Winter World_ The Ingenuity of Animal Survival - Bernd Heinrich [83]
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