Least Weasels in the Subnivean Zone

Sometimes, when temperatures struggle to rise above zero, it takes all of my willpower to get out for a simple walk. Even when cheeks sting and the ice in my eyelashes sews up the corners, I know that getting outside will do wonders for my mood. When possible, I do try to plan any outdoor excursions for “the heat of the day.”

So it was noon on one of those bitterly cold days by the time I wrapped up some cookie baking and clomped down my driveway. Coyote tracks quilted the snow between the tire tracks, but they were softened by time and flurries. Nothing new.

And then, there was something new! Several pairs of tiny tracks stitched a line across flat ground and up the bank before disappearing into a hole no bigger than one gloved finger would poke under a mess of dead branches. Each set of prints could have fit on a nickel. In a few places, parallel lines connected the tracks to each other as the critter’s jump didn’t quite rise fully above the snow. Any signs of tiny toes were obscured within the texture of snowflakes.

Tracks of a least weasel disappear into the snow—into the subnivean zone.
Photo by Emily Stone.

“Least weasel!” I exclaimed before taking a few quick photos. Then my phone died from the cold.

With a maximum size of 8 inches long—including tail—least weasels are the smallest weasels in Wisconsin. Female weasels are smaller than males, tending toward 6.5 inches long, and weighing only 37-40 grams. Small animals tend to lose heat quickly, because of their large surface-area-to-volume ratio, so it’s odd that the smallest of the dozen or so least weasel subspecies lives in northern Russia, and the largest lives in North Africa.

There is one major benefit to being small in winter, though: least weasels can fit easily into the narrow passages and tight spaces of the subnivean zone. Inside this magical space where ground meets snow, a crystalline blanket traps the earth’s warmth, and temperatures hover near freezing. The subnivium is so important to Mustela nivalis, that their scientific name means “weasel of the snow.”

Earlier that morning, when the air temp read -13, my thermometer buried under just 6 inches of fluff registered a balmy 28 degrees. Being small means that the least weasels almost never have to venture up into the frigid, windswept winter world.

In fact, their entire body is designed to exploit the underground. A narrow head, lithe body, short legs, and flexible spine allow weasels to maneuver easily in tight spaces. Sensitive whiskers and a great sense of smell guide weasels in the darkness. They don’t dig their own tunnels, but then, why would they? Mice, voles, and chipmunks can be found in the dens they’ve dug themselves—in either snow or soil.

With a bite to the neck, least weasels are even able to take on adult rabbits—prey that is at minimum 16x bigger than this feisty little predator. In fact, least weasels are the world’s smallest carnivore. When relaying this fact, I’m often asked, “What about shrews?” Shrews are insectivores, and members of the now-obsolete taxonomic order Insectivora instead of the weasel’s order of Carnivora.

Unlike wolves—another member of Carnivora—who often gorge on a kill and then go without eating for several days, weasels cache their prey and come back for smaller snacks 8-10 times each day. To feed their fiery metabolisms, weasels eat at least half of their own weight daily, and more in the winter.

Least weasels are prey as well as predators, and they attempt invisibility by transitioning from summer brown to winter white fur. Unlike their cousins, the short-tailed and long-tailed weasels, least weasels do not keep a black tip on their tail. In the larger weasels, that bit of misdirection tricks raptors into aiming for the wrong end. In least weasels, their head and their tail are just too close for comfort.

Least weasels change from brown fur in the summer, to all white in the winter.
Photo by Cecil Sanders, flickr.com.

Speaking of comfort, I found myself a bit jealous of the “weasel of the snow” today as I ventured into a sparkling, -21-degree morning. While my car and lungs protested equally at being asked to work in such conditions, my subnivean thermometer had risen to 31 degrees under the added insulation of a fresh foot of snow.

I probably won’t have another chance to see tracks or sign from my tiny neighbor this winter. At least I know that they are staying warm under the snow as I imagine their adventures in the subnivean zone.

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

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