Coping With Cold

When Vice President Dick Cheney sneered that energy conservation was nothing more than “a sign of personal virtue,” he spoke exclusively from the point of view of rich American homo sapiens. For virtually every other form of life — from the daffodils in our front yards that manufacture energy for next year’s blooms from this year’s sunlight to the chipmunks who fill their bulging cheeks with the sunflower seed we set out for the titmice and chickadees — energy conservation is a matter of life and death.

Our fellow living beings have evolved multiple strategies to survive the season of want, the time when their particular corner of earth is tipped away from the sun. The scope of this article is limited to Mr. Cheney’s (and our) closest relatives: mammals, large and small, who winter in the southern Appalachians and do not have the luxury of purchasing their energy dependence from propane dealers and grocery stores.

Snowbirds & Residents

The most obvious means of coping with the cold — and with the disadvantages that cold weather brings, the greatest of which is the elimination of or reduction in food supply — is to avoid it altogether, to migrate somewhere else. Our mammal migrants include three species of solitary bats: red, hoary and silver-haired.

Like the colonial bat species of the southern Appalachians, these three subsist on insects that fly at night (though red bats sometimes alight on vegetation to pick off ants, leafhoppers and beetles). In the summer months, these solitary bats roost on trees and foliage. When food supplies diminish, they move south.

Not so the eight or nine colonial species who spend the winter here in hibernation. In the summer, some of them roost in caves (eastern and Virginia big-eared), while others nap under the bark of dead trees (Indiana), in old buildings (little and big brown), or in cracks and crevices in bridges and rock outcroppings (eastern small-footed and northern).

During the winter months, however, they take advantage of caves and abandoned mines, where they “form species clusters, from a couple of individuals to several thousand,” says Chris McGrath, faunal diversity biologist for the North Carolina Wildlife Resources Commission.

“Some caves and mines support only one kind of bat; in others, several species coexist. The different species tend not to be side by side; they may be widely separated or just a few feet away from one another. Bats select hibernation locations that meet their temperature and humidity needs, which are different for different species.”

Hibernation, like any survival strategy, has associated risks. While a hibernating mammal may use only one-seventh the energy burned by an animal that stays active all winter, hibernators are extremely vulnerable — to freezing, when fat stores are used up, and to disturbance by predators. Though most hibernators wake very slowly, bats revive relatively quickly if disturbed. A rise in temperature can cause a bat to emerge from hibernation, or to burn its stored fats faster than normal.

True Hibernators

Our nonmigratory bats are among the very limited number of “true hibernators” living in the southern Appalachians. True hibernators are distinguished from “partial hibernators” or “denners” by the metabolic changes that take place when they turn in for the winter. Our most common true hibernator is the groundhog—or woodchuck, or “whistle pig” (so nicknamed for the piercing whistle it sometimes emits when alarmed).

In hibernation, a groundhog’s temperature plummets from about 97 degrees to less than 40. Curled in a ball on a grassy mat deep in its winter burrow, its heartbeat slows from over 100 beats/minute to four. It “takes one light sighing breath every six minutes,” writes Diana Kappel-Smith in Wintering.

“A biologist once took a hibernating groundhog and rolled the stiff, cold, furry ball back and forth on his living room rug, and a stiff, cold, furry ball he remained. It takes hours to emerge from such a deathlike trance. He will keep his own appointment with the year, his metabolic clock set for sometime in March or April,” when the grasses, alfalfa, clover that are his usual diet again become available.

Our other true hibernators are jumping mice — “beautiful yellowish or reddish mice with very long tails, large hind feet, and deeply grooved orange incisors,” according to my Audubon guide. Jumping mice earn their name from the very large leaps (from 3-8 feet) they are capable of making.

Our two species — the meadow and the woodland—can spend six to eight months of every year in hibernation. Curled in tight balls in their underground winter quarters, heads tucked under their bodies, long tails curled like watchsprings, they may perish in their nests of shredded grass if their accumulated fat runs out before winter does. (Unlike many other mouse species, they don’t cache food.)

Several hibernating meadow jumping mice were “dug out of a loose clay bank along Noland Creek” in Swain County on Nov. 7, 1935, write Alicia and Donald Lindsey, authors of Mammals of Great Smoky Mountains National Park. “Each was located in a separate compartment (hibernaculum) lined with dry leaves, approximately 18 inches below ground level.”

Meadow jumping mice, who add about 6 grams of stored fat in the fortnight prior to hibernation, feed on a varied diet, depending on the time of year. After emerging from hibernation, they rejuvenate on protein-rich foods including caterpillars, beetles and other insects, then feed on seeds of grasses and other plants during the summer months. Just before retiring for the winter, their diets also include a species of subterranean fungus.

Woodland jumping mice, residents of coniferous and hardwood forests, also subsist on underground fungi (which accounts for about a third of their diet), as well as berries, seeds, caterpillars and beetle larvae. About two weeks before they hibernate, they begin accumulating their winter fat.

The Deep Sleepers

“Between hibernation proper and plain sleep there are a lot of choices, a hall of darkness with many doors,” Diana Kappel-Smith writes. “Torpor is the name for them; a slight but controlled lowering of core temperature, drowses deeper than most, useful for the short haul, and the long haul too.”

Black bears, who gorge themselves on wild berries, grapes, acorns and other nuts in the late summer and fall, building up the fat layer that will see them through the winter, are what N.C. Wildlife Resources Commission wildlife biologist Mike Carraway calls “partial hibernators.” Though they spend most of the winter asleep in their dens, their body temperatures and respiration rates only slightly depressed, they “can and will wake up on warm winter days, ramble around some, and eat a little, but not much,” he says.

