After a lingering fall and frigid December it finally feels like winter is in full swing. The ground has been covered with snow for several weeks now, but you would never guess that under the fallen snow is an entire ecosystem. In the small, narrow space between the ground and the snow pack lies the subnivean zone, which is inhabited by rodents like mice, shrews and voles who graze on grass, seeds or insect eggs. All winter they remain busy under your feet carving out elaborate tunnels. Not only do these animals not hibernate, but their cold weather survival actually depends on the presence of deep snow.
The subnivean zone offers protection from cold temperatures, bitter winds and hungry predators for these small mammals. The creation of this layer begins with the first snowfall that remains on the ground. As snow lands on plants, leaves, sticks and overhanging rocks it is prevented from accumulating underneath and the snow gets physically held up off the ground. Meanwhile, radiant heat from the earth causes the snow that lands on the ground to sublimate; that is, change from a solid directly into a gas without melting and becoming a liquid in between. When this warm, moist water vapor rises into the bottom layer of snow it cools, condenses and refreezes into tightly packed, rounded ice crystals that then act as an insulating roof. The result is an insulated, humid, open space with relatively stable temperatures around 32°F (no matter how cold it gets above) that can be used by small mammals as winter habitat. This fairly constant, relatively warm air also supports insects and microbes able to survive harsh conditions, which, in turn, provides food for the main tunnel dwellers.
It only takes about six inches of snow for a sturdy roof and ample living space to be formed. Some of the tunnels created by these small mammals are quite elaborate, consisting of a number of chambers for different functions like eating, sleeping, caching food and urinating. They even build air shafts to the surface so that fresh oxygen to breathe can get into the tunnels. Most of these air holes are made next to tree trunks, large rocks or thick bushes for protection and ease.
While living under the snow offers protection it is not without risk. Owls can hear rodents scurrying in their tunnels from 30 yards away. Forming a ball with their feet, they punch through the top crust and all the layers of snow to grab their prey. Foxes and coyotes detect animals in the subnivean zone by scent and will pounce and dive right in for their meal.
Come out to Kroening Interpretive Center at North Mississippi Regional Park to hike around the prairie look to for air tunnels to the subnivean zone or check out a pair of binoculars and go bird watching. Join us on weekends for free activities! Saturdays from 11 a.m.-3 p.m. stop by our Naturalist’s Nature Table where we’ll have themed activities and a craft. Sundays from 12:30-3:30 p.m. drop in for Family Funday, grab an activity bag full of fun for the whole family and head out into the park for some nature adventures. We are currently hosting Putting Down Roots, an evolving exhibit of artistic fiber representations of prairie plants. Local artists used their talents to create pieces that help bring awareness about these beneficial plants, their relation to water quality and biodiversity; available for viewing free of charge throughout our center.
January public programs (all children under 12 are free, but must be accompanied by a registered adult):
Jan. 14 Bushcraft –Practice some winter wilderness survival skills. 1-3 p.m., free for all ages.
Jan. 14 and 28 – Snowshoeing – borrow snowshoes to explore the park. 11 a.m.-3 p.m., free for all ages.
Jan. 31 Senior’s Day – Prairie Ecology Presentation and Hike. 1-2:30 p.m., free ages 55+.
School Release Day activities: Friday Jan. 27 – Wild Winter Survival, 8 a.m.-4 p.m., ages 6-12, $20.
For more info or to register for programs at North Mississippi visit minneapolisparks.org and like us on Facebook to stay in the loop about what’s happening in our park!