About the Deer Mouse
The deer mouse (Peromyscus maniculatus) is so named because it has a chestnut coat, a white vest, and eyes and ears disproportionately large enough to resemble a miniature deer. Deer mice weigh only 20 grams, so it comes as a surprise that they don’t hibernate in the winter. The key to their winter survival, like most other Adirondack species, is energy economy.
Finding Shelter and Food
Deer mice are nocturnal creatures. During the day, they primarily live in deep nests in tree holes left by woodpeckers or natural knots in trees. There is also anecdotal evidence that deer mice are more likely than their rodent counterparts to find their way into log cabins or basements to keep warm. At night, they’re faced with the difficult task of finding food and staying warm.
Deer mice are known to take over bird nests (or create their own), not for means of shelter or to nurse offspring, but rather to serve as a spot for grain storage during the winter. Deer mice tend to “dome over” nests to hide their seeds. Since they live with family and non-family members alike, and prefer to keep their hard-earned seeds to themselves, deer mice are stubbornly protective of their food cache. They will bear the energy cost of storing food in inconvenient and distant places so long as no one else takes it.
Deer mice in the Adirondacks make use of 3 strategies to keep warm: huddling, nesting, and torpor. Huddling is the most basic of strategies. Deer mice huddle on a frequent basis, if not to keep warm then just by chance, as they live together in big groups and can’t help but be in close quarters. The second decision made by deer mice is whether or not to build a nest. On warm winter days, deer mice can manage to stay warm enough just by huddling together and saving the energy it would take to build a nest. Below a certain temperature, deer mice generally take a day or two building deep nests to keep them warm in the long run. If temperatures drop low enough, some deer mice will become torpid, or physically inactive, dropping their internal body temperature down to 68ºF. Deer mice have the best chance of survival in winter if they use all threes strategies.
Before entering torpor, deer mice make a peculiar shift from carbohydrate metabolism to fat metabolism, an adaptation usually reserved for organisms engaged in prolonged exercise. They stay in torpor from daybreak to late afternoon when temperatures are at an extreme low. No one knows exactly what prompts deer mice to enter or awake from torpor.
A final adaptation unique to deer mice is their ability to add red blood cells to their blood. To survive in the lowest mid-night temperatures of winter, deer mice add new red blood cells, thus increasing their hemoglobin content, which allows them to increase their metabolic rates. Deer mice use this burst of energy to shiver throughout the night. Shivering keeps the body active enough to initiate a slow warming process.