winter well-fed and well-nourished stand a much better chance of surviving the winter and first few weeks of spring than bears that find thin meals in the fall.

            Fat reserves are critical for females to ensure cub production and survival.  During years of low food availability in the fall, cub mortality can reach 50 percent; in most years, it’s around 20 percent.

            Another strategy of bears is to conserve energy. While a black bear periodically awakens in its den and can be roused from its sleep (making it not technically a “true hibernator”), its system slows way down. Its heart rate will slow to around 8 beats per minute, compared to 40 or 50 beats per minute in the summer.

The Deer-Bone Femur Test

            While the white-tailed deer doesn’t sleep away the winter, it shares two basic strategies with the bear: Fatten up and conserve energy.

            Like the bear, much of a deer’s fall is spent consuming acorns and other nutritious foods to lay on those fat reserves. During winter, fat reserves will supply as much as 40 percent of an adult deer’s daily energy.

            So, conserving energy becomes crucial for a deer’s winter survival. Deer seek out good winter cover – like dense softwood stands – where they can avoid high winds, cold and deep snow. If they’re lucky, they also have nearby access to plentiful forage and room to spread out to avoid becoming a coyote’s prey. The less they have to roam, the better, especially near winter’s bitter end.

            By then, a deer’s fat reserves really start to dwindle. Here’s a morbid way to see this point: If you’ve ever encountered a deer carcass in winter, take its femur and crack it in half. Just do it. Check out the marrow inside the bone. If the marrow is bright red and thin, almost like jelly, the deer was malnourished when it died. If the marrow is fatty and white, it died pretty well-fed. By late-March, when snows are deep and winter has played its course, many deer are digging deep into those marrow fat reserves.

The Pantry Approach

            Personally, I think beavers have the right idea when it comes to winter food. They keep a well-stocked pantry.

            As beavers go, they’re really busy in the fall, felling trees and gathering the tops. They haul the limbs across the water to just outside their lodge, where they stick the butt ends into the underwater mud … and leave ‘em. This aggregate of tops is a beaver colony’s winter cache.

            Once the ice comes in, a beaver colony pretty much stays in its lodge for the rest of the winter, laying low, but awake nonetheless. When they get hungry, they go out to the pantry and return with a stick. They eat the bark and return the de-barked stick to the water, where it often floats up to the underside of the ice.

            Beavers will also draw from their fat reserves, and they have a neat place to store it: their tails. Aside from serving as a rudder, a beaver’s tail is a big, leathery hunk of fat. Near the end of a lean winter, a beaver’s tail will become thin as the animal draws fat from it.

Care for a Late-Night Snack?

Chipmunks also like to have a well-stocked pantry. All fall, you may have seen the chipmunks gathering acorns and other seeds in their fat cheeks and stashing them into their underground burrows. Although the chipmunk is a hibernator, it is not dormant all winter. A chipmunk will occasionally wake up and dip into its cache.

Susie Spikol, a teacher/naturalist at the Harris Center for Conservation Education, likes to have students imagine they are chipmunks. “If you were a chipmunk,” Susie says, “you might wake up once in a while, reach under the bed and grab a snack.”

When in Doubt, Snuggle

Obviously – despite the title of this article – not all critters fatten up for winter. The flying squirrel wouldn’t glide too well from tree-to-tree with a December build-up of a few ounces. It would probably drop to the ground like a rock!

The flying squirrel and other tree squirrels (like the gray squirrel) are out and about all winter, gathering food. The flying squirrel’s winter strategy, however, is to snuggle in numbers. This aggregate of flying squirrels inside their nest – usually inside a hollow tree – keeps each individuals warm, like bunched-up penguins.

In his book, Winter World, author Bernd Heinrich describes banging on a tree with an axe in late-winter. One-by-one, out of a hole came flying squirrels – 10 in all! – a little upset but safe, nonetheless.

That Ancient Genetic Code

All species have their own approach to winter. Bees stock up on honey; turtles dig deep into the mud and shut down; chickadees lower their night-time body temperatures. The list goes on, and it’s all about adaptations that work for that species.

So what about us? Do people fatten up for the winter just like many other critters? The days get colder, the nights get longer and we just want to hunker down and eat! There may in fact be some ancient genetic code telling us to have yet another piece of turkey … oh, and more stuffing, too … and pass the gravy!