What's Going on Below the Snow at Voyageurs?
A complex world sprawls beneath the snow, a survival network for Voyageurs’ smallest mammals. For the mice, voles, and shrews facing a long, harsh winter, the snowpack offers safe haven – a warm, resource-rich ecosystem for them to avoid predators and thrive until spring.
The subnivean zone’s name gives us clues to where to find this hidden maze. Subnivean (sub-NIV-ee-an) comes from Latin for ‘under’ (sub-) and ‘of snow’ (niveus). The subnivean zone is the bottommost layer of the snowpack (snow that stays on the ground throughout winter). Here, warmth from the ground gets trapped by the blanket of snow above. It typically is formed when there is at least six inches of snow on the ground – enough layers for snowpack to form this unique habitat.
Photo by Kate Van Daele
An icy crust forms in lower layers of the snowpack when the warmth radiated up from the ground causes snow to convert from solid (snow) directly to gas (water vapor) in a process called sublimation. When that water vapor hits the snow above, it refreezes and forms a thin, icy layer that helps create a pocket of warm air that can be up to 32° warmer than the air temperature. After sublimation, the snow in the subnivean zone is less dense than the layers above, making it easier to burrow through.
Within this coziest layer of the snowpack, shrews, mice, and voles have access to the grass and seeds that were scattered on the ground when the snow started flying. Some of this ground cover can create small pockets of warm air, such as when a patch of grass folds over and prevents the snow from getting to the ground. Energized by this food and warm air, subnivean occupants dig through the snowpack, creating tunnels and chambers to survive and thrive during the winter.
As plentiful and protective as the subnivean zone is, sometimes predators are able to see, or hear, through the protective snowpack. Foxes and birds of prey can hear small rodents’ subnivean scurrying and will pinpoint the location of their next meal. Foxes dive through the top layers of snow after hearing squeaking and scratching at the bottom of the snowpack. Scientists believe that they also utilize alignment with Earth’s magnetic poles to catch their prey. Similarly, birds of prey, most notably owls, will hear a subnivean rodent and use their talons to extract it from the snow. Evidence of this catch can be found in the form of wing marks surrounding a hole in the snow, kind of like a hunting snow angel.
Want to explore the subnivean zone for yourself? While we ask you to please not dig down into the snowpack in search of small mammals, there are plenty of signs that hint at the world beneath your feet (or skis, or snowshoes). Air holes, tunnel routes, and tracks to openings in the snow are strong indicators that you are on the roof of a subnivean living complex.