March 2, 2018
While we are out and about in the winter trying to stay warm and keeping our feet under us as we navigate the ice and snow, how many of us really contemplate life under the snow? While many creatures and plants become dormant for the winter, there is plenty of activity under the blanket of snow when it is there.
The “subnivian layer” is the insulated space between the snow cover and the frozen ground beneath. Many animals, as well as plants, take advantage of this thin layer of warmer air and space to continue with daily activities. Though the temperatures under snow may be at or near freezing, this layer provides a place for both plants and animals to survive the harshest winter winds and below freezing temperatures at the surface. As snow settles and ice crystals reform, air spaces and humidity provide another world for animals to move around without being exposed to the extremes in weather. Many of the creatures taking advantage of this “insulating blanket” are small mammals actively searching for food to keep their metabolic furnaces fired. For these creatures, as with any winter active species, they must balance the cost of being active and losing heat energy, with the amount of heat energy obtained from food.
These subnivian dwelling small mammals include various species of mice, voles, moles and shrews. Small, furry rodent – like creatures often get lumped together (“LBJ’s” or “little brown jobbers”), but each of these winter active animals is just a little different in physical appearance and behavior.
Mice and voles are classified in the order Rodentia; they are the true rodents. They have small orange incisor teeth as do all rodents (like miniature beaver teeth; NH’s largest rodent!). The common deer mouse and white- footed mouse look very similar, with large eyes and ears, and a long tail. They are gray/brown on top and white below. Differences between the two species are subtle and it is not easy to tell them apart even when in hand. In the winter you may see the tracks of mice on the surface of the snow as they leave zigzagging patterns across an opening before pushing under the snow at the base of a tree or downed log. The large back feet leave prints in front of the smaller front feet, and often their tail leaves a drag mark as they hop on the surface of fresh snow.
The voles are chunkier looking, and often appear smaller in length. They have smaller eyes, rounded ears tucked in their fur, and short tails. They are reddish brown or gray with a gray belly. In NH there are a variety of voles, but the most common in many areas is the red-backed vole, and the meadow vole. Voles in winter spend most of their time in the subnivean layer, leaving tunnels in the snow that become exposed as layers melt. If you feed birds you may see vole tunnels and tracks as they push their way through the snow in search of what the birds have left behind. Once the snow is gone in spring you will see evidence of voles as open trenches in the grasses with piles of grass cuttings clumped together as nests. It is easy to see how common they were under the snow cover where they appeared to enjoy the freedom to move around under the insulating blanket safe from cold and predators. Look for vole signs along trails and the edges between fields and forests.
Also under the snow, and more often found working through tunnels in the soil, are the shrews and moles. These are member s of the Order Insectivora, high strung small mammals that are constantly on the move in search of food to support their high metabolism. Tiny eyes (with poor vision), small ears hidden well into the fur of their head, long pointy noses, and mostly gray fur distinguish these subnivian and fossorial (adapted for a burrowing life underground) creatures. Most of these mammals with poor eyesight use some form of echolocation to help them navigate their world.
The Order nicely suggests their diet; they eat mostly insects, earthworms, snails, and a variety of other invertebrates. Their high metabolism necessitates a near constant search for food to provide fuel – a shrew’s heart beats as much as 1200 times in a minute! The short-tailed shrew has venomous saliva which is used to immobilize its prey.
The fur of insectivores is short and is two directional to allow for ease of movement in tunnels. The moles tend to spend the winter further underground, while the shrews work their way through the leaf litter at the forest floor in the subnivian layer. You may see shrew tunnels similar to vole tunnels in the snow, but much narrower, with any prints small and close together. Later in the winter or early spring you’ll see signs of the moles as the familiar mounds of dirt where they come to the surface.
All of these LBJ’s have an important role in the environment, though most of us would rather the moles dig somewhere other than our gardens, and the mice live somewhere other than the insulation in our homes! They are near the bottom of the food chain and provide a source of nourishment for the many predators that spend the winter hunting them for food. Owls, hawks, weasels, foxes and coyotes all take advantage of these many subnivian small mammals. Without the LBJ’s it is likely we wouldn’t have the diversity of owls and hawks we have here! These small mammals also have a role in controlling insects and slugs in our backyards and spreading seeds and fungal spores. Whatever you may think of these “rodents”, their ancestors were here long before we were and they have a job to do!