Early mammals evolved in tropical habitats, where living was relatively easy, at least as far as the climate was concerned.  However, with the cooling and drying out of the world during the Tertiary and the Eocene (40-55 million years ago), and concurrent radiation of species, many mammals of these non-tropical areas had to become more specialized than their tropical relatives in their ability to survive under conditions of seasonal food shortage brought on by a winter season.  

Mammalian adaptations to the stresses of winter generally take on three different strategies; migration, hibernation, or continue in a greater or lesser level of winter activity, by developing mechanisms to minimize the impacts of both the stresses of the cold and lack of food sources.

Migration and hibernation are costly strategies. In the case of migration, a large amount of energy must be consumed to make the trip south and the returning spring trip back north. And, of course, migration to warmer latitudes is a feasible alternative only to those species capable of long distance travel. In the boreal regions of northern Canada, carribou are an example of a migratory mammal, known for traveling up to 600 miles between the summer range on the tundra and the wintering grounds in timbered areas.  Other ungulates, like elk, are known to migrate between high and low elevations. But, with these exceptions (and marine mammals), our only migratory mammals are three species of bats that inhabit our insect-laden deciduous forests. These bats; the red, hoary, and silver-haired bats, travel to southern regions of eastern U.S. and Mexico, and then, normally will hibernate. However, some of these species will stay in their northern (summer) ranges and will hibernate there in protective locations. To make the distinction between migrating and hibernating species more ambiguous, all species of bats have some individuals that will migrate, with individuals of many species migrating 100 miles (more or less) before entering hibernation. 

Hibernation, in the truest sense, is only practiced by woodchucks, bats, jumping mice and ground squirrels (there are no ground squirrels native to our eastern forests). This strategy is costly in the need to store an adequate amount of energy, in the form of brown fat that will fuel the body's system over the duration of the winter season. This strategy is only suitable in mid-latitudes, since more northern habitats preclude animals from safely sleeping through the winter without freezing or running out of stored energy. (Even here, the exception to the rule exists.  The Arctic ground squirrel, known locally as the sik-sik, is the most northerly hibernator.  It hibernates for eight months in the coldest hibernaculum known, maintaining temperatures of -12 F.  This creature doubles its weight on the polyunsaturated fat, linoleic acid, it gets from local nuts before waddling into its burrow for the winter.  In addition to being able to burn its brown fat, this ground squirrels can generate heat by burning its white fat and skeletal fat.  It also performs a  highly risky trick of supercooling its body fluids.  In such a way, the hibernator can allow its body temperatures to drop five to seven degrees below freezing while not having the fluids freeze into a solid.  Of course, if you break the skin and poke them with an icicle, they will freeze catastrophically and die.)

Woodchucks are the textbook hibernators, with the oldest (and fattest) entering the burrow first (late October). In autumn, after gaining 30% of it's summer weight (mostly in the form of a half-inch layer of brown fat stored over much of the body, especially the back and shoulders), the ground hog will line it's hibernaculum with grass and leaves and then plug the entrances (to maintain constant temperatures and to keep out curious visitors) before curling into a ball. During hibernation, heart rate drops from 100 to four beats per minute, respiration rate drops to one breath per three or four minutes, and the body temperature drops from 98 to about 40 degrees. Males usually exit a few days earlier than females in March (similar to the hibernating jumping mice). They will awaken occasionally throughout the winter (about every two weeks) to defecate. This may occur in special chambers below ground or, less frequently, on the surface. They will emerge having lost approximately one half their fall weight.

All bats of the eastern US are capable of hibernation in winter, and they will also reduce metabolism and enter torpor during cool days, as an energy-saving mechanism. Brown bats store energy as brown fat. About 75% of the stored fat is used during the wakeup periods, with the other half-gram serving to maintain the bats life functions through the entire winter of hibernation. Many studies of big brown bats in hibernation have been conducted. While normal awake body temperatures are 99 degrees, apparently the body temperatures match the environment down to a temperature of 30 degrees while breathing can drop from 200 times a minute to once every four to eight minutes. As all true hibernators, bats will occasionally awaken during winter to prevent muscle atrophy and defecate, and may even change caves.

