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The Subnivean Zone
Subnivean, Subnivian, Subniviun, or Subnivium?
There are many different spellings of the same word, but they refer to the same thing. When it snows, a special zone is created called the subnivean zone. This insulated space between the bottom of the snow layer and the top of the frozen ground provides a safe space for small mammals.
Two Ways to Form
Leaves and small branches near the ground may hold up the snow, or the snow is warmed by the ground, which creates these small spaces. Heat released from the earth is trapped by the snow, creating a stable temperature right around 32⁰F. Many small herbivores use these tunnels to move around. It provides them some protection from the cold and predators. However, predators like fox and coyote, can hear these animals moving around under the snow. They will pounce to get their prey.
Black-capped chickadees (Poecile atricapillus) caches food for the winter. They will also fluff up to create an insulated layer to help stay warm. If outside temps drop very low, they can reduce their body temperature and go into a state of torpor.
Eastern cottontails (Sylvilagus floridanus) are active during winter. They use shelters of thick brush or thickets. They will eat buds and twigs and sometimes will eat their own scat. This recycles nutrients they did not get the first time around from the woody plants.
Bumble bee queens burrow into the ground and await the spring. Only the queen survives the winter, and she goes into diapause. Once spring arrives, she lays eggs to start a new colony.
Mice, voles and other rodents build tunnels in the subnivean to move around to places where they have cached food and their home. When the snow is gone, you can see the trails they created.
Salamanders brumate underground in burrows, under logs, or in leaf litter below the frost line. They can come in and out of brumation when it warms up for a few days.
Woodchucks are one of the only true hibernators in Illinois. They curl up in their burrow and sleep through the winter months. They survive off their stored fat and lower their body temperature, heart rate, and metabolism.
Aquatic turtles brumate by burying in the mud underwater in lakes, ponds, or slow-moving water. If it warms up, they may come out for a few days and return to inactivity when it turns cold.
Subnivean trails revealed after snowmelt outside IDNR building, January 2025.
Torpor vs. Hibernation in Mammals
Torpor
•Lower body temperature slightly
•Lower breathing rate
•Lower heart rate
•Lower metabolic rate
•Appears to be an involuntary state
•Can last from a few hours to a few weeks
•Can arouse relatively quickly
•Responds to external stimuli
•Wakes up to eat and drink
Hibernation
•Deep sleep or dormancy
•Long periods of inactivity
•Low body temperature
•Low heart rate
•Low metabolic rate to 1-5% of normal
•Respiratory rate drops
•Last several weeks to months
•Largely unresponsive to external stimuli
•Relies on fat reserves