On snowy mornings, you may wake to discover your yard crisscrossed with the trails of tiny footprints. Likely, they are the handiwork of North America’s smallest mammals: mice, lesser-known shorter-tailed voles, and thumb-sized pointy-nosed shrews.
Examine low-growing bushes bare of leaves. Often, even just outside cities, you will find a cuplike birds’ nest roofed over with shredded material—the renovation efforts of the white-footed mouse. Look for meandering two-inch-wide paths in snow-covered meadows; these are the tunnels of voles, revealed when the surface snow has melted. And at night, if you live in the country, listen for faint whispering sounds up in the trees. The “tick” of tiny toenails hitting bark will alert you to flying squirrels landing. All these signs testify that although they seem fragile, little animals thrive despite winter’s snow and cold.
In fact, these small mammals have an easier time in winter than many larger creatures. Deer, for instance, suffer immeasurably, struggling to lift their legs clear of deep snow. Big ravens and crows shiver continuously when not flying to raise their body temperature. Even the big hibernating groundhogs will be the worse for wear in winter: they emerge from their sleep looking emaciated, having lost up to one-third of their body weight. But the smallest mammals are usually still fat by winter’s end, if they haven’t been eaten. The tiny shrew, despite a hyperactive lifestyle, actually gains weight over the winter.
One way to beat the cold is to huddle. In North America, thirteen species of normally solitary small mammals, including the ubiquitous gray squirrel, share sleeping quarters in winter to stay warm. Cold-resistant biologists who measure such things have discovered that the temperatures in these shared nests will often be twenty-five degrees warmer than outside air. The underground communal nests of meadow voles may warm up to fifty degrees Fahrenheit even in January’s chill. Normally solitary flying squirrels will sleep together in groups of up to twenty individuals in an elaborate nest, often in a hollow tree lined with birds’ feathers and laced with grapevines.
This concentration means that if you spot signs of one flying squirrel in winter, you are likely near a mother lode. In northcountry woods and fields, look for a “sitzmark,” or landing spot, in an open area, with tracks leading away from it.
If these dusk- and dark-loving little animals nest near your yard, you may well attract them to your bird feeder on winter evenings. Leave the porch light on after dusk so that you can watch them glide right to the feeder.
Of course, another way animals can keep warm in winter is to come into your house. You may wake one winter morning to watch a mouse march a muffin across the kitchen counter. These rodents’ caches of food are often impressive. One naturalist reported finding several gallons of seeds and nuts stored in one of his closets. (Outside, mice bury these stores one or two inches below ground.) Usually, mice remove the outer hulls of seeds and nuts before storage, presumably to save space.
The mouse’s wintertime penchant for human habitation helped welcome cats into American homes. In the 1700s, reports British author and historian George Ordish, New Englanders even cut “cat holes” into the interior walls of their houses so that the felines could pursue rodents into the spaces between ceilings and floors.
If you buy a Havahart mousetrap, you can release unwanted mice outside (move the mouse at least two hundred feet from your house if you don’t want it to come back). If the mouse is gray with a white belly, it is probably a sweet-smelling deer mouse or white-footed mouse and can survive quite well outside. If the mouse is brownish, it is the smellier, more destructive house mouse—not a Native American but a European interloper that came here with the Colonists. Most often found in cities and suburbs, this mouse, if released, will simply find another house to move into.
Unlike mice, voles, and squirrels, shrews disdain both human habitation and the company of their own kind. Yet Joseph Merritt, director of the Powdermill Nature Reserve in Rector, Pennsylvania, who has studied shrews with radiotelemetry, calls the shrew “a champion at winter survival.”
Most species of North American shrews live alone. The fiveinch-long short-tailed shrew (it weighs as much as two quarter coins, making it one of the largest of the twenty shrew species on our continent) sleeps in an elaborately insulated grapefruit-sized nest, a hollow ball of sedges and grasses hidden underground or under logs, stumps, or boards. By day, it probes its garden-hose-sized underground tunnels with ultrasound, using the same echolocation ability as bats and whales. At night, the shrew emerges, leaving its five-toed tracks in the snow. (Mice and voles, by contrast, leave four-toed front footprints and five-toed hind prints.)
This pointy-nosed predator needs to eat its own weight each day to fuel its hyperactive body. (The heart of the short-tailed shrew, reports Merritt, beats 760 times a minute—ten times faster than a human’s.) A shrew will fearlessly attack animals much larger than itself. New Hampshire naturalist Meade Cadot will never forget the first time he saw a shrew, when he was six years old. The animal was hanging by the teeth from the tip of his grandfather’s finger. “The question was whether my grandfather had caught a shrew,” Cadot remembers, “or whether a shrew had caught my grandfather.”
The shrew can afford to be fearless. It is one of the world’s few venomous mammals. Although it poses no danger to a big mammal like a human, with one bite, the shrew releases a toxin that will send a mouse into a lingering coma and immobilize insect larvae for days. Thus it can keep large caches of high-calorie, high-protein prey fresh. It protects its cache by urinating and defecating on it, which makes the food unappetizing to other predators. And it has another noteworthy winter adaptation: a great capacity to generate heat from a special kind of body tissue called brown fat, which kicks into highest gear in January, when it is needed most. Taken together, the shrew’s special adaptations showcase what Joseph Merritt considers “one of nature’s best plans for life in the winter.”