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Why brave the cold and dark when you can sleep through the worst of winter? That's exactly what some members of the animal kingdom do. Called hibernation, this blissful state of inactivity functions as a way of escaping sub-zero temperatures, conserving energy when food stocks are low, and even gestating young while pregnant.

Browse the gallery and find out which animals like to doze when the temperature drops.

▲You often hear these prickly mammals before you see them, snuffling unseen in the undergrowth. By fall they are very busy, constructing nests out of grass, dry leaves, and other vegetation in readiness for the long winter ahead.

▲During hibernation, the body temperature of a hedgehog can decrease to about 2°C (36 °F). They tend to wake up regularly during hibernation, but rarely leave the security of their nest.

▲Only four species of bear are hibernators, and the eponymous grizzly is one of them.

▲That said, brown bears can be woken easily from their winter den, which is often a cave, a hollow log, or cavernous tree root large enough to accommodate their huge bulk.

▲A nocturnal species, the common poorwill is the only bird known to go into a torpor—a state similar to hibernation—for an extended period.

▲The bird spends much of the winter inactive, effectively hibernating by concealing itself in piles of rock or undergrowth for weeks and even months at a time.

▲Not only do these tiny long-legged customers hibernate, they actually freeze while doing so. But don't worry, these little guys have an astonishing freeze tolerance.

▲After finding a place to settle down, the frog starts to nod off, its blood and other tissues actually freezing as it begins to hibernate. Their hearts stop beating as ice crystals form in the blood. Eventually as the weather warms up, this amazing creature starts breathing again as its tiny hearty starts pounding. A wonder of nature!

▲Among the smallest and one of the most beautiful of the world's bird species, hummingbirds have the highest metabolic rate of any animal.

▲When it's cold or when food is scarce, the tiny bird goes into torpor in order to preserve energy. This slows their metabolic rate to 1/15th of its normal rate, and during this time they often hang upside-down as if dead.

▲The largest bear species lives in some of the most inhospitable terrain on Earth. It's generally a solitary creature, and classified as vulnerable because of habitat loss caused by climate change.

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In fall, pregnant female polar bears will dig a maternity den where she'll enter a dormant state similar to hibernation, and where the bear's heart rate slows from 46 to 27 beats per minute. Cubs are born between November and February.

▲Not all snails hibernate, but this species tends to avoid the cold winter months (typically October through April in the Northern Hemisphere) by finding places under rocks, in leaves, or in the snug corners of buildings.

▲How do they sleep? Well, they retreat into their shells and close up the opening with a thin layer of mucus made of their own slime. This not only traps heat but protects them from the elements, and prevents them from drying out. Clever, eh?

▲Turtles that live in warmer, sunnier climates do not hibernate. But since they can't produce their own body heat, it's a must for turtles in colder climates to hibernate when temperatures start dipping.

▲Box turtles are native to North America and occupy a wide variety of habitats. Prior to hibernation however, they tend to move further into the woods, where they dig a chamber for overwintering. During hibernation their heart rate drops to just one beat every 5-10 minutes and they don’t have to breathe in air at all (although they do take in oxygen through their skin).

▲Known as ladybirds in the United Kingdom, these distinctive members of the beetle family are as familiar in summer as sunshine itself.

▲When it stars getting cold its common to see these bugs gather, or aggregate, in their hundreds or even thousands on the south sides of trees or houses during the winter months. This mass bed-in is the easiest way to stay warm.

▲The chipmunk—a smaller and less bushy-tailed version of the ground squirrel—is native to North America. It's a popular pet choice in some countries.

▲The diminutive rodent with its striped coat is not a true hibernator, and only the eastern chipmunk beds down in winter to enter a state of torpor.

▲Only pregnant females and mothers with yearling cubs will enter hibernation. Considered true hibernators, female American black bears enter their dens in October and November, having bulked up on extra body fat to survive the long winter ahead.

▲Hibernation typically lasts lasts 3–8 months, depending on regional climate. Tree cavities, caves, and under logs or rocks are preferred den sites.

▲Do bees hibernate? Well it really depends on the species. Honey bees for example are generally perceived as workaholics and tend to see out winter on the wing.

▲The bumblebee does hibernate, but only if its a queen. By fall, all regular bumblebees in a colony will have died off, leaving the queen safe and cozy in a hole in the ground ready to begin a new colony in the spring.

▲Endemic to Madagascar, the fat-tailed dwarf lemur is unique in that it is the only tropical mammal and only primate that hibernates, and this despite the fact that Madagascar experiences relatively warm winters. (Photo: Flickr/CC BY 2.0)

▲The species can hibernate for up to seven months, during which it can't control its body temperature. This means that if the tree hole it uses to nestle down in is not adequately insulated, the mammal's body temperature will fluctuate according to the outside temperature.

▲The life a moth is pretty short, and that means they don't necessarily live through the winter months. In fact, few species actually fly and reproduce through winter.

▲However, moths that do brave the cold will enter a period of inactivity. (Photo: Wikimedia /CC BY-SA 3.0)

▲This handsome lizard is venomous (one of only two known species of venomous lizards in the world), but it's hardly a monster!

▲Despite being a native to the southwestern United States and northwestern Mexican state of Sonora, the reptile is not a fan of mid-summer heat and will seek shelter underground. But it's not too taken by the cold either, and by November it's permanently under the surface, hibernating until February.

▲Also known as a woodchuck, groundhogs are one of the few species that enter into true hibernation. Traditionally relied upon to predict the weather, these large sociable rodents build separate winter burrows as the temperature drops and snow begins to fall.

▲The animals hibernate from October to March or April, during which their breathing rate drops to one breath every six minutes. This produces a heart rate of around 4–10 beats per minute.

▲Like hummingbirds, bats preserve heat and energy by slowing down their metabolic rate to enter a state of torpor. Not all bat species hibernate, but those who do can spend up to 60 days just hanging around.

▲Most hibernating bats however do so for anywhere between a couple of hours and two weeks, waking up only to drink. Interestingly, they use the heat stored in their bodies to help kick-start themselves back into flight mode.

▲Think skunk and stench immediately comes to mind. Known for their ability to spray a liquid with a strong, unpleasant smell, these animals are not true hibernators in winter, but do den up for extended periods of time.

▲Though dormant, they'll occasionally wake up to forage depending on their needs. As many as a dozen females may den up together during winter while males often see out the cold alone.

▲In regions where winters are colder than snakes can tolerate while remaining active, local species will brumate (when physical activity is temporarily suspended).

▲Brumating snakes remain awake but appear inanimate. Dens can contain just one snake, or accommodate several hundred. Watch where you tread!

▲With their beady eyes and cute noses, these adorable little creatures tend to snuggle down in winter and do as little as possible, surviving on their stash of food collected over the summer months.

▲Nests can be found anywhere that's warm, which includes your own home. Deer mice don't go into complete hibernation, but you'll not see much of them before spring.

▲Known also as the "moon bear" or "white-chested bear" for its characteristic white V-shape on its chest, these bears are classified as vulnerable due to habitat loss and, sadly, hunting for its body parts.

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Only pregnant females hibernate, entering their dens mid-October and only emerging the following March, offspring in tow.