In winter, we often notice the signs of animals that are active on the snow’s surface, such as tracks left behind by a deer or fox. But another set of animals is active right beneath the snow, in the pockets of air between the snowpack and the ground.
Small rodents like mice, moles, voles, and shrews all enjoy the relative warmth and safety of this seasonal habitat, called the subnivean zone. The network of small open spaces and tunnels that forms between the snowpack and the ground gets it name from the Latin sub (under) and nives (snow).
“Snow is like a blanket—it has insulating qualities,” says naturalist Nancy Ritger, AMC’s huts and Cardigan program manager. “It traps air in all the spaces between the snowflakes, and that air traps the heat that comes up from the ground.”
In the Northeast, where we do not have permafrost, once the snowpack is about 6 inches deep, the air beneath it doesn’t get colder than 32 degrees Fahrenheit, Ritger says. This remains true even when the top layer of snow and the air above it are much colder.
The heat from the earth changes the composition of the snow, transforming snowflakes closer to the ground into ball-like ice crystals that are easier for small animals to push aside. Tunneling through the snow allows these creatures to travel and find food in relative warmth. They also create ventilation tunnels to get fresh air, so that they won’t suffocate.
The tunnels also offer protection from the watchful eyes of predators like owls, hawks, and foxes. But the animals traveling in them are still vulnerable. Short-tailed weasels (also known as stoats or ermine) are thin enough to squeeze into the tunnels. Red foxes have such keen hearing that they sometimes hear mice or other animals beneath the snow and pounce, with all four feet landing on one spot, to collapse the tunnel and trap their meal.
When the snowpack in New Hampshire, where Ritger is based, gets to be less than 6 inches deep, the air and ground beneath get colder and animals must burrow deeper underground to stay warm, she says. They therefore miss the chance to find food in the subnivean zone. In contrast, if the snow gets deeper than 6 inches, it becomes challenging for small animals to come to the surface. This isn’t always a problem; some animals are content to stay under the snow until spring.
When the snowpack is at least 6 inches deep, you are unlikely to disturb any tunnels beneath your feet while out snowshoeing or skiing. But once the snow begins to melt, you may notice the tunnels or packed grass where they once were. When you recognize the signs, you can read a story in those remnants of the subnivean zone.