Wednesday, February 5, 2014

Next stop, the Subnivean Zone!

Last week I wrote a post about the winter habits of the Eastern Cottontail Rabbit.  In this post I mentioned that some smaller mammals make use of a space called the subnivean zone.  The word subnivean comes from the Latin root words sub "under" and nives "snow" - so subnivean translates to "under the snow".

The subnivean zone is mostly hidden from humans.  We might look at a snow covered field and see this.

A snowy field with evidence of rabbits.

Rabbit tracks, feeding evidence, and scat only show part of the story about what is going on in the field.  The snow in this picture is more than 12 inches deep.  The rabbit that left its tracks was only interacting with the surface.  More of the story is buried under the snow.

Try this some time.  Wait a few days after a snowstorm and then go find a grassy field that is covered with more than six inches of snow.  Look for an area with no visible signs of animals.  Take a flat shovel and scrape off the top inch or two of snow from a path six feet long (make sure you don't step in the path of your shovel).  You may not find anything unusual at this level.  Continue scraping off layers of snow from this path.  At some point the path you shovel will probably intersect a horizontal tunnel through the snow.  This tunnel will probably be about 1 - 1.5 inches across. 

Found it - a tunnel in the snow
Once you find one of these tunnels, gently use your shovel to follow the tunnel.  Scrape snow from the surface along the direction of the tunnel to expose it to the surface.  Be careful not to fill in the tunnel with the snow you are removing.  This tunnel will likely travel for many feet in one direction.

The subnivean world - tunnels within the snow.
Sometime those tunnels will intersect with perpendicular tunnels.  In other places the tunnels will go vertical with a shaft rising to the surface or diving downward to the ground.

The subnivean world - cross-tunnels and vertical shafts.
Find one of the spots where the tunnel drops vertically to the ground.  Dig downward at this point until you come to the bare ground.  Look closely at the sides of the hole that you just dug.  The snow is probably not the same density from the top to the bottom.  The upper few inches are probably fluffy and not compacted.  At the level where you found the horizontal tunnels, you will probably find that the snow has formed a hard crust.  This crust usually forms the floor of the tunnels.  Below this crust, the snow will often be densely compacted.  In areas without vegetation, this layer of compacted snow may go down all the way to the upper surface of the soil.  In areas a dense covering of plants, the vegetation will suspend the snow up above the level of the soil.  A lens of ice will form at this level, supporting the layers of snow above - creating a space between the snow and the surface of the soil.  This space is the true subnivean zone.  This subnivean zone will not be free of ice and snow.  Hoarfrost often develops here as water vapor rising from the soil hits the ice lens where it freezes.

A hole dug through 12+ inches of snow to reach the subnivean zone.

Here is the same image with the lines to show the crust and ice lens.

The subnivean world - snow crust (blue line) and ice lens (red line)
So what animals are creating these tunnels and using the subnivean world?  In Mid-Michigan, the most likely suspects are several species of mice and voles (kind of like a chubby mouse with a short tail).  Other animals will use the tunnels to hunt including shrews and sometimes weasels.  Even with this intrusion by predators, the subnivean world provides  freedom of movement and safety to the rodents that utilize it.  Mice and voles can move about this zone without being seen from above - they can still be heard and sometimes you will find where a fox or owl has intruded feet first into the subnivean zone to snatch a meal. 

This zone can also substantially warmer than the world above the snow.  The snow and ice eliminates the effects of windchill to the inhabitants of the subnivean zone - allowing them to retain more heat than they would if they were  inches trapped above the snow.  Snow is a great insulator and the layers of ice within the snow prevent heat from the earth from rising upward, creating a pocket of warmer air trapped just above the soil's surface.  When I was taking these pictures I sunk my shovel down into the soil with no resistance - the ground in this field has not frozen.  In areas where the snow cover is thinner (or nonexistant) the two months of cold temperatures have caused the ground to freeze to a depth of 6 inches or more. 

So next time you go outside and think that those barren snow-covered fields are devoid of life you will know that there is a whole world going on within and under the snow.

The subnivean world - tunnels criss-crossing within the snow.

Welcome to the Sunbivean Zone...


  1. Nicely done, Mike - great descriptions and illustrations.

    1. Thanks. Sometimes curiosity and boredom can be a wonderful combination.