Walking on Snow


(This column was first published in the January 14, 2002 Buffalo News.)


Last week I wrote about the activity of insects and mammals under the snowpack in tiny caves called pukak. But not all animals -- and in particular not larger ones -- have access to those caverns.


The rest, humans included, have to make our way cross-country at this time of year as well as possible -- hopefully over the snow but more often through it. We urbanites and suburbanites don't often think about this because we enjoy the benefits of automobile transportation and even when we walk, snowplows create pathways for us. Wild animals don't share these benefits, nor do we if we hike across open fields or through trackless woods.


As I write, the snowpack in my yard has generally reduced through evaporation, sublimation and melting to a depth of about a foot, but rewind the calendar to December 28th and recall how that 82 inch snowfall left us with 44 inches on the ground. (The Lewis County town of Montague in the Tug Hill Country east of Lake Ontario had still more: 127 inches of snow that produced a snow depth of over six feet.) Consider the problems faced by different species crossing such fields.


Winter-active arthropods like stoneflies, crane flies and springtails face none at all. They are light enough to walk on the surface of the snow. So too are a few small mammals like mice and, when the snow has settled enough to give it a little more firmness, chipmunks, red squirrels and weasels can also make their way.


The quality of the snow makes an important difference. Many of us recall walking on crusted snow that, at least when we were children, bore our weight. Occasionally we broke through and had to pull up our leg to move on, many times breaking the crust under the other foot in the process. The early soft snow – especially lake effect snow -- offers no such support. It is light as foam. I recall the first time I tried a new pair of snowshoes in such snow. They offered no support at all and I might as well have left them home. But over time snow thickens and provides something between those two extremes.


What is it that allows some animals to progress better through snow? The answer derives from simple physics. Two factors come into play: weight and the surface area pushing down. Small animals are lightweights and their legs are so short that their entire bodies "float" on the surface. They can effectively swim over the snow.


Not so larger animals. Clearly, light weight and big feet serve best, thus the outsize feet of animals like snowshoe hares, lynx, mountain lions and wolves. Deer, on the other hand, are terribly served in this regard. Their tiny hooves (and to some extent their lower legs) break through even crusted snow. And, recalling that lovely sequence from the movie "Bambi", even ice gives them problems.


Coyotes and wolves, for example, enjoy a big advantage over deer. (In pounds per square inch, the canines' 1.8 is a vast improvement over the deer's 6.3.)


Remarkably, and even though our weight is carried on two rather than four, our still bigger feet make us equal to these wild dogs. And we can do still better with snowshoes. A mid-sized pair reduces that pressure to less than a half-pound per square inch. This gives us a tremendous advantage over all other medium or large-sized North American animals. We can quite literally walk on snow.


Of course, most wild animals overcome their disadvantage through additional strength, agility and condition.