Black bear den: There is often very little structure to a black bear’s den, as illustrated by the one in this photograph, which is located under the branches of a fallen tree. (Photos by Mary Holland)

Beaver lodges are both impenetrable and well insulated.

Gray squirrel drey: A gray squirrel’s nest, or drey, is constructed mostly of leaves and sticks.

A muskrat lodge can be as big as 8 feet in diameter and 5 feet high.

If you were faced with living outdoors all winter, what survival strategy would you choose? Some of our mammals, such as the red fox, typically sleep out in the open, curled up with their head tucked under their tail and little else sheltering them. Most of our mammals, however, seek more protection from winter's freezing temperatures, cold winds, and snow. The nature of this protection ranges from the use of existing nooks and crannies to the construction of complex structures.

The den of a black bear is surprisingly minimalist. Although bears do occasionally excavate holes in banks, most of their dens are as simple as a dirt scrape, a cavity under a stump, log, or brush pile, or a hollow tree or rock crevice. None of these sites totally shuts out the winter elements or provides much insulation; dens with open entrances are about the same temperature as the air outside.

Hibernating black bears tend to roll up into a tight ball, with their heads between their front feet and their heavily furred backs exposed to the worst of the cold. Thanks to their inches-deep layer of fat and their fur, black bears can withstand the rigors of winter under these conditions.

During the spring and summer, porcupines spend most of their time in trees, feeding on leaves, buds and inner bark. During the colder six months of the year, female and juvenile porcupines live in dens during the day, and venture out to nearby trees to feed at night. (Males typically do not use dens, frequently spending several days feeding in one tree.)

Porcupines do not construct their dens; they use crevices and cavities in existing rock ledges and hollow trees as shelter. These dens provide them with a windbreak, but not much else, as they are not insulated and the entrances are open. The same dens may be used by the same porcupines (up to a dozen may gather in a single den) year after year.

For reasons yet to be determined, within a given winter it's not unusual for porcupines to change dens about every three weeks, rotating among two or three.

Gray, red and flying squirrels are among those animals that build winter shelters if natural cavities are not available. Their nest, or drey, consists of a bulky ball of leaves interwoven with sticks which hold it all together. It is so tightly constructed that it is actually waterproof.

A drey measures roughly 12 to 19 inches in diameter and is quite obvious, especially after leaves have fallen off the trees. The insides are lined with soft material, such as shredded inner bark, grass, ferns and moss. Often a squirrel has more than one such shelter within its territory. Gray squirrels often build their dreys out on the limbs of deciduous trees, whereas red squirrels tend to build theirs near the trunk of a coniferous tree, but there are plenty of exceptions.

Often a group of several squirrels will occupy a single drey in winter.

Muskrats build their lodges in May or early June, and then again in October. They construct them with plant material such as cattails, reeds, sedges, and phragmites, as well as mud. Muskrats heap this vegetation up into a pile in a marshy habitat and then hollow out a cavity above the water level in the center of the mound. In winter the entrance is plugged with vegetation which is replaced every day. The chamber, where up to ten muskrats reside, remains about 36 degrees above outside temperatures, even during the coldest months.

Porcupines often den in hollow trees. If you want to locate one, search for a large pile of pellets or scat at the base of a tree and look up.

Beavers are well known for their architectural prowess, as illustrated by their dams and lodges. In addition to bank lodges, beavers build lodges in the water; they pile vegetation on the bottom of a pond and hollow out a central chamber in which they live year round. Unlike muskrats, beavers use woody plants—branches and trunks of saplings–as their building material, and apply a thick layer of mud to the outside of the lodge. Once this mud is frozen, the lodge is impenetrable, except from underwater entrances.

The lodge insulates the beavers extremely well from the outside temperatures, with the interior remaining about 34 degrees warmer than the outside air. Up to ten or so beavers can inhabit one lodge, each one acting as a little furnace.

When building a lodge, beavers construct a "chimney" or vent that runs up the center of the structure. Because they don't use mud in this location, it serves as a flue, ventilating the lodge. Snow is usually absent at the top of the vent/peak of the lodge if it is occupied in winter, due to the warm air of the chamber rising up through the central vent.

Given the lack of hands, tools, and ability to read "how-to" manuals, four-legged mammals do quite well for themselves when it comes to constructing winter shelters. In most cases, far better than this two-handed, semi-literate, tool-box-owning, two-legged mammal would.

Mary Holland is the author of "Naturally Curious: A Photographic Field Guide and Month-by-Month Journey Through the Fields, Woods, and Marshes of New England." She has a natural history blog which can be found at www.naturallycuriouswithmaryholland.wordpress.com.