The cold facts of winter.

It was the winter of 1978-79, the coldest on record in North America. I was camped by the Lewis River in Yellowstone National Park. My goal: to learn how animals survive in a place where temperatures routinely drop below the -40 [degrees] F mark.

As I welcomed the first rays of sunlight, a coyote pranced into view. It paused, listened, and then pounced, sticking its nose beneath the snow in pursuit of a vole. I knew that if I followed that coyote's tracks, they would show signs of the thick hair on its feet. The added fur is an adaptation that reduces the animal's heat loss in winter.

The growing sensation of cold in my own toes reminded me to wiggle them as protection against frostbite. At temperatures like these the body quickly loses heat.

Conserving heat - energy, really - is the key to winter survival. Think about it: Winter itself results from a loss of solar energy. As Earth revolves around the Sun, the tilt of its axis causes the Northern Hemisphere to slant away from the Sun during the winter months. This means that every sunbeam covers a larger area in winter. In other words, the energy is spread out. Each plant, each animal in each patch of wintry Earth gets a smaller share of the total sunlight hitting the planet, compared with what they get during other seasons.

If you want to get technical, you can measure the incoming energy - in Calories, or Cal. (It makes sense to use the same Calories we mean when we talk about how much energy there is in food. After all, all food energy produced by plants and eaten by animals comes originally from the Sun.)

In June, every square yard of the Yellowstone area receives a daily average of 7,685 Cal of solar radiation. That's equal to the number of Calories in 27 regular Milky Way bars (see P. 4). In December, by contrast, every square yard of the Yellowstone area receives an average of just 5,288 Cal of solar radiation - or the number of Calories in 18 regular Milky Ways.

The difference may not sound all that big to you. But keep in mind that Earth is constantly radiating energy outward (some 5,337 Cal per square yard per day). That means that in winter, Earth suffers a net energy loss. In other words, it loses more energy than it gains.

In Yellowstone, the most obvious results of this energy loss are the polar air masses that pour in from the north and the snow that covers the region for six months or more.

Keeping your energy budget balanced during winters like these is a terrible struggle. For the animals that live here, a few Calories may mean the difference between dying or lounging in the green grass of spring. Calorie-counting is truly a matter of life and death.

SURVIVING WINTER

Faced with such a dramatic energy loss each fall, animals in the park resort to one of three basic strategies: 1 They migrate. Great blue herons, for example, fly south. Elk simply head for lower elevations, winding their way down from summer meadows in search of places where only a thin blanket of snow covers the grass. 2 They hibernate - the animal equivalent of serious "vegging." Like ground squirrels, they may retire into an under-ground den and go to "sleep," reducing their energy needs so much that they can survive until spring on just their fat reserves. 3 They face the cold and snow head-on, relying on special adaptations to keep from losing more energy than they take in.