How Do Deer Survive a Polar Vortex?

Fortunately, whitetail deer are uniquely adapted to handle just about anything Mother Nature can throw at them. Whitetail deer can withstand a wide variety of climates and conditions, which is likely why they’re found as far south as Mexico and as far north as Canada, with some predicting they’ll soon make it to Alaska.

Harsh winter conditions, like what we experienced during the polar vortex, will certainly kill deer each year. In Montana, for example, the annual deer “overwinter mortality” is estimated to be 25 percent.

Still, whitetails are remarkably capable of handling winter extremes with behavioral and biological advantages they’ve developed over 4 million years.

Biological Defensives
Each year as winter draws closer, whitetail deer undergo a physiological shift in preparation for the tough times to come. Most noticeable is the growth of their winter coat—which is darker in color and thicker in volume than the thin orange coat you’ll see in the summer.

This is one of the whitetail deer’s first defenses against the cold. The winter coat’s darker color absorbs sunlight better, and the thick undercoat comprised of hollow hair shafts provides insulation similar to down feathers in a puffy jacket. Oils within a deer’s skin provide a certain level of water repellency to this winter coat as well.

A common belief is that whitetail deer lower their metabolism during the winter months to reduce energy expenditure. But after my curiosity was piqued during the vortex, I discovered that this is a myth.

According to Kip Adams, director of conservation for the Quality Deer Management Association, this misunderstanding stems from a study conducted decades ago during which a series of errors with experimental design and analysis led to misleading outcomes. It was proved in later studies (some of which Adams took part in while doing graduate work at the University of New Hampshire) that a whitetail’s metabolism is consistent across all seasons.

According to Adams, rather than a shift in metabolism, the greatest biological adaptation deer have for winter survival is their ability to build a healthy fat supply leading up to winter. Studies from the University of New Hampshire showed that an adult doe will rely on this fat supply for nearly half of its winter nutrition, with that same doe typically entering winter with a 90-day supply.

With this being the case, winter survival is largely determined many months ahead of time by the available nutrition and carrying capacity of local habitat.

Deer do, of course, still need to feed during the winter to survive, but forage this time of year across the whitetail’s northern range (stems, buds and bark) is largely void of nutrition. Amazingly, whitetails still derive value from these depleted food sources since digesting food produces internal heat as a byproduct, helping maintain body temperature.

Winter Whitetail Behavior
In addition to these physiological adaptations, whitetail deer have also adopted a number of unique behaviors to help weather extreme winter conditions. Most notable of these is their use of wintering or “yarding” areas.

Yarding areas are specific habitats that are optimal for whitetail survival during times of extreme cold or heavy snow, and therefore are used most by whitetails in the northern reaches of their range. Typical yarding habitat is comprised of coniferous trees such as spruce, fir, cedar or hemlock, which can block wind, catch snow and provide a certain level of thermal retention.

Deer will congregate in these ideal locations, sometimes traveling from extreme distances to get there. In northern New Hampshire, for example, it’s not uncommon for deer to migrate as far as 20 miles from their regular fall range to reach such a place.

“Research has shown that deep snow is the most important factor in determining winter severity,” said Terry Minzey, regional wildlife supervisor for the Michigan DNR. “That research indicated that energy expenditures associated with movement increased by 50 percent when snow depth reached 14 inches and 100 percent when snow depth reached 21 inches.”

In addition to seeking out especially protective habitat, deer will also self-regulate how they travel across their landscape to reduce energy expenditure. Deer spend more time bedded during the winter, and will adjust their bedding locations to be closer to whatever food source they’re using. This reduces the necessary time spent moving between bed and food. In the name of even greater energy efficiencies, whitetails will also often travel in single file across areas of heavy snow, creating beaten trails that they will seldom leave after creation.

Space Equals Survival
As impressive as it is that so many whitetail deer survive relentless winter weather, it’s important to remember that they are not invincible. Conditions such as those we experienced during the recent polar vortex still pose a significant threat to many deer, especially those that are malnourished or injured.

For that reason, it’s important to be mindful of the impact we might have on local deer battling to make it through the tough conditions. Any time a human spooks or pushes a deer, it forces that animal to expend precious energy that might have otherwise helped it survive the remaining winter months. During particularly tough spells of winter weather, it’s worth considering how you might be able to reduce your impact on those deer.

This might mean canceling a scouting trip or delaying shed hunting. As much as whitetails provide for us—food, enjoyment, humility and wonder—the least we can do is not stress them out during the most stressful time of year.

Feature image via Matt Hansen.