MTPR

Forget The Powder; Why Some Animals Prefer Crusty Snow

Feb 24, 2019

This morning, the valley sits under a blanket of fresh snow, and another few inches is forecast for tomorrow. I love the first day or two after a snowfall when everything is still white and clean, before the snow’s presence in town becomes more of a hindrance than a facelift.

After a few days, the formerly bright and fluffy snow fades to brownish under a constant stream of people and car traffic, or melts to a shiny crust under the rays of the sun. It’s usually at this point that we start noticing the inconvenience of muddy, crusty snow, and begin to long for another fresh white layer to top us off.

This year, I’ve found myself asking: Is fresh snow always the best snow? Surely there are some benefits to old snow, or crusty snow, or snow that has been melted into an icy sheet. And for even casual observers of nature, it should be no surprise to learn that yes, indeed, there are.

Once snow lands on the ground, the individual snowflakes begin to settle, melt slightly, and subtly restructure themselves. These melting snowflakes join together and the air spaces between them disappear, resulting in tightly-packed snow with a higher density. This dense snowpack is stronger and sturdier than fresh powder, and as more snow continues to fall and build up, its insulating properties increase, too. With a sufficient depth of dense snowpack, the temperature at ground level remains a balmy 32 F, even when the outside air temperature is much lower.

All this melting, refreezing, and condensing opens up spaces between the now-sturdy snow and the ground, in an area often referred to as the subnivean zone. This mini-climate is a winter necessity for many small mammals, who create tunnels, nests, food caches, and more under the insulating cover of the snowpack. The subnivean zone also provides protection from predators above, whether they hunt by foot or by wing (although some predator species are adapted to use their finely-tuned sense of hearing to hunt the subnivean zone by ear alone).

There’s another way that crusty snow benefits animals in winter. Not only does it shelter creatures that live underneath it, but it can also provide better transportation for those that travel above it. Predators such as coyotes, wolves, wolverines, and others find a thin snow crust especially helpful. Their paws distribute their weight over a large surface area, enabling them to move quickly atop the snow rather than struggle through deep powder. Their prey, especially ungulates such as deer and elk, aren’t so lucky, and struggle to move quickly as their hooves punch through the crust. The result is a bit of an advantage for these predators who need all the calories they can get during the coldest time of year.

And carnivores aren’t the only ones who benefit from an easy saunter across snow crust. Small mammals such as mice and voles also take advantage of this phenomenon, using the crusty surface to emerge from their subnivean shelter and walk atop a layer of snow that may be quite deep. In this way, they can reach vegetation that is inaccessible during the summer months, providing a whole new level of foraging options.

So while it’s true that sometimes nature looks its best when covered in a new layer of winter white; old snow has its value too. Next time you’re struggling to walk atop a layer of snow crust, or maybe even noticing the remnants of subnivean tunnels after a spring thaw, consider all the creatures that make their home within this special natural structure.

'Field Notes' is produced by the Montana Natural History Center

Broadcast: "Fieldnotes," 2/10/19 and 2/15/19. You can hear the program on the radio Sundays at 12:55 p.m., Tuesdays at 4:54 p.m., and Fridays at 4:54 p.m., or listen via podcast.