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One hard swing of the axe split the log with a satisfying thunk. Loose bark fell from each half, exposing a spidery network of beetle tunnels etched into the sapwood. It’s a familiar sight to anyone whose winter warmth depends on firewood, particularly if they’ve run short before spring and had to scrounge half-rotten dregs from a slash pile. Dead or dying timber attracts dozens of beetle species that lay their eggs in the protected and nutritious layer between bark and wood.

Previous experience told me this was the work of the Douglas fir engraver, a tiny beetle whose tracks radiate outward from a long, central chamber. Look closely and each winding trail widens as it spreads, enlarged by the growth of the constantly chewing larva inside.

I reached down to position another chunk of wood, and there lay one of the grubs, rudely knocked loose by my axe work. It twitched helplessly on the cold chopping block, a pale, maggoty thing that any non-entomologist would likely brush aside with a shudder. But I knew it held an important lesson in winter survival, and a single thought ran through my mind: Now’s my chance.

It was a moment I’d been putting off for 15 years, ever since taking a graduate course from famed University of Vermont biologist Bernd Heinrich. With books like Ravens in Winter and Winter World to his credit, the unassuming, German-born Heinrich stands in the vanguard of scientists fascinated by cold. His winter ecology class explored the myriad ways animals survive frigid temperatures, from the true hibernation of black bears and painted turtles, to the controlled shivering of songbirds, to the way a wood frog simply freezes solid and waits for the weather to warm. Beetle larvae, he told us, make their own antifreeze. Lots of insects do. As temperatures drop, they produce quantities of glycerol and sugars that effectively lower the freezing point of their body fluid. “You don’t believe me?” he’d asked the class. “Taste one. They’re sweet!”

As a Peace Corps volunteer in Africa, I had eaten the occasional termite or fried grasshopper to be polite, but in general I don’t think of insects as food. Steeling my nerve, I plucked the grub from the chopping block and popped it into my mouth. Once past the initial crunch, I knew that Bernd had been right. My taste buds registered a burst of nutty sweetness. It gave me a sudden insight into the lives of woodpeckers—in winter, apparently, every meal is dessert.

Any walk through the winter woods offers adventures that a leafy summertime hike could never provide. But getting past the silence and empty branches requires a different pace and a new way of seeing. Bernd gets there by plunging headlong into the season, living and studying in a remote cabin and doing what animals do to survive: If you get cold, run to warm up. If you’re hungry, try eating grubs or a plate of fried voles. And if you want to find wildlife, track it down. Toward the end of Bernd’s course, I devoted a whole day to doing just that.

Tracks in snow have a lot more to tell than merely who walked by; they let us picture who was going slowly, who was running, who was being chased, and whether or not they got away. Any snowfall tallies these dramas, but that day’s conditions were perfect—a light dusting over older, compacted material. Behind Bernd’s cabin I crouched down by the tiny prints of a deer mouse, its hopping gait made clear by the broad stance of the rear feet. I could even make out the faint trace where its tail had brushed down with every leap and landing. The mouse had been moving fast, and for good reason. Crossing open ground is always risky, but on snow a rodent’s dark scurry stands out like a beacon to any predator. More than once I’d seen a mouse’s tracks simply disappear, and then noticed the telltale brushstrokes of owl wings, framing its last step like feathered parentheses. Not that an owl is limited to prey on the surface: With asymmetrical ears tuned to differences in sound, large northern species like the great gray owls can triangulate the exact position of a tunneling rodent and snow-plunge as deep as 18 inches to make a kill.

This mouse had been lucky, dashing successfully from the base of a maple to a hole by a nearby stump, a reminder that in spite of how much goes on at the surface, an equally busy world plays out beneath the snow. Biologists call it the , a place that hardly anyone paid attention to until quite recently. Now studies have found that it’s alive with grazing voles, seed-eating mice, predatory weasels, shrews, spiders, and vast mats of bacteria and fungi whose respiration emits enough carbon to affect climate change.

In fact, the subnivean zone can be a surprisingly cozy place. As survivalists and hibernating polar bears know well, a thick pile of snow makes excellent insulation. Winter nights in the North Woods can reach a frigid minus-20 degrees Fahrenheit, or colder. But where the snowpack is deep and long-lived, temperatures in the subnivean zone rarely stray from the freezing point, a full 50 degrees warmer than the air above. By trapping the earth’s radiant heat at ground level, snow creates a stable, relatively warm environment that helps a wide range of animals, insects, and microorganisms stay active all winter. The timing and condition of the snowfall are critical.

