Sleeping winter away – secrets of hibernating wildlife


If everything out there died when winter came, well, you know. So wildlife have devised some surprising, even astonishing ways to save themselves through the harshness of winter weather. 

The great pretenders?
When you think of hibernating wildlife, do bears come first to mind? They’re well-known for hibernating all winter long. They crawl into dens and spend the winter there, mostly sleeping. Bur, do they meet the definition of hibernating? Scientists aren’t so sure they do. Bear don’t follow the flow of so-called true hibernators. Their metabolic rate and temperature are much less depressed than with other hibernating animals. They awaken easily and may occasionally leave their den. Pregnant females deliver and care for their cubs in winter dens.

Still, they go up to 100 or more days without food, slowly burning body fat for energy. They also go without water, and it’s a bit of a mystery how they stay sufficiently hydrated. Only four species actually spend winter in a den, the American Black Bear, Asiatic Black Bear, Brown Bear and female Polar Bear. Look inside a bear’s den

Hey! Where’s my parka?
Butterflies and moths don’t have insulating fur or feathers and being “cold-blooded,” they can’t regulate their body temperature. If the weather’s cold, they’re cold. If it freezes, they freeze. So where are these lovely insects we see fluttering about our garden in early spring coming from?

Edwards' Hairstreaks, Satyrium edwardsii

Edwards’ Hairstreaks (Satyrium edwardsii) overwinter as eggs (aecole2010 / Flickr; cc by 2.0)

Some are the new generations of butterflies that migrate south in the fall and move north in the spring. (The Monarch is a famous example.) But most of these very fragile insects stayed in our yards all winter, hibernating in one stage or another of their life cycle. Some species hibernate as adults in any spot that keeps them out of sight of predators and away from winter winds — places like tree cavities, beneath loose tree bark and in unheated buildings. Others hibernate as eggs, caterpillars or pupae.

Okay, but how do they do that without some sort of “winter coat?” The secret is in their body chemistry. Before winter arrives, they begin secreting a natural antifreeze into their body fluids to keep them from freezing. It’s a combination of chemicals that includes sorbitol and glycerol and works like antifreeze works in our cars.

Mourning cloaks, Nymphalