Exploring the Subnivean
Words By: Hanna Holcomb
While snowshoeing under bright sunshine along Greenhorn, I spotted the barbell-shaped tracks from a short-tailed weasel. The tracks moved erratically across the snow’s surface, then disappeared. When I walked to where the tracks stopped, I saw a mouse-sized vertical shaft that dropped into the snowpack.
The weasel had gone hunting in the subnivean environment, the area in and below the snowpack where small mammals, like mice, voles, and shrews endure the winter. Spending winter in the subnivean helps these animals stay warm and fed during a time when food is not readily available and temperatures dip below 0℉.
The subnivean begins to form when snow falls and lingers in the Tetons. Overhanging rocks and vegetation prevent the falling snow from reaching the ground below them, leaving pockets of air in the snowpack. Simultaneously, the Earth’s heat sublimates snow that lands on the ground into water vapor. This warm, humid air rises into the lower layers of the snowpack and refreezes, forming an icy roof above the air at the ground. As snow continues to accumulate, it acts like an insulating blanket. About a foot of snow is sufficient to keep the subnivean environment around 30℉, even on frigid days.
Peering into the tunnel connecting the snow’s surface to the subnivean, I imagined the weasel’s journey through the snowpack. With their long, skinny bodies, weasels can maneuver through tunnels and chambers built by their prey. Small mammals build various rooms for eating, sleeping, and defecating and have access to grasses, bark, seeds, and insects to eat.
If the weasel gets lucky, they might locate a mouse’s nest. They are fierce hunters, who will kill the entire family, eat what they can and cache the extra. They will then line the nest with the mice’s fur, hunker down, and enjoy the warmth and protection provided by the subnivean environment
Other predators hunt in the subnivean as well. Owls use their excellent hearing to locate their prey, then crash through the snowpack with their talons and scoop up dinner. Foxes and coyotes hunt the subnivean by sound and smell. They pounce headfirst into the snowpack, collapsing the mammals’ tunnels and walking away with a meal.
When snowshoeing at Grand Targhee, I love to pause and think about all of the activity occurring right underfoot. Through tracks and signs in the snowpack, we can see evidence of how local wildlife survives the winter.