In years when the mast supply is poor (not the case this year, when bears were “big and fat and healthy”) they have to feed in the winter. In our region, females den in mid-December; males follow suit toward the end of the month. A few days before they den, they stop eating their normal food to consume leaves, grass, pine needles — even their own hair — to form a “fecal plug” that keeps them from evacuating in the den. This plug, up to a foot long, is voided after the bear emerges in the spring.

Like bears, eastern chipmunks spend extended periods of the winter asleep. “They make complicated burrows, with a nest room in at the center in which they pile a hoard of seeds, nuts and other dormant edibles; and then they make a dry grass and leaf nest on top,” Kappel-Smith writes. Secure in its winter bed and larder, the chipmunk curls in a ball, tail wrapped around feet and nose.

Even the eastern chipmunk’s Latin name, Tamias striatus (striped storer), alludes to its hoarding propensities. The chipmunk is “a perfect miser in respect to provisions, gathering and secreting much more than it can possibly eat, and never seeming to be satisfied so long as another acorn or nut can be obtained,” writes J.G. Wood in Homes without Hands. Within one chipmunk burrow, he reports, two quarts of buckwheat, some grass seeds and Indian corn, nearly a peck of acorns and a quart of beaked nuts were discovered.

Mammals who remain somewhat more active during the winter months also rely on stored fat (and, in many cases, cached food) to see them through the lean season. Fat reserves allow raccoons and skunks to spend extended periods of time sleeping in dens, while opossums, whose body stores fat less well, have to venture out more often. With their naked extremities, opossums abroad in cold weather commonly lose parts of their ears and tails to frostbite, a hazard these short-lived marsupials increasingly suffer as they expand their range from the Southeast into New England and the Midwest.

Raccoons, possums and skunks may use the same dens, though not at the same time. All three “have similar intermittent periods of activity during winter,” Donald Stokes writes, in A Guide to Nature in Winter. “Many of them frequently switch dens after a night’s ramblings. Because they do this, they will get to know the best dens over an area of many hundred acres, and one den may be inhabited by skunk one night and raccoons the next. Skunks tend to use ground dens, opossums tree dens, while raccoons use both.”

Northern and southern flying squirrels accumulate body fats in the fall and “hunker down during harsh weather,” says Chris McGrath, who has been monitoring the former species since 1994. But only the southern flying squirrel caches food. In areas where the ranges of the two overlap, it’s not uncommon to find nest boxes filled with its caches of acorns.

“Northern flying squirrels can go a couple of days without leaving their nests by slowing their metabolism down,” he says, but must return to foraging for the lichens and fungi they live on before their southern cousins.

Both flying squirrel species engage in another fuel-conserving behavior: communal nesting. This practice aids other small mammal species as well. Deer mice “pack together in a communal nest, each one squirming toward the center in turn and elbowed out to the cooler rim in jerky convection currents,” Kappel-Smith observes. Heat loss is a bigger problem for small animals than large ones, because they have a greater proportion of heat-losing surface to heat-making volume. Five deer mice huddled together each need 30 percent less food to survive than a single mouse going it alone. (Curling in a ball is the way a solitary animal reduces its heat-losing surface.)

Huddling in communal nests is also practiced by red-backed voles, who also store bulbs, stems, nuts and tubers for winter consumption. Unlike most mammals, these voles shed rather than gain weight, preparing for winter. By losing up to 30% of their body mass, these tiny animals reduce their caloric needs. They compensate for this loss by huddling.

Cache In The Bank

Food caching takes many forms beyond the hoards house mice stash in the backs of our bureau drawers and the large caches of cones and nuts red squirrels hide under tree roots or in their burrows. Gray squirrels bury each acorn they collect individually. They’re quite careful about it, digging a pit, putting the acorn in it, covering it up and tamping it down. In winter, relying on their sense of smell, they return to dig the acorns up.

“One attempt to check their ability to do this found that out of 250 nuts stored, only two were uneaten by spring and one of those was rotted,” Donald Stokes notes. But Henry David Thoreau, noting that young oaks grew far from mature oak stands, wondered in his journal why we do not credit “these squirrels, these planters of forests? We regard them as vermin, and annually shoot and destroy them in great numbers because — if we have any excuse — they sometimes devour a little of our Indian corn, while, perhaps, they are planting the nobler oak-corn (acorn) in its place.”

Bark-eating beavers, who must continually chew in order to keep their constantly-growing incisors in check, cut branches of trees and shrubs to store underwater near their lodges. Bobcats, who kill animals as large as deer in their nocturnal ramblings, cache uneaten portions under snow or vegetable debris. Their cached leftovers are an important food source for other meat-eating mammals who are active in winter, including red foxes, who cache their own kill in a similar manner.

Some mammals even cache food while it is still alive. Some moles, who subsist primarily on earthworms, cache stocks of living but immobilized (by a bite, near the front of their bodies) worms in their burrows. Caches of more than 1,000 worms have been discovered.

Many mammals — deer, hares, minks, otters and bobcats among them — add a heavier coat during the winter months. Winter coats are often double-layered, with an outer layer of long, water-shedding guard hairs, and an inner layer of fluffy fur that traps body heat.

Despite the perils that winter poses, many mammals breed—and even give birth—during the months of cold. The first of two breeding periods for southern flying squirrels begins in February. Coyotes form pairs and breed in January and February. Bobcats’ breeding season peaks in late winter. The does of white-tailed deer go through the winter pregnant, after breeding in mid-to-late fall.

Black bears give birth in their dens in January or February. Interestingly, though bears mate in the late spring or early summer, embryos do not begin to develop until females den for the winter, after which they develop rapidly. The young, at birth, are rat-sized, blind and weigh less than a pound. In years when mast supplies are small and a mother has gained insufficient weight to sustain herself and her young, her embryos do not implant. Yet another ingenious way to save energy!