Life Under the Snow – The Subnivean Zone

When the snow blankets the mountains, every animal that lives there must figure out a way to survive.  For some animals this means migrating to lower elevation, while others hibernate and sleep until warmer weather arrives.  But what about the small mammals that stay and are active throughout the winter?  For them, the deep snow is important to their survival, as they seek shelter in what’s known as the “subnivean zone.”  Based off the latin words for snow (nives) and under (sub), subnivean literally means “under the snow” and refers to the area between the surface of the ground and the bottom of the snowpack.

Small mammals such as mice, shrews and voles spend much of the winter in this hidden, subnivean zone, as it provides protection and insulation from harsh winter weather and hungry predators.  This zone starts to form as snow begins to fall on rocks and brushy vegetation, creating open pockets of air where the snow is blocked from falling.  As winter progresses, the ambient heat of the ground causes some of the snow closest to the ground to melt and an ice layer forms, further solidifying the air pockets and gaps in the snow.  The mice and voles then dig tunnels complete with air shafts to help them move around underneath the snow pack.

Snow is a fantastic insulator, which means that with only 8 inches of snow above them, the tunnels and pockets in the subnivean zone will remain a toasty 32 degrees no matter how cold it gets outside.  Plus, there is easy access to food with seeds, grasses, insects and bark remaining unfrozen below the surface of the snow.  Some of the tunnels are quite extensive, featuring different chambers and even toilet areas.

While hidden underfoot, it’s easy to spot the entrances to these tunnels.  Look near rocks, shrubs or trees for a tell-tale air hole, especially if you see little critter tracks near it.  If you dig a pit down next to the hole, you may see an elaborate system of chambers and tunnels.

 

This invisible world is crucial for the voles, mice and other tiny mammals who live in the snowy regions of the Sierra and is another example of the ingenuity and adaptability of mountain wildlife.  Next time you’re out hiking, snowshoeing or skiing through the forest, take a moment to look for signs of these animals and realize that the quiet snow beneath your feet may not be as quiet and peaceful as it seems.