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home : outdoors : outdoors April 29, 2016

2/22/2013 5:50:00 AM
Getting through the winter: It's somewhat of a mystery
Animals deal in different ways
The gray tree frog, like some other frogs, produces a glucose-based antifreeze that keeps freezing temperatures from destroying its cells.
Craig Turk photograph
The gray tree frog, like some other frogs, produces a glucose-based antifreeze that keeps freezing temperatures from destroying its cells.

Craig Turk photograph
The flying squirrel stays active during the winter, but often shares a den with others for body heat.
 Craig Turk photograph
The flying squirrel stays active during the winter, but often shares a den with others for body heat.
Craig Turk photograph

Craig Turk
Outdoors Writer/Photographer

Animals do many different things to get through the winter.

Some, most notably birds, migrate to places where the weather is warmer and they can more easily find food.

Many remain and stay active in winter. There may be behavioral changes or even changes to their bodies. For instance, fur may become thicker, or even change color, as is the case with weasels and snowshoe hares.

Some animals hibernate for part or all of the winter. Hibernation is deep sleep in which the animal’s body temperature drops and its breathing and heartbeat rates slow, conserving energy. Many animals count on fat reserves. Just how hibernation works is still a bit of a mystery.

There are even different “levels” of hibernation. True hibernators sleep so deeply that they are difficult to wake up. Others are much lighter sleepers.

The eastern chipmunk, for example, spends most of the winter in its burrow, but is not a true hibernator. It occasionally even makes appearances above ground.

While some Wisconsin bat species migrate south, others hibernate. They are true hibernators – very lethargic during the winter months.



Mammals known as rodents are characterized by continuously growing incisors which must be kept short by gnawing – mice, for example. The Northwoods boasts a number of interesting rodents. Like the vole (aka field mouse).

The tiny vole’s life changes during the winter. Voles do not hibernate, but generally stay active under the snow where it is a few degrees warmer. Vole activity is easy to find in the winter. It’s identified by tunnels they leave in the snow.

Vole tunnels will be lined with grass and other material. Shallow winding trails that appear on lawns after snow-melt are a result of this work.

The chipmunk partially hibernates, but also wakes to eat food it has cached in underground tunnels and occasionally leaves its burrow to forage. The chipmunk accumulates little body fat prior to winter and must eat.

The little ground squirrel might be observed searching for food in a patch of bare ground under a tree or other protected area during the winter.

Our tree squirrels remain pretty active during the winter. Both red and gray squirrels molt and have a winter coat that varies slightly in color from their summer coat.

Squirrels, of course, cache food for the winter. They rely on a keen sense of smell, not memory, to locate the caches.

Gray squirrels build leafy nests for use in the summer, but prefer the protection of a den in the winter. These are often located in a hollow in a tree, but may be in the eaves or attic of a house.

Red squirrels may be found in nests built in a tree or in an underground chamber they’ve excavated. Logs, stumps, bird houses and buildings represent other possible homes. Red squirrels are generally most active at midday during the winter, while they are most active early in the morning during warmer months.

Flying squirrels remain active during the winter as well. These nocturnal rodents usually den in tree cavities and they often den with other flying squirrels to share body heat. They may be spotted gliding to your bird feeder at dusk or after dark.

Cottontail rabbits remain active all year. They are most active at dawn and dusk. Signs of rabbits feeding on bark in the winter may be evident three feet up on young trees when the snow is deep.

Cottontails have excellent sight, which helps them avoid predators. They often crouch down and freeze when threatened. Their dark coloring can be a disadvantage in the winter, as they are more easily spotted by predators such as owls and hawks.

The snowshoe hare, on the other hand, changes color. It sports a white, fluffy winter coat. This white coat helps the hare blend in – assuming there’s snow on the ground. In fall, the new white coat comes in, regardless. The transformation to white takes about 10 weeks.

Beavers don’t hibernate. They store food for winter both as fat on their bodies and in a food cache. Tree limbs are trimmed into convenient sizes and stored in underwater piles near their lodge so they can get to them easily even when their pond is frozen over.

