Winter Adaptations: New fur coat and different menu for active mammals

The long-tailed weasel can climb trees, bark and swim, and in winter, in certain latitudes, its dark, furry back turns all white except for the tail.

Changing to a new fur coat is one of various adaptations used by mammals that are active year-round in Illinois to cope with winter. Others include changing diets and sleeping arrangements, plus storing food in a natural pantry. These adaptions enable them to remain active in the coldest months, even as other mammals including wood chucks and ground squirrels are hibernating.

A member of the mustelid family, the long-tailed weasel is 8-10 inches long with a reddish-brown body and a light-colored belly. It also has a 3-6 inch long black-tipped tail. It’s small, but pound for pound (or ounce for ounce), it is one of the fiercest predators to be found in Illinois.

Year-round, the long-tailed weasel keeps small mammal populations such as mice, voles and rabbits in check. Yes, the long-tailed weasel has been documented attacking and eating the much larger cottontail rabbit. Its smaller counterpart, the least weasel, also dines on mice and voles.

“The long-tailed weasel has a reputation for occasionally killing chickens on a farm, but they also do a lot good by getting rid of rats and mice around the farmsteads,” said Stan McTaggart, program manager for the wildlife diversity program at the Illinois Department of Natural Resources.

In more northern latitudes, shorter days and falling temperatures leads many weasels to change from their summer brown/tan coat to a white coat for winter. The change of its fur to white to mimic the color of snow in winter serves two purposes.

“It’s a long-term adaptation that helps them blend into the snow to avoid predators (such as owls), and also ambush and hunt their prey,” McTaggart said. “Depending on the latitude, when they molt or shed in fall, they then put on their new, white winter coat,” he said. “There’s a hereditary component to the adaptation, based on where the weasel is living.”

Researchers have moved long-tailed weasels from their northern homes to places with no snow in winter, and their coats still turned white, he said. The reverse is also true with weasels living in warm climates being brought up north and not changing color in the winter.

Though the long-tailed weasel mates in summer, it’s not until spring that the young are born. The long delay is not just the gestational period, it is because of another adaptation called delayed implantation. Delayed implantation is relatively common in the weasel family and after an egg is fertilized, it doesn’t implant into the wall of the uterus to begin developing until spring. This is likely an adaptation to increase the number of offspring a female can produce, and possibly an added benefit that young weasels aren’t born in the dead of winter but arrive closer to spring when prey is abundant.

Vegetarian Beavers

Another furry mammal that remains active during the winter is the beaver, and its diet is completely different from that of the weasel. Beavers are vegetarians.

McTaggart said the beaver undergoes seasonal behavior changes. In fall, when the amount of daylight diminishes, species like beavers and river otters and cottontail rabbits are triggered to develop a heavy winter coat for warmth in winter as well as to gorge on food to develop a fatty layer.

In Illinois, beaver generally live in bank dens or traditional lodges. A bank den is a burrowed-out cavity in the bank or shore that maintains a covered underwater entrance. A traditional beaver lodge is surrounded by water and is made of sticks that allow the beaver to get out of the water to rest and eat, and offers protection from predators and the weather.

“The top of the dens or lodges are often covered with and patched by a hodge-podge of sticks and mud,” McTaggart said.

In spring and summer, the beaver has a wide cornucopia of vegetation from which to choose, including wetland plants, such as water lilies and cattails, as well as tree bark, with willow being one of their favorite. But in winter, beavers depend on the bark of woody sticks for survival.

Starting in early fall, beavers begin to store food for the winter in a stockpile called a cache.

“They cut up limbs into manageable size pieces to store underwater in the cache,” McTaggart said. “Then, if or when ice becomes too thick to break through, they can enter the water from inside the dens or lodges, swim underwater to their cache and bring sticks back to eat in the safety and comfort of their den. This behavioral shift of having a fully stocked pantry is a really cool adaptation that allows beavers to survive long, cold winters.”

Rabbits and Otters

Like beavers, cottontail rabbits also have less choice of nutritious food in winter. Though they may eat green plants all spring and summer, they begin munching on the bark of woody stems and buds in fall and into winter. In January, a rabbit will eat anything it can find and digest, including the thin sticks of newly planted trees and shrubs. Dense shrubs, stands of conifers, and brush piles covered with insulating snow give rabbits shelter in winter.

Otters seek abandoned beaver dens and other sheltered places to spend time in during winter, but they are still out and about seeking their favorite foods including bullheads, sunfish, suckers and crayfish. In winter, they often move to places where water is still open, for example, springs, fast moving sections of rivers and creeks where food is available.

Learn more about the mammals of Illinois at Wildlife Illinois.

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Sheryl DeVore writes environment and nature pieces for regional and national publications and has had several books published, including “Birds of Illinois” co-authored with her husband, Steven D. Bailey.