Winter World: The Ingenuity of Animal Survival - Bernd Heinrich (2003)
MICE IN WINTER
Even before I finished building my cabin in Maine, I could see that it had potential. Bubo, my tame great horned owl, chose to perch on the rafters rather than out in the woods, where he or she was harassed by the blue jays. Similarly in June the hordes of bloodsucking blackflies and horseflies left off their hot pursuit as soon as I crossed the doorstep. The cabin was a sanctuary for me in the summer. When winter came to the Maine woods, however, it suddenly became appealing to the wild local fauna, and many adopted my haven as their own.
Masked shrews and red-backed voles, my occasional winter visitors from the subnivian zone, were only transient visitors. In contrast, white-footed mice took up permanent winter residence. For some reason they find the cabin congenial. But before I tell you more about them, I need to describe and identify them. According to Mason A. Walton, the so-called Hermit of Gloucester, who in 1903 wrote about them in his book, A Hermit’s Wild Friends or Eighteen Years in the Woods: “The white-footed mouse, unlike the house mouse, is a handsome fellow. He sports a chestnut coat, a white vest, reddish brown trousers, and white stockings. His eyes and ears are uncommonly large, causing his head to resemble a deer’s in miniature. This resemblance has bestowed upon him the name of ‘deer-mouse.’” (p. 118)
Deer mice juveniles have lead-gray pelage and white bellies. Unlike meadow voles or field mice, they also have long legs that allow them to bound like deer. There are, however, two species of closely related deer mice, and only one of them is the official deer mouse, Peromyscus maniculatus. The other is the white-footed mouse, Peromyscus leucopus.
Differences between the two species are subtle. One field guide I consulted indicated that in white-footed mice the tail is longer than the body, while in the deer mice it is shorter. But those that I measured at my cabin had tail lengths about equal to their body length. Only experts can distinguish the two, and the defining characteristic used for differentiating them is a molecular variation in their salivary amylase, an enzyme in their saliva that helps digest starch. Bill Kilpatrick, the mammologist I consulted, told me that mine were indeed deer mice, Peromyscus maniculatus. This information makes a difference to me, because here in the East, only Peromyscus leucopus is known to carry the Hanta virus, which is lethal to humans. (However, I’m not convinced that a virus capable of jumping from P. leucopus to H. sapienswould be incapable of transmission to P. maniculatus.)
Even with Hanta virus out of the picture, deer mice can be objectionable in a cabin, and in the winter they enter in droves. I can’t blame them, though. The fault is mine. I should have used dry, nonshrinkable ceiling boards to foil these partly arboreal mice. Nor should I have used Styrofoam panels for ceiling insulation. I had not been warned that Peromyscus systematically shreds Styrofoam into chips. The chips drift down like unmeltable snow through cracks between the boards and fly up into the air when one tries to sweep them up. The mice, once inside, also raid one’s dry goods, and use one’s shoes, and bed, to hide them in. The Hermit of Gloucester, who lived before the age of Styrofoam, had dozens around him simultaneously. He was entertained by them, yet even he acknowledged, “A few mice for company on winter evenings would not be objectionable, but I draw the line when I am forced to eat and sleep with them.” Relocating them, Walton learned, has little effect. One night he caught twenty-eight deer mice in his cabin and released them a mile distant. The whole crowd returned by the second night, noisily announcing their presence with the drumming of their tiny feet (a sound by now familiar to me). Deer mice, which utter no vocal sounds perceptive to our ears, use these drumrolls to communicate messages to one another—messages that remain undeciphered by man.
Deer mice are cute, and grudgingly I admire their liveliness and resourcefulness. They live full-time in the woods, where even without the handy materials of Styrofoam or crumpled sweaters, they manage to build excellent nests. At our home in Vermont, they have neither, and most stay outside. Recently at least one set up housekeeping inside the brain cavity of a moose skull that has long hung on the chicken house. (The skull is an eighteen-year-old memento. It came from the poached moose whose carcass attracted a crowd of ravens that started my study of ravens in the wild.) The entire nest in the cramped quarters of the moose skull was a solid ball of Rhode Island red chicken feathers no doubt gathered from inside the chicken house. On December 6, 2001, the skull was unoccupied, and I checked the birdbox in the woods adjacent to the raven aviary to see if the mice had changed residence. It seemed that the roomy log birdbox might be preferable to a moose skull, as woodpecker holes make natural nesting places for the mice. The birdbox consists of a small section of hollow log drilled through with a hole with one board nailed over the base and another board set on the top and secured with wire so that it could be opened.
I worked clumsily to remove the top. When I was finally able to look inside I saw the typical domed-over nest of a deer mouse. This one was made almost entirely of fur. Nothing stirred in the nest, so I started pulling out felted wads, when two Peromyscus immediately shot straight up and almost into my face and then bolted for the woods in long bounds. A third mouse poked its head through what was left of its nest, looking at me intently with its large black eyes. I replaced the cover at once and stepped back. The remaining mouse then poked its head out of the hole before running off as well.