The beaver lodge consists of limbs and logs secured with mud and sod. The above-water inner chamber has an air vent and is lined with grasses or shredded bark. Lodge entrances are underwater.

Another Northwoods rodent, the porcupine, seeks better shelter in the winter and will den in places like hollowed trees or even culverts or your hunting blind between November and April. They don’t hibernate, and their tracks and droppings will be evident near these dens.


Other small creatures

Black-capped chickadees, and other small birds that do not head south, have their own ways of dealing with winter. Fluffing out feathers increases air pockets, adding insulation. Chickadees also shiver, increasing their metabolism. Small birds need to eat far more in the winter to get the calories needed to maintain their body temperature.

True hibernators, at least the ones that don’t migrate, bats gather together in caves called hibernacula. They need to find a winter temperature range that’s warm enough to prevent freezing, but cold enough to keep them from expending too much energy.

Bats have been known to find that this ideal “cave” exists in your attic. They may have brief periods of activity during the winter months.

The tiny shrew, like the vole, may tunnel through snow in search of food. A shrew tunnel will be less than one inch in diameter. Unlike the vole, the shrew is not a rodent. It is of the genus Sorex and has sharp, shredding teeth.

The shrew’s mostly carnivorous diet consists of things such as insects, spiders, slugs, worms and small mice, but might also include things like berries, seeds or nuts.

Shrews, with their super-high metabolic rate, have to eat every few hours and remain active year-round.

Weasels (ermine) turn from brown to white as winter approaches. They are not hibernators, but conserve body heat in winter by curling into a ball and also lower their metabolism.

Raccoons are light sleepers, not true hibernators. They breathe more slowly and lower their body temperature a few degrees, but they wake up to forage from time to time during the winter months.

By November, raccoons have fattened up for this period of inactivity. They den up for warmth and, frequently, a number of raccoons will share a den.

Raccoons do not construct den sites and are flexible in their selection of shelter. Hollow trees are common, and so are barns, attics, crawl spaces, sheds, abandoned vehicles, brush and burrows created by other animals.

Skunks are similar in that they don’t really hibernate, they just stay in their dens during most of the winter, occasionally wandering out on a mild day. Like raccoons, their dens are often shared.

Skunks will dig their own burrow, but might also den in another animals’ burrow or seek shelter in another protected spot, such as in a pile of lumber or beneath a building.


Cold blood

Cold-blooded animals such as amphibians and reptiles have special ways of dealing with the cold.

Frogs may burrow in mud or hide under rocks, wood debris or fallen leaves in the bottom of a water body, such as bullfrogs and leopard frogs do. Many frogs can absorb oxygen right through their skin.

Other frogs, like the gray tree frog, produce a glucose-based antifreeze that allows their cells to withstand freezing during the winter months. The gray tree frog may hibernate in leaf litter, loose soil or behind the sloughing bark of a tree.

Another amphibious Northwoods resident, the American toad, burrows beneath the frost line to avoid freezing.

The blue-spotted salamander that is common to the area is also not freeze-tolerant. Like the toad, this salamander spends its winter beneath the frost line.

Our area turtles, similar to many frogs, take their winter refuge by burrowing into the substrate at the bottom of lakes, streams and ponds.

Most snakes hibernate when the temperature falls below 50. Snakes are sensitive to cold – their winter survival means keeping themselves from freezing.

Snakes will crawl into holes, stumps, rock crevices or similar spots where frost does not penetrate. They do not need much food to keep functioning and often go long periods without eating.



There is much that remains unknown about animals, their winter habits and hibernation (or “torpor” – a dormant, inactive state) as it relates to various species. 

As anyone researching the subject will find, not everyone agrees on the intricacies of all these behaviors.

And, of course, there’s a lot more wildlife to cover here in the Northwoods. 

What have you seen? As always, feel free to share your stories and pictures.

Craig Turk may be reached at cturk@lakelandtimes.com.

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