As snow blankets New England, how do voles, mice, and shrews stay warm? They burrow under the snow, into a hidden world of tunnels and rooms – the subnivean zone.
Named from the Latin “sub” (under) and “nives” (snow), the subnivean zone is a space that forms between the ground and the bottom of the snowpack in two ways. With the first snowfalls of the season, vegetation and rocks act like umbrellas, blocking snow from piling up underneath. The snowpack also holds in the earth’s heat like a blanket, heating up the snow that does make it to the ground and transforming it directly into water vapor (a process called sublimation). The warm water vapor rises and freezes onto the bottom layer of the snowpack like a ceiling. The subnivean zone offers shelter from cold and wind, and provides access to food caches stored in the fall. With eight inches of snowpack, the subnivean zone stays right around 32 degrees Fahrenheit, regardless of air temperature.
Subnivean-dwellers dig a series of long, narrow tunnels under the snow, connecting known sources of food to chambers for sleeping, eating, food caching, and defecating. Most tunnels begin at a bush, tree trunk, or large rock, and these entrances double as ventilation shafts.
While small rodents in the subnivean zone are hidden from view, they are not safe from predators. The ermine’s slim build allows them to follow prey right down into their tunnels, sometimes napping in their victim’s tunnel before moving on to the next meal. Fox can hear their prey under the snow – tilting their head to triangulate the sounds (and perhaps tuning into the Earth’s magnetic field), leaping and pouncing head first into the snow. Owls are also tuned into the subnivean sounds, plunging into the snow talons-first.
Winters of repeated thawing and freezing can be difficult for subnivean-dwellers. Their winter homes can be flooded by abrupt snow melt. Sudden ice formation or snowpack collapse can trap animals in their tunnels and lead to suffocation. The hard ice layer that accumulates on top of snow during repeated freeze-thaw cycles makes hunting challenging for subnivean predators. Overall a thinner snowblanket is less insulating, and requires animals in the subnivean zone to eat more food in order to maintain their body temperature.
See our previous blog post about making a snow cave, or “quinzee,” to have your own experience of getting cozy under the snow!