No food was stored in either the skull or the birdbox, nor in any of the dozens of others that I have examined. Yet deer mice do cache their food. I have found their hoards of seeds not only in shoes in the cabin, but also under loose bark in the woods and in abandoned bird nests that were specifically domed over to hide the seeds (see Chapter 5). Why don’t they store their food more conveniently, right in their own nests? I suspect it has to do with private property. Deer mice in the winter huddle not only with relatives, but also with nonkin. Not even deer mice are interested in working hard for an investment that others (especially nonrelatives) might reap as their own.
Deer mice have been intensively studied for more than a half century: everyone wants to know how they survive northern winters. Researchers are in agreement that deer mice don’t hibernate. The relevant question then becomes how such animals weighing only about 20 grams each manage to survive without hibernating? As is usually the case when an animal is up against a difficult problem, it uses every trick available to solve that problem.
As we’ve seen with birds, amphibians, even insects, the key to winter survival is energy economy. To this end, deer mice employ several strategies, each of which has been well studied and documented.
Deer mice are nocturnal, and when not active, they retire to the snug nests they generally build in woodpecker holes and other tree holes. Captive Peromyscus leucopus kept at 25°C build deep nests in a day or two. At 30°C, however, they don’t bother (Glaser and Lustick 1975). It costs energy to build a nest, but it yields energy savings in the long run by reducing the fuel costs of keeping warm. Even more energy is saved by huddling with others (Sealander 1952). Some individuals take it even further, by becoming torpid and reducing their body temperatures to near 20°C in the daytime. The various behaviors are adaptations, since deer mice from northern areas are more prone to enter daily torpor, build larger nests, and store food than those from more southerly areas (Pierce and Vogt 1993). Combinations of these several adaptive energy-saving strategies make a big difference in the winter, when energy supplies are often limited. For example, at 13°C, those mice that are nontorpid, nonhuddling, and nestless expend 2.5 times more energy per day than those that employ all three energy-saving strategies (Vogt and Lynch 1982).
Normally the torpor of the mice begins at daybreak and is over by late afternoon. Thus far, no one knows what cues the mice to enter or arouse from torpor. Curiously, they shift from carbohydrate to fat metabolism several hours before entering torpor (Nestler 1990), in a metabolic shift reminiscent of that which occurs in animals engaged in prolonged exercise.
Although the strategies of the deer mice act to save energy and thereby preserve their energy stores, winter adaptation in deer mice paradoxically involves being able to preserve energy expenditure. Normally the mice are active at night or at least during part of the night. At that time low temperatures cannot be avoided, so the mice must acclimate to them (Sealander 1951). Key to that adaptation is the adding of new red blood cells that have a higher hemoglobin content (Sealander 1962) to the circulation. As a consequence, the mice can increase their metabolic rates and hence their ability to tolerate and keep warm by shivering at low air temperatures. By living in my cabin and in other human dwellings during the winter, deer mice exhibit another manifestation of their energy-saving strategy: Temperatures in the cabin are not as low as those outside the cabin, fuel is plentiful, nesting material and sites are conveniently available, and so the mice can afford to remain nontorpid and active longer.
Deer and white-footed mice contrast with another species pair of similar mice common in New England, who like Peromyscus are not permanent residents of the subnivian zone. But these two species, the meadow jumping mouse (Zapus hudsonicus) and the woodland jumping mouse (Napaeozapus insignis) never enter our houses. They are deep hibernators and stay outside. Neither species has cheek pouches for carrying food, like some other hibernators that store food (pocket mice, kangaroo rats, chipmunks, hamsters), and they do not store food. Instead, they fatten up prior to hibernation. Like Peromyscus, both species have yellow-gold pelage and a blackish stripe on the back. When in a hurry, say, making an escape, they move in successive leaps of about four feet long each, aided by powerful hind legs, their long, white-tipped tails extended out behind. They are seldom seen, although Carolyn Sheldon who studied both species near Woodstock, Vermont, from 1934 to 1937, reports that the meadow jumping mouse was familiar to the local farmers.
Sheldon’s (1938a, b) study involved capturing and marking numerous individuals of both species near Woodstock, Vermont, to determine their home ranges. She also tried to raise them in captivity. Zapus never mated in captivity, and a pregnant female that was captured and gave birth to seven young paid no attention to them despite their cries. By three days, they had died. Napaeozapus, on the other hand, mated readily in captivity and produced many litters, although Sheldon reported that the mother “nearly always” destroyed (ate) them within twenty-four hours. In captivity both species fed on seeds, berries, powdered milk, insects, and their own young when available. Obviously their environment in captivity was not like that to which their psychology is evolved. Either a key element was missing or a harmful one was present.
In the field, Zapus builds aboveground summer nests of closely woven fine grass and dry leaves, with a small difficult-to-find entrance to the side. In the fall the mice leave these summer nests and burrow into the ground, where they hibernate. In captivity, mice began digging and making underground nests for hibernation during the last part of September, when temperatures dropped to 5°C. At first the mice’s periods of torpor lasted only a few hours: They awoke frequently, for a few hours and then a few days, before settling in to prolonged torpor of more than two or three days at a time. In Ithaca, New York, hibernating mice have been dug up as late as the end of April (Hamilton 1935), and Sheldon (1938a) even found them in torpor in her live traps in the last part of May. Napaeozapus also “became sleepy” in September and October, and like Zapus they then also left their aboveground nests and built new underground nests for hibernation. Like deer mice, jumping mice find new quarters in the winter. Theirs, thankfully, are not with me.