by Larry Weber
Woodcuts by Betsy Bowen
Sometimes in winter, Minnesota's snow-covered land looks as lifeless as the moon,but get out for a closer look and you'll see that animals are alive under the cold sheets of ice and snow. With practice you'll learn to read their many signs.
The characteristics of snow make this frozen world livable for many small critters--and some not so small. The two most important characteristics are insulation and reflectivity.
Though it seems like a contradiction, 20 inches of packed snow can act like a fluffy down comforter and keep animals warm--much warmer than the air above. Snow cover in January can keep the ground at a steady temperature, even while the air temperatures above the snow rise and fall, from 40 degrees above zero to 40 below.
Warmth under this cover comes from deep within the earth. Trapped under the snow, this heat melts a space where animals can move about.
Snow also reflects sunlight, keeping the sun's warmth from reaching deep into the snowpack and melting it. Thus the snowy world endures.
A few insects and spiders dwell beneath the snow and leaf litter of the forest. We might see them when they move over the snow's surface on mild winter days.
Perhaps the most noticeable snow-dwelling critters are small mammals,such as voles and mice. These rodents dig a network of tunnels and rooms under the snowpack. Here, in snowy fields, meadows, and lawns, they find seeds and stems to eat.
Although the tunnels are nearly dark, enough light penetrates the snowpack to make life here comfortable. Early in the winter, we can see the voles' air shafts to the surface. About as thick as one of your fingers, these vents probably provide fresh air. By late winter, as the snow melts down to the tunnels, we might get a glimpse of just how abundant this vole network can be.
We notice the presence of voles beneath the snow, and so do predators. Shrews and ermine take advantage of their own small size to pursue the voles through the tunnels. Shrews sometimes stay beneath the snowpack for a long time. Ermine come and go in search of mice to eat. Foxes, coyotes, and great gray owls give voles quite a surprise when they dive for dinner in the snow. Though we seldom see the catch, we can see the telltale impressions of feet, wings, and bodies, where a predator has plunged into the snow.
Red squirrels often descend from trees and go under the snow to visit their food caches. They might also find that traveling beneath the snow is easier, safer, and warmer than scampering across it.
Ruffed grouse roost beneath the snow to escape bitter cold. The birds fly into loose, deep snow, then tunnel a few feet before settling down for the night or the duration of the cold snap. The snow offers shelter and warmth but, unfortunately, little if any food. Grouse abandon their hiding place after a few days to eat tree buds.
The noisy explosion of a disturbed grouse flying from its roost site has startled many a skier or snowshoer. Tunnels and droppings show where the grouse rested.
In winter, fish and other animals that must breathe under water have to cope with water that never freezes but hardly ever rises in temperature above the mid-30s. Here, beneath the ice, in semidarkness, the critters must use the dwindling oxygen and food supply for as long as six months. They must change their normal living habits to survive.
Their main problem under ice and snow is not the cold, but the lack of oxygen. During the rest of the year, wind, waves, and current constantly mix air into the water. Underwater plants also produce oxygen by the process called photosynthesis. But during the winter, ice seals off this source of oxygen and cuts off the sunlight so plants no longer photosynthesize. As animals breathe beneath the ice, they gradually use up the oxygen dissolved in the water. The decay of dead plants and animals also uses oxygen. The levels of oxygen go very low.
Fish, frogs, turtles, insects, and other cold-blooded organisms slow down, becoming lethargic or even dormant. Because they are less active, they can survive for a long time without food or oxygen.
Some wetland residents live both above and below the ice. For example, beaver families remain snug in the warmth of a lodge. Up to 10 beavers winter together. An observant naturalist might spot the breathing hole and perhaps hear the family moving inside the lodge.
Every day or so, beavers swim from their lodge to retrieve and eat twigs and branches of birch and aspen that they cached beneath the ice in the fall. They stay among the twigs for only a short while. Their quick return to dry shelter seems to help them survive winter.
The beavers' smaller cousins, the muskrats, approach winter in a more daring way. Because they do not store food, they must swim and forage each day, sometimes quite far. Though they usually find plenty of roots and stems, they must deal with the water's chill and the challenge of getting enough oxygen. As they swim, they breathe air bubbles trapped beneath the ice. Often they enter feeding shelters, built before the freeze-up, for a snack and a break from cold water.
Water-dwelling animals must watch out for a couple of sleek predators. Mink and otter prefer fish, but they will eat any small aquatic critters during lean times. They usually travel alone. Though at home in water, they go far over land at times. With thick fur, they do not seem bothered by cold water. Sometimes you can see the tracks of the otter's toboggan runs and the mink's hopping gait, often leading to holes in the ice.
Larry Weber, Barnum, is a science teacher and author of Backyard Almanac.
Betsy Bowen, Grand Marais, is the author and illustrator of Antler, Bear, Canoe and Tracks in the Wild.
A complete copy of the article can be found in the January - February 2000 issue of Minnesota Conservation Volunteer, available at Minnesota public libraries.