Mammals spend fall growing thicker coats, but beyond that survival preps vary. Several hunt or forage all winter long, which means they don’t need to ready themselves for dormancy or hibernation. Predators such as coyotes, weasels and bobcats continue to search for meals, even ones hidden under the snow.
Deer, tree squirrels, mice and voles continue to eat bark, twigs, nuts, dried grasses and seeds . (This makes late fall and winter excellent times to find tracks.) But all of these mammals need shelter when temperatures are especially cold. White-tailed deer gather together under trees with low-hanging branches to avoid the wind. Mice and voles create tunnels in the snow, which insulates their network of travel routes and hides them from predators on the ground and in the air.
Gray and fox squirrels den in tree cavities when conditions are poor or on branches in “dreys,” which are made of twigs and leaves and lined with soft grasses and fur. They usually den alone, but during particularly cold spells they may share dreys to conserve warmth, especially male and female pairs during the December and January breeding season.
Some animals enter a state called “dormancy” if the weather becomes too severe. Their body temperatures drop, slowing metabolism and sending the animals into a sleepy, energy-conserving state. It’s different than hibernation because although movement slows significantly, the animals are still awake and slightly active. Raccoons spend dormancy in trees or other high places, usually alone, although females may huddle with young they had earlier in the year. Skunks, sometimes in groups of a dozen individuals, move into abandoned woodchuck burrows or brush piles. Opossums, which are equally social, may change shelters several times a week, but chipmunks merely hunker down in the confines of their underground winter homes.
Only a few DuPage mammals are true hibernators. Among them are bats and woodchucks, which eat as much as they can to pack on the pounds that will sustain them through the weeks ahead. During hibernation, they can be motionless for days at a time. Their breathing, metabolism and other processes slow so significantly that body temperatures can drop by half. Heart rates of woodchucks in particular can fall to only four beats per minute.
To conserve body heat, many bats hibernate in groups — some numbering in the thousands — in caves or hollow trees called “hibernacula.” Most of the 12 bats native to Illinois migrate to southern Illinois and beyond, where they use abandoned silica mines, but big brown bats are different. These cold-tolerant flyers remain minimally active for most of winter, although two to five will hibernate together for short periods when the weather is exceptionally bitter.
Unlike bats, woodchucks hibernate alone and spend fall digging wintering burrows, long tunnels with chambers lined with soft grasses. Once a woodchuck’s work is done, it moves inside and closes the entrance with soil.
Winter can be a difficult time for animals, but in fall the native animals that call DuPage forest preserves home show they have lots of interesting ways to prepare and survive.