Snowbound plants have also adapted to the subnivean lifestyle. Available light decreases as the snow layer builds, but the wavelengths that penetrate farthest are ideal for photosynthesis, and signs of activity have been recorded at depths of nearly 20 inches. Seeds are even more light-sensitive, responding to the angle of the sun, day length, and other germination cues through a snowpack of more than six feet. Though it was only January, I knew that somewhere beneath my snowshoes the trillium, trout lilies, and spring beauties might already be stirring. Protected from wind, cold, and water stress, they could afford to start growing early and would sometimes even blossom before the snow all melted away. I left the mouse to its subnivean pursuits and pressed on, soon spotting a set of good-sized prints with faint nail scrapes at the tip of each toe pad. They had to be canine but showed none of the romp and exuberance of a domestic dog. These tracks descended from the slope above me in a purposeful line, moving at a steady trot. I turned my snowshoes down the hill and fell in behind. With coyote, red fox, and gray fox at home in these woods, there was only one way to find out what this was.

Winter ecologist James Halfpenny calls tracking a “detective story” where the shape and context of every footfall unmasks another clue. A good set of prints reveals far more than an animal’s identity—it’s an intimate glimpse into its habits and behaviors. Differences in gait, depth, and position show an animal’s size and speed, whether it was hunting or traveling, and even where it was looking. Melt patterns in and around the print tell when the animal passed, and what the weather was like before and since. Where summertime gives us brief glimpses of wild animals, snow allows us to spend hours with them, reliving their every move.

Today’s mystery canine seemed to have a destination in mind. Its tracks led straight downhill, skirted the clearing where Bernd’s cabin stood, and then started up the opposite ridge. Red squirrels, mice, voles, and other potential prey crisscrossed our path, but its steps never wavered. Not even for a thicket of wild rose, where I had to duck low and eventually crawl—and where I found a tuft of long orange hair that finally identified my quarry: a red fox. Emerging from the thicket, I spotted a rose bush festooned with tufts of another kind, like clumps of withered moss growing straight out of the stems. These “Robin’s pincushions” are a winter home for tiny insects, and a reminder that Bernd Heinrich wasn’t the only famous scientist to roam the North Woods.

To most people, the name Alfred Kinsey brings to mind several famous studies of human sexuality published in the 1940s and 1950s. Entomologists, on the other hand, view “The Kinsey Reports” as an unfortunate distraction from an otherwise illustrious career. Before dabbling in sex research, Kinsey spent two decades studying gall wasps, the curious creatures responsible for those deformed rose bushes. As a parasites. Put one of these galls in a jar and you never know how many species might emerge in the spring. Kinsey may well have found human mating rituals dull stuff in comparison.

I left the rose thicket and followed my fox uphill to a rocky ledge, where the tracks made a wide circle around an odd depression in the snow. Suddenly, I knew exactly what that fox had been up to. After climbing to its favorite viewpoint, it turned and looked in all directions. Satisfied that the coast was clear, it then curled up nose to tail, just like a dog, and took a nap. Its sleeping body left such a perfect impression in the snow I could see holes before each nostril, melted by the warmth of its breathing.

Finding fox breath in the snow may be reward enough, but the impacts of winter ecology stretch far beyond any single field walk. In medical research, people often look to winter animals for new approaches in treating human ailments. Mimicking the isometric twitching of slumbering bears and prairie dogs could help patients (and even astronauts) with limited mobility retain muscle mass. The way hibernating ground squirrels tolerate deficient blood flow to their brains promises a new treatment strategy for stroke victims, and many hospital emergency wards have learned that chilling patients can slow the damage caused by cardiac arrest. Engineers study winter ecology, too, leading to everything from air conditioning based on how birds warm their feet in snow to a thermos cozy inspired by polar bear fur.

Crouching beside the fox bed, I felt my own feet beginning to chill. As I huffed back toward Bernd’s cabin, the silent woods seemed full of life, from the beetles and wasps tucked into their winter homes to all the hidden animals. Suddenly, my path crossed the snowshoe tracks I’d made earlier in the day—one more story added to the winter’s cloak of snow and stillness.

This story originally ran in the January-February 2014 issue as "Cold Case."