Field Notes

Subnivean Zone

February 20, 2019

Do you ever wonder where all the critters go to make it through the winter? Survival techniques vary, of course, but some utilize the subnivean zone.

This cozy area deep beneath the snow’s surface is used by mice and voles during the cold winter months. The zone forms between the ground and snow pack in one of two ways: Snow is either upheld by low branches and other vegetation, creating a natural umbrella effect, or the ground warms the bottom layer of snow causing it to sublimate (changing from a solid to a gas). The space remains approx. 32⁰F regardless of outside temperatures. These temps are comfortable for mice and voles, who create a system of tunnels that allow access to food and warm sleeping quarters.

On your next winter hike, look for holes (often with many tracks leading to and from it) in the snow’s surface which typically mark entrances to the subnivean zone. In the spring, look for remnants of elaborate tunnel systems at the bottom of the melting snow layer or on the ground in the grass and think about all the work and activity these small mammals do to make it through the cold winter months.

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