Jumping mice are profound hibernators. Meadow jumping mice put on a substantial layer of fat (about 6 grams - increasing in total body weight from 15 to 21 grams) and wake every two weeks to stretch and defecate. Woodland jumping mice put on 7-8 grams (from 20 to 28 grams total weight). The woodland jumping mice will spend from October to April or May in deep torpor in underground moss/grass/leaf nests. They will put on one third their weight within two weeks in fall. The biggest cause of death in jumping mice is the inability to put on enough brown fat in the fall to carry them through to spring.

Hibernation is a strategy with great advantages, but also with severe costs and problems.  First, great volumes of weight must be put on in the fall (from a third to a half the animals' summer weight).  This requires tremendous expenditures of time foraging for carbohydrate-rich nuts and berries.  Much of the weight gain is in the form of brown fat, which can be oxidized to produce heat directly, without muscular contractions (shivering).  But the burning of brown fat requires the withdrawal of water from the bloodstream, so that a major problem of hibernation comes from problems associated with dehydration.  Coincidently, the burning of muscle adds water to the bloodstream, so that a combination of both occurs throughout the winter for hibernators.  The end result is the loss of both stored body fat and muscle mass, leaving the animal in a stressed condition come springtime.  

One question has always been why hibernators must wake up every few weeks throughout their dormancy.  It has been thought that the brain must be "exercised" to keep everything in working order.  It has been believed that something goes wrong at low body temperatures that take rewarming to fix.  Recent studies have found this to be only partially correct.  EKG's on hibernating mammals finds virtually a flat line; a condition in which a human would be considered brain dead.  Hibernators, in fact, become sleep deprived.  Thus, the animal must "wake up" in order to go sleep before reentering the true  physiological stage of hibernation.  

Whether other animals, like bears and chipmunks, hibernate or not depends on your source and definition. Animals (as well as plants and all living material) don't play by any definitive rules, and thus individual species (in fact, individuals within a species, as our bats have illustrated) have developed characteristics that are more of a "smearing" of possible strategies and physical adaptations, covering an endless number of variations on a common theme. There is a continuum of sorts between the true hibernation of  woodchucks, bats and jumping mice, in which all bodily functions are greatly slowed, the deep sleep of bears and chipmunks, and the occasional sleep of raccoons, opossums, and gray squirrels.

Hibernation is the extreme end of this continuum of dormancy strategies.  All of these strategies are geared to reduce metabolic rates.  Every 18 F drop in body temperature halves the rate at which energy is used simply to keep things running.  They can be lumped under the general term "torpor".  Torpor is a condition of deep sleep, with very slow breathing (one per minute) and heartbeat (four to eight per minute)  at body temperatures close to the ambient temperature.  The purpose is to conserve calories at times when they are hard to come by.  

There are several patterns of torpor  .Daily torpor is practiced by bats, which on cool days allows the animal to drop it's metabolism significantly, saving many calories while the bat rests.  Similarly, hummingbirds do the same thing on a nocturnal level, saving these small creatures from significant caloric loss at night.  Seasonal torpor, such as hibernation (from the Latin word for winter), or aestivation (from the Latin word for summer), enables animals to basically shut down their life cycle during times when the costs of maintaining an active life style offset the benefits.  Occasional torpor is a last-ditch response to food shortage or extreme temperatures.

Bears are said not to truly hibernate, because although their bodily processes are slowed, they are not suppressed to the extent found in the deep hibernators. Their metabolism drops by half, and their digestive system tightens into a knot, with the limited waste products reprocessed into the bloodstream in the form of proteins.  Black bears, however, if not true hibernators, are certainly close. Various terms - dormancy, ecological hibernation, and carnivoran lethargy, for example - have been used to indicate the black bear's various modifications to hibernation. They can exhibit continuous dormancy for up to about seven months, without eating, drinking, defecating, or urinating. (In the Shenandoah National Park, bears enter their dens around December 1, and emerge in mid-April.) Their temperature does not fall to the extent it does in true hibernators, nor is respiration as greatly retarded. Body temperature decreases from a summertime 99-101 to 88-95, and the heart rate drops from about 40 to about 8 to10 beats per minute. Bears feed heavily and become extremely fat, thus well equipped for winter. They may increase their weight by up to 100% (one bear went from 110 to 220 lbs). Average loss of mass in one study was 260 grams per day; over the winter, that amounted to a drop of 23.1% from peak weight. Most hibernators wake periodically, but bears do not wake during winter unless disturbed. If they are disturbed, they will rouse readily. The ability of bears to recycle urea and to desist urinating, defecating, eating, or drinking during the entire denning period is unique. It has been said that, while bears may not be true hibernators, they are digestive hibernators.

