By Bryn Scriver
Earlier this winter, I was walking along a snowy prairie trail when I was surprised by a small dark object that popped out of the snow right in front of me (I almost stepped on it) then quickly disappeared into a small bank of snow. After the initial surprise, I realized it was a vole. Then I remembered the animal habitat that only occurs in Wisconsin in winter—the subnivean. The subnivean is the area between the surface of the ground and the bottom of the snowpack. In Latin “sub” means beneath and “nive” means snow.
The subnivean forms after a snow “lasts” and stays on the ground. As the snow lands on grasses, shrubs, or rocks it forms pockets of air. If the plants are frozen (and stiff) when the snow falls it can hold up snow even better creating spaces for hideaways and tunnels. These areas mostly form in prairies and grasslands.
After the vole I saw on that walk disappeared I looked closely at the ground and sure enough there was an “entrance” of frozen grass covered by snow.
In Wisconsin, subnivean animals include mice, voles, and shrews. They rely on winter snow cover for survival. First, it helps them stay insulated from the cold. Once snow cover has reached a depth of 6 inches the subnivean maintains a temperature of around 32 degrees F. Second, animals can find food in the subnivean such as grasses, bark, seeds, and even unfrozen insects. Third, the subnivean provides protected spaces for animals to tunnel under the snow to avoid predators. Voles will actually form underground complexes with areas to eat, sleep, and go to the bathroom. Sometimes, if the snow isn’t very deep yet or the snowpack starts to melt, you can see the tunnels that animals have created.
The subnivean is not completely without danger though. Foxes, coyotes, and large owls can hear their prey under the snow and pounce from above to catch a meal. Also, as snow starts to melt tunnels can collapse or be flooded with water trapping or suffocating residents. Overall though the subnivean is an important winter habitat—a habitat that is threatened by climate change as Wisconsin winters become warmer, icier, and less snowy.
UW-Madison Professors Mark Zuckerberg and Jonathan Pauli are collaborating together on research to better understand how climate change will affect this seasonal refugium. To learn more about their research visit the Zuckerberg Laboratory Warming Winters page.
This winter Madison is fortunate to have snow. Why not bundle up and take a walk along the trails in the Preserve’s community gardens and Biocore Prairie and see if you can find evidence of the subnivean.