Every animal must develop its own way of dealing with winter. Migrate, hibernate, or insulate; these are common strategies. For a few small mammals, survival depends on the snow itself, and the deeper the better.
The subnivean zone is the area between the surface of the ground and the bottom of the snowpack. The word subnivean comes from the Latin “sub” (under) and “nives” (snow). Mice, voles, and shrews retreat here for protection from cold temperatures, bitter winds, and hungry predators. Food is right at hand: grass, leaves, bark, seeds, and insects are free and unfrozen. Under the snow, these tiny mammals create long tunnel systems complete with air shafts to the surface above.
The subnivean zone begins to form with the first snowfall that lingers. Two things happen. First, some of the snow lands directly on hardy vegetation and overhanging rocks, blocking snow from accumulating underneath. If the plants have already frozen by this time, they are able to hold up subsequent snows like little umbrellas. Eventually the weight causes the slender tips to droop to the ground, creating a protected area perfect for hideaways and runways.
At the same time, the snow that lands on the ground sublimates; that is, changes from a solid into a gas without going through the melting stage. Sublimation is prompted by heat radiating from the earth. Warm, moist water vapor rising into the bottom layer of snow cools, condenses, and refreezes into tightly packed, rounded ice crystals.
It takes only six inches of snow for mice, voles, and shrews to have a sturdy roof over their heads and roomy living quarters below. Add another two inches and the subnivean zone remains within a degree or two of 32°F, regardless of the temperature and weather conditions in the outside world.
It is easy enough to study this hidden habitat. Look for an air hole in the snow and dig a pit straight down next to it. If you’re lucky, you may discover an intricate system of rooms and hallways. The most elaborate contain a sleeping area, a breakfast nook, a food cache corner, and a latrine. Long, narrow tunnels connect everything. For convenience, most tunnels begin where there is a tree trunk, large rock, or thick bush. These dark surfaces also absorb solar heat, helping to moderate the temperature of the animals, the plants, and the ground itself.
Living under the snow is not without risk. Owls can hear mice and voles running around underground from thirty yards away. With balled-up feet, they crash through the top crust and all the layers of snow to grab their prey. Foxes and coyotes detect by scent. With an acrobatic pounce, these predators will dive right in for their meal. Suffocation is a hazard for those left behind in a collapsed tunnel.
Another predator is the ermine, a white weasel with a black-tipped tail. Its long, slender body can easily squeeze down narrow air shafts. It boldly enters a mouse’s domain and fills up on its favorite food. To add insult to injury, it will sometimes build a nest of mouse fur and usurp the tunnel system for its own. It doesn’t stay long, but unfortunately no one is left to appreciate that.
Come spring, the subnivean dwellers (and any weasel still pursuing them) face other challenges. A sudden thaw or early rain can drown them as water floods their home. The conditions that stir sugar maples to life also cause the snow to alternately thaw and freeze, weakening its structure. One day the snowpack will collapse, often with a pronounced “whumph.” There does not seem to be any depletion of the mice and vole population (at my house, anyway); perhaps the awakening of dormant roots and shoots has alerted the mammals that it’s time to vacate the premises.
As spring progresses and all but the last few inches of snow melts, the ingenuity of subnivean survival is spelled out like a map: telltale humps appear where tunnels had crisscrossed the field from tree to rock and beyond. And unlike an abandoned bear den or the deer browse line, all evidence of this survival strategy will disappear when the grass greens up.
Barbara Mackay is a teacher and naturalist who lives in northern Vermont. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: firstname.lastname@example.org