Chipmunks do not develop a layer of brown fat as does a true hibernator. Unlike true hibernators, who overwinter on the brown fat alone, chipmunks must wake occasionally to eat from their caches. They will go through periods of torpor of variable length and frequency during periods of stressful weather. During these periods of torpor, their body temperature drops, and their heart rate and respiration slows for several days. Chipmunks' respiration rate drops from 60 to less than 20 per minute and a drop in temperature from 100 degrees to 45 degrees. Chipmunks will come out in mild weather any time. In fact, the strategies  used by chipmunks can vary from year to year, as well as with elevation and latitude.  Within a given population, some chipmunks will hibernate while others will opt to stay active all winter, and, if the weather moderates, hibernators can de-hibernate, and become active for the rest of the winter season.  Some chipmunks have genetically totally lost the ability to hibernate, even within a population where most do hibernate.  They are also known to overwinter among rattlesnakes, benefiting from their warmth, while entering and leaving the rattlesnake hibernaculum as the snakes are dormant.

By far, most mammals "stick it out" in winter by various adaptive methods. Development of brown fat is not uncommon. Many mammals produce brown fat, full of mitochondria, the engines in cells that convert food into energy, whose only function is to generate heat.  Thicker winter coats, or pelage, provides further cold protection (the winter coat of the mink is highly prized and is much thicker and heavier than the summer coat), as does the winter white fur of weasels and snowshoe hare. White hairs, without the pigment melanin, have more air spaces within the hairs and thus has greater insulation Winter grouping allows for mutual warmth sharing, allowing a further northern range. A change in diet is also utilized to provide the necessary energy supplies.  Most carnivores assume a larger home range to assure an adequate food source. Some animals will cache food for winter food sources.  Some animals become more diurnal. And practically all mammals lay low and minimize energy loss during extremes of winter cold and food scarcity.

Below, is a general description of the various techniques exhibited for winter adaptations by orders of mammals.

An interesting secondary adaptation to winter stress developed by many mammals are the phenomena of delayed fertilization and delayed implantation. 

Delayed fertilization occurs among most of our native bats. Delayed fertilization is seemingly a highly advantageous adaptation in mammals with long periods of dormancy. In the case of bats, copulation, requiring considerable energy, occurs in the late summer and autumn, when males are in excellent condition and have abundant food, rather than in spring when the animals are in their poorest condition and when food (insects) may not yet be abundant. Ovulation and zygote formation occur almost immediately upon emergence from dormancy, rather than being delayed until after males attain breeding condition and copulation occurs. Perhaps the major advantage is that of hastening the time of parturition and allowing the longest possible time for development of young before the winter period of dormancy. 

Delayed implantation occurs in bear, marten, fisher, badger, river otter, mink, long-tailed weasel, certain seal and roe deer. The advantages are similar in nature to those found for delayed fertilization. In this case, fertilization occurs normally with early cell cleavage, but then, the embryonic development is arrested at the blastocyst stage until a more suitable time of development and ultimate birth can occur. In the case of the black bear, mating takes place in early summer, when the males are in prime condition and food supplies are not limited. Without delayed implantation, mating would have to occur in the fall, when bear must concentrate its efforts on building up a fat reserve to allow it to pass through the winter dormancy. The delayed implantation of five to six months allow for a late fall implantation and mid-winter birth, allowing the maximum length of summer growth period for the newborn cubs before the subsequent winter. The longest period of blastocyst dormancy is attained by the fisher and river otter, which is ten to eleven months. (White-tailed deer don't follow this trait since they are active all winter, commensurate with the availability of their food source.)


