New! SVT Trail Guide: 40 Walks West of Boston

Under the Snow: Mice, Voles, and Shrews

Mouse photo by Dan Stimson

I suppose most adults have mixed feelings about snow. Who doesn’t enjoy the quiet as it falls, or relish the transformation of familiar landscapes to purest white? But snow gets in our way, impedes travel, and it often becomes ice, threatening unpleasant surprises for those of us afflicted by gravity.

Because it hides them from predators, small mammals ought to be grateful for snow. Although they are abundant, shrews, voles, and mice are seldom seen. Glimpsing one, most of us would identify it vaguely as a “field mouse”—but these little beasts are as different as leopards and sheep.

Shrews are tiny tigers. They hunt for a living, catching mostly bugs and worms. Voles are half-pint-sized cows: they eat grass. White-footed Mice, and their near relations, Deer Mice, store and consume seeds. All three eat a little of everything. And all three are eaten by larger animals such as foxes, weasels, coyotes, snakes, hawks, and owls.

Masked Shrews, White-footed Mice, and Meadow Voles are active throughout the winter, conducting the business of their brief lives in tunnels through leaf litter, grass, soil, or snow, and scampering about the surface, where they are apt to be picked up by predators for forwarding through the web of life. Snow offers some safety—but tunnels near the surface are vulnerable, because owls can hunt by sound. An ill-timed squeak can bring talons punching from above. On windless mornings when the snow is powdery you sometimes find an impression of owl wings on either side of the hole made by their feet. As snow melts, the runways in which these little creatures travel are revealed.

A study at an Eastern Massachusetts wildlife refuge concluded that, across a variety of habitats, we have about five mice to every three voles and to just one shrew. All are numerous. On average, there are about 25 voles per acre, but they can crowd to several times that density in their preferred marsh habitat.

Marshes, such as those along our rivers and streams, are subject to flooding, which can set back the vole population. Temporarily. Meadow Voles have heroic powers of reproduction. With plenty to eat, females give birth to six or seven young, have them out of the nest in a few weeks, and promptly replace them with a new batch of siblings. Young female voles are fertile in three weeks, and are never coy. It doesn’t take much skill with a spreadsheet to show that a pair of voles could theoretically spawn millions of descendants per year. In practice that means that pockets of surviving voles speedily repopulate a marsh when the waters subside.

Whereas voles prefer grassland, White-footed Mice live mostly in the woods. They often hop on the surface, where their three-quarters of an ounce leaves tracks only in the softest snow. Voles are a little heavier than mice, and far outweigh the tiny Masked Shrew, which tips the scales at a tenth of an ounce, and occupies fields and forests at densities of 15 to 50 per acre. Its cousin, the Short-tailed Shrew, which is also common here and weighs as much as a mouse, has the distinction of being one of the world’s few venomous mammals. It eats its weight or more every day. Both kinds of shrews eat invertebrates—and Short-tails also eat mice and voles!

When a shrew skull turns up in the pellet of bones and fur cast by an owl, it’s easy to see its carnivorous heritage. Shrew teeth are pointed, like a dog’s, whereas voles and mice have the prominent incisors characteristic of rodents.

I suppose these little creatures live exclusively in the present. Even if they had the capacity to anticipate the relative security of deep snow, their life expectancies do not run to multiple winters. Although individuals yield their lives quickly, these species have proven very successful. Because they convert plants and insects into food for larger predators, mice, voles, and shrews are key components of the natural communities around us.