Survival of the Fittest

A brief explanation of how whitetails weather winter

In their tactical bid to survive tough northern winters, whitetails instinctively make a number of trade-offs in terms of nutrition, shelter and avoiding predators. Their behavior is so efficient that it seems as if the animals are consciously calculating biological energy cost/benefit ratios and assaying potential predatory risks with every move.

The whitetail's winter survival traits include an array of physiological and behavioral adjustments that permit it to better cope with harsh winter weather, to live on limited amounts of poor-quality food and to survive near the northern limits of their geographic range. Here's a quick primer on the transformation.

Preparing for Winter
In autumn, whitetails molt into an insulative coat and accumulate heavy fat reserves. Because these events are hormonally driven, triggered by a decrease in the amount of daylight (the "photoperiod"), they occur around the same time each year, regardless of temperature or available nutrition. Even undernourished young animals sacrifice body growth in order to put on at least some fat for the winter.

Weight loss is inevitable in whitetails wintering on northern ranges, which is why an individual animal's survival prospects hinge heavily on the amount of stored fat and the rate at which it is burned. Animals that enter winter with their fat reserves topped off can suffer upward of a 30 percent weight loss without dying.

When winter arrives, whitetails respond to low temperatures and cold winds by seeking protective conifer cover. On average, northern whitetails travel 8 to 10 miles from their summer to winter grounds. But some travel in excess of 50 miles.

This "yarding" behavior probably evolved as an energy-conservation and predator-defense adaptation. Historically, those deer that made the physical changes just described survived to reproduce and perpetuate those basic traits, whereas the line of those that did not adapt died out.

Safety in Numbers
Related does and fawns typically band together in autumn and spend the balance of the winter together. Associating with older deer helps the young learn migratory routes to wintering areas as well as techniques to survive on limited food, often for several months at a time. This herding behavior also improves the group's chances of detecting predators. And multiple deer do a better job of keeping trails packed and passable, which are critical to the whitetail's mobility and ability to escape predators.

Deer Yards
Dense swamp conifer stands dominated by mature northern white cedar provide the best protection from cold temperatures and windchill. They've been known to hold several hundred deer per square mile in winter months. A tight tree-canopy closure also intercepts much of the snowfall, allowing ground accumulations to pack more firmly, again making travel easier.

In regions where the weather is less severe, dense stands of hemlocks, jack pines and other conifers attract wintering herds.

In mountainous areas of the Northeast, deer often concentrate on steep southern and southeastern slopes during winter to take advantage of a warmer microclimate provided by the maximum available sun exposure. While in sparsely forested areas of the northern plains, isolated marshes, wooded draws and brushy stream bottoms offer the best shelter, provided a good food source is nearby.

Reduced Metabolism
During their initial adjustment to winter weather, deer undergo a number of physiological changes, including reduced rates of metabolism and thyroid function. By midwinter, they gear down to an almost semi-hibernating state, reducing their food intake by about 30 percent, regardless of what nourishment is available, and decreasing their movement by at least 50 percent. These adaptive tactics enable them to minimize wasteful expenditures off energy and to coexist in a crowded, hostile environment.

Once they're acclimated to a chosen wintering area, deer can become incredibly stubborn and reluctant to deviate from their established routine. When they become severely stressed, whitetails have been known to limit their activity to less than 80 acres (down from several hundred acres in summer), feed almost exclusively during warm daytime hours and refuse to travel more than a few hundred feet from shelter to feed.

Even given the whitetail's dialed-down winter lifestyle, the animal's energy requirements will exceed what it takes in by eating woody browse. In fact, northern white cedar may be the only browse that by itself can sustain deer through a long yarding season. Hemlocks, maples, ash, birch, aspens and various shrubs are sufficient only when they're available in great variety and abundance, or when they're consumed along with more nourishing foods. No matter what the food, it's not uncommon for yarding areas to become "browsed-out" when deer numbers are too high.

When corn, apples, rye grass, alfalfa, soybeans, winter wheat and other cultivated crops are available in winter, whitetails will feed heavily on them. If snow cover is scant, they also paw for acorns, beechnuts and low-growing herbaceous plants. Even small amounts of these high-energy foods enhance first-stomach digestive function and, along with woody browse, permit deer to survive winter when their shelter is of relatively poor quality.

Starvation
The whitetail's otherwise impressive strategy for winter survival weakens around mid-March when the animal's metabolism kicks back into high gear in response to lengthening daylight hours. The change accelerates food demands and rapidly saps the animal's remaining energy reserves. Deer are more vulnerable during this period than at any other time of year. When a prolonged winter coincides with a delayed breakup of wintering herds, the results can be devastating. Hundreds of thousands of whitetails can die across the northern range during prolonged severe winters, which happened most recently in 1996 and 1997. That's nature's dispassionate way of culling the weakest individuals from the herd to ensure the long-term survival of the species.