Opossum - Opossum put on brown fat; up to 30% of their body weight, and can lose up to 45% of their body weight over the winter seasonOpossum have been known to den communally.  Has even been known to share a winter den with rabbits, skunks, raccoons, and woodchucks.  One extensive burrow system in Michigan was simultaneously occupied by an opossum, a woodchuck, a raccoon, and a striped skunk.  They also tend to lose skin on their ears and noses from frost-burn.

Shrews and Moles - Shrews must be active; finding enough dormant insects to supply their needs. With such a high metabolism and high surface area ratio, they are unable to hibernate; they would burn up too much calories and lose too much heat through their skin.  To survive cold periods, they must simply turn up the metabolism and burn more energy; a costly requirement.  To do this, shrews and moles must maintain a very high metabolism to produce adequate heat (the short-tailed shrew has a heart rate of 760 beats a minute at rest).  Therefore, they must eat all the time.  The short-tailed shrew is well adapted for winter survival, including a thickening of its winter pelt, ability to cache food, and restriction to the subnivean environment (under the snow). Winter grouping by the social least shrew allows for mutual warmth-sharing, allowing a further northern range. All shrews produce a layer of brown fat, accumulated over the shoulder blades, which, in the generation of heat, keeps the adjacent internal organs at a minimally-acceptable temperature. Research has found that the heat producing breakdown of the brown fat is so effective, that shrews actually gain weight during winter.  It has been shown that their brown fat reaches its maximum heat-generating capabilities in mid-January, when the outside temperatures are coldest.  Water shrew can reduce their own metabolic demands, so as to be able to dive in cold mountain streams even in winter, and they will swim under the ice.  Moles will burrow deeper in winter.  The star-nosed mole stores brown fat in its tail.

Bats - All bats of the eastern US are capable of hibernation in winter, and they will also reduce metabolism and enter torpor during cool days, as an energy-saving mechanism. As all hibernating mammals, hibernation includes waking every two weeks or so to prevent muscle atrophy. Brown bats store energy as brown fat.  There is a great reduction in metabolism during hibernation, in which the rate of oxygen consumption is only about one-hundredth of the normal active rate.  About 75% of the stored fat is used during the wakeup periods, with the other half gram serving to maintain the bats life functions through the entire winter of hibernation. Red bats respond to subfreezing temperatures by raising the rate of respiration, assuming a spherical body shape, and stretching the furred tail membrane over the ventral body surface like a blanket.  This protects them from waking too frequently, and thus, wasting energy in the winter. 

Rabbits and Hares - Rabbits and hares will use burrows of woodchucks and skunks during periods of bad weather, never venturing far inside.  Not only a defensive aid, the hollow white hairs, without the pigment melanin, have more air spaces within the hairs and thus has greater insulation.  Snowshoe hares' white winter pelage has 27% better insulative qualities than the summer brown coat. 

Rodents - Ground squirrels, woodchucks and, to a lesser extent, chipmunks hibernate; with chipmunks waking periodically and eating stored food over the winter, while the ground squirrel and woodchuck use stored fat for maintenance energy during hibernation.  Gray squirrels are active year-round, foraging in mid-day in the colder months, and will den together when extremely cold. They cache acorns and maple seeds. Red squirrels cache cones and nuts in seep or wet areas where they keep for literally years. Fungi is allowed to dry hung in trees, thus available well into the winter. Red squirrels cache their food supplies in "middens" while gray and fox squirrels prefer to store their acorns individually. Southern flying squirrels enter a torpid state in winter (body temp drops only to 72 degrees), but northern flying squirrels do not. Southern caches acorns, northern doesn't; eats more fungi and lichen.  Southern communally nest (up to 50 in one nest), more so than the northern.

Jumping mice are profound hibernators. Meadow jumping mice wakes every two weeks and puts on a substantial layer of fat (about 6 grams - from 15 to 21). Woodland jumping mice put on 7-8 grams (from 20 to 28 grams total weight). 

Voles and mice build communal nests and tunnels under the insulating blanket of snow (subnivean tunnels).  Even in air temperatures as low as 10 degrees F, measurements over a two-week period in January showed that temperatures hovered around 32 degrees F in these surface tunnels.  In the nests, temperatures often reach 50 degrees.  White-footed mice conserve up to 75% of energy needs by 1) communal nesting, 2) nesting below ground, and 3) torpor. Only some go into torpor, most do not. Communal nesting begins in tree nests in fall, then goes below ground in colder months. White-footed and deer mice cache nuts and seeds. Southern red-backed voles cache wintergreen leaves and beechnuts. Bark and roots are prime winter food. Meadow voles eat bark, blanched grass stems, rootstocks and seeds. 

Muskrat lodge temperatures can be 36 degrees warmer than outside temps. 

Beaver maintain brown fat in its tail and cache food underwater for winter use.  Among the many  mammals with exposed feet (or the beaver's tail - or birds), an interlaced network of blood vessels acts as a heat exchanger to block excessive loss of heat to the environment.  In the exposed extremities, surface veins constrict, shunting blood toward the center.  There, the warmth of arterial blood is transferred to the cooler blood in adjacent veins.  This keeps the extremities cool but preserves the animal's core temperature.  (A beaver may lose 25% of its body heat through its tail in summer, but is loses only 2% in winter.)  

Porcupines have a dense coat that allows it sufficient protection from the cold. Males can roost in trees for days, eating bark. Females and juveniles sit it out in dens and nearby areas, changing dens every three weeks or so. Several can share a den, but usually of the same sex.   

Carnivores - Carnivores will use dens for winter lethargy.  A change in diet is also utilized to provide the necessary energy supplies. (Red fox consume primarily insects and fruits in summer and fall, and then rodents in the winter. The mink's diet in spring and summer includes muskrat, crayfish, frogs, fish, snakes, small mammals, and waterfowl. Winter mink diet focuses on muskrat, but small rodents make up a portion of their diet. Raccoons will feed on anything being omnivorous, but favor crayfish in the spring, and adding insects in the summer, but switch to primarily acorns and corn in the winter.) Bears feed heavily and become extremely fat, thus well equipped for winter. They may increase their weight by up to 100% (one bear went from 110 to 220 lbs.).  

Red wolves, gray wolves and red foxes become more diurnal in response to weather conditions and the nature of their winter prey (red foxes feed almost exclusively on voles). 

Raccoon are known to den together in winter communal "piles" of up to 23 individuals in a single den. They will den for a month or more in cold climates, living off up to 30% total body weight stored fat. Respiration is not greatly affected, and the raccoon will arouse quickly from dormancy. Raccoons will feed on anything, being omnivorous, but favor crayfish in the spring, and adding insects in the summer, but switch to primarily fat-rich acorns and corn in the winter. In winter, the pelage of weasels turn white in the northern region, not in the south. 

The long-tailed and short-tailed weasels coats are all white, except for a black tip on the tail, a defensive mechanism to throw off the attention of predators from the weasels body to its tail. The smallest least weasel is so small (7" including a 1" tail), that the black-tipped tail would not protect it, thus, it has no black on its tail. However, some scientists believe the winter white may not be so important as a camouflage, but for its insulative capacity.  In it's winter white pelage, each hair, without the color pigment melanin, has more air spaces within the hairs and thus has greater insulative abilities.  Weasels are more active in the summer than winter. They also will hunt their prey in winter in subnivean snow tunnels. Members of the weasel family also cache their prey. ( One winter mink cache was found to contain over a dozen muskrats, two mallard, and an American coot). River otter are active year round, but must travel more in winter to find open bodies of water where fish can be caught. They will also root out overwintering frogs and soft-shelled turtles from the mud. Foraging on land is also common in winter. 

Skunks don't hibernate, but will become inactive or even enter a winter sleep during bad weather. They will gather in communal dens in winter, but usually only females and young. They also add a layer of fat and drop body temp slightly. 

Bobcats (and mountain lions) are active year round, who will occasionally attack deer in deep winter snow, when smaller game is in short supply, and the deer would supply its needs for a longer period of time. Bobcats have much larger home ranges in winter in order to secure adequate food.  However, mountain lions have smaller winter ranges, due to the concentration of their food source (mainly deer) in yards.

Ungulates - White-tailed deer change from grazers to browsers in winter. Winter hair is hollow, thus providing more insulation. Many elk and some deer populations will migrate to lower elevation grounds and congregate in "yards" which can afford more protection from predators.