Life in the subnivian lane


For those of us living in temperate parts of the world, winter means our yards grind to a halt. Plants and trees stand bare. Butterflies are hibernating and honeybees stay in their hives. Snails and earthworms are deep underground. Even squirrels, raccoons and opossums will stay in their nests for days if the weather’s cold enough. Even at our feeders, where birds still whoosh in, they’ve no time to stay. They jet back to warmer resting spots quick as they can.

But winter does have its own special charm. It’s quiet out there and rather peaceful, too. And, inconveniences aside, who doesn’t appreciate the beauty of freshly fallen snow, where for days, nothing may mar its pristine surface but the skinny four-toed prints of birds eating below our feeders. Then, if we’re lucky, we may spot raccoon or fox footprints there, too. And, that’s about it.

Well, not entirely…if we were to travel down under the snow, we’d find a busy highway of activity, with animals moving about in the relatively cozy world of “subnivian spaces.” Subnivian means “under snow.” It happens that snow flakes have air pockets in them which hold heat and provide roughly the same insulation value as wood chips or brick. Even when the surface temperature is subzero, below the snow it’s almost always above freezing. Snow also reflects the sun’s heat, keeping it from melting too fast deeper down.

Field Voles (Microtus agrestis) at the entrance to their snow tunnel. (Tomi Tapio / Flickr; cc by 2.0)

So, many animals make use of the special protection snow offers them. Mice and voles dig mazes of tunnels through the snow to find the foods they eat. And there’s usually plenty to be found: Mosses, lichen, grasses, roots and other plants are there under the snow, along with plant seeds and nuts. These industrious rodents dig chambers for food storage and resting places and punch small holes to the surface, probably to draw down fresh air. It’s likely a liitle light from the sun also penetrates the snow, giving them some visibility.

The subnivian world may almost be better than summer for the mice and voles — they stay hidden from most predators. Not all, though. Foxes have such acute hearing, they can hear movement under the snow and will dive into it to grab a meal. Tiny carnivorous shrews live under the snow and use snow tunnels to look for mites, spiders, beetles and…mice and voles.


Snow tunnel. (Sara Hollerich, USFWS. cc by-sa 2.0)

All this is going on right in our snow-covered yards, so look for evidence — small holes at the surface. And as the weather warms and snow thins away, tunnels will become exposed.

Tree squirrels, too, dig tunnels. They have such a sharp sense of smell, they can detect a buried nut through a foot of snow. And, they’re capable of digging down to it, but they’ll often just tunnel under snow and reach nuts that way. That’s how the squirrel in the photo at the top of the page got where he is without leaving prints. Tunneling might keep them warmer, as well as safer from predators. All about tree squirrels

Ruffed Grouse use the insulating properties of snow to keep them warmer while they roost. They dive into soft, deep snow and then dig down a bit farther to settle into it for the night. You might also be surprised to occasionally see insects and spiders on the snow. They’ve wintered under leaf litter and snow will sometimes rouse themselves and come to the surface to take advantage of warmer days.

*Top photo: Dawn Huczek / Flickr; cc by 2.0