Each year as the snow begins to accumulate across our Michigan landscapes, many of us assume that the animals have bedded down for the winter, sleeping away the cold winter days and nights awaiting the spring thaw. In reality, the snow actually creates a sheltered environment for some small animals, allowing them to stay fairly active during the winter months.

Next time you take a winter walk through your woods, Michigan State University Extension recommends that you pay attention to the tiny tracks in the snow. Look closely, these tracks may actually be the tracks made my mice and other small animals as they travel back and forth across the snow. Follow the tracks a little further, and you will likely see that the tracks disappear into a small hole in the snow. You have just found an entrance into the “Subnivean Zone”, a place under the snow where mice, voles and other animals make their cozy winter homes.

The Subnivean Zone

The word “subnivean” comes from the Latin words for under (“sub”) and snow (“nives”) and refers to the open, shallow layer that usually forms under deep, layered snow. The layer can form two ways. The first is when vegetation, leaf debris or trunks and branches physically hold the snow up, which creates an open space that can be used by the small mammals. The subnivean layer can also be created as the snow is warmed by the ground, and sublimates into water vapor that moves up through the snow pack. This sublimation, or the transformation of solid snow particles into the moist gas, changes the lowest snow layer into small ice particles that then act as an insulating roof. The sublimation also occurs when the snow is physically help up, providing further insulation. The result is a humid winter habitat with relatively stable temperatures around 32 degrees.

What types of animals live there?

There are a variety of animals that live in and depend on the subnivean zone for winter survival. The most common are small mammals including mice and voles. These animals spend most of their winter in the subnivean zone, eating plants, seeds, bark from bushes and shrubs. Both mice and voles will sometimes cache, or store up small amounts of food, to ensure a steady supply. While these animals are active throughout the winter, they do spend small amounts of time huddled together in a deep sleep, waking occasionally to feed.

The mice and voles develop a series of tunnels under the snow to make travel easer. The tunnels lead from entrances to sleeping areas and to known sources of food. The entrance holes double as ventilation shafts, allowing the carbon dioxide created from animal respiration as well as carbon dioxide released from the ground to escape. This helps to keep the concentration of the suffocating gas to non-lethal levels.

In the spring or during a thaw, the tunnels become visible. This allows us to ponder the winter movement of the creatures living in the subnivean zone. The evidence of the tunnels can either be hardened snow in winding patterns that lingers after a thaw or trails of beaten down or chewed grass. Whatever the evidence you find, it is awesome to imagine life under the snow.

This is the first of two articles about animals in the subnivean zone. The next article focuses on the perils of predation these animals face, even under the cover of snow.

More information about the subnivean zone and other scientific marvels of winter can be found in the National Park Service’s Winter Ecology Field Guide.