Tag Archives: sub-nivean zone

The sub-nivean zone

Well, the great blizzard of Ought Fifteen was a bit of an anti-climax here in my neck of the woods. Instead of the 27″-33″ that had been predicted, we got about 5″. I’m not complaining – the poor folks in Nantucket and Boston really got hammered – but rather am admitting to a mild feeling of let-down after all the water I bottled, the extra wood we sawed, and the emergency flashlights and candles I purchased. Ah well, at least we have another 5″ of snow, and, counter-intuitively, that actually makes for better winter conditions for our small wildlife friends, including moles, mice, voles (and their predators), various insects, and an assortment of bacteria and other microscopic life. This is because of the creation of the sub-nivean zone, which occurs after about 8″ of snow accumulation. (In case you are doing mental math, we had 3″ or 4″inches of previous snowfall, plus the additional 5″ish, leading to a total of 8″ or 9″.)

Entrance hole to the sub-nivean world. Photo by the Seney Natural History Association via Wikimedia Commons.
Entrance hole to the sub-nivean world. Photo by the Seney Natural History Association via Wikimedia Commons.

The sub-nivean zone (“sub”= under, “nivea” = snow) occurs in a protected area between the upper-level snow pack and the earth’s surface, where the temperature hovers around 32 degrees (F). The upper levels of snow provide insulation, while the lowest level melts slightly due to the heat from the earth’s surface, providing water and unfrozen food sources. Indeed, a whole winter ecosystem exists here, a fact that was not widely known until relatively recently. Rodents living in this zone create tunnels, air holes, latrines, and nesting areas, feeding on grasses, insect eggs, and whatever else they can find during the winter months.

The upper layers of snow hide rodents from the sight of predators, although owls can hear them running from 30 yards away, and foxes and coyotes can smell them. These predators will pounce through the snow, attempting to catch their prey with claws or mouth. You have no doubt seen videos such as this one, of a fox leaping into the air to try to pin a rodent through the snow:

(As a side note, according to the Discovery Channel, new research shows that if a fox is facing north (at least in North America), he/she is 75% more likely to be successful in catching prey through the snow than if facing any other direction. The reason for this is not fully understood.)

Weasels, more sneakily, slide down the rodents’ air tunnels and chase their victims through their own homes. (Don’t feel too bad for the little critters, though; if it weren’t for predators like weasels, our world would shortly be overrun with mice, voles, and other rodents, given how fast they tend to breed.)

Next time you are out walking in deepish snow, keep an eye out for traces of predators, such as fox jumps or owl/hawk punches. (In the photo below, note the marks left by the wing feathers and the hole punched through the snow by the bird’s feet.

bird wing snow
Photo by U.S. Fish and Wildlife Service via Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Snow_angel_%2811585439005%29.jpg

Alternatively, look near the base of trees or rocks for air holes leading to the sub-nivean world. (Please don’t disturb the holes, however; the creatures that made them have enough to contend with without our interference.)

For more information on the sub-nivean zone, see here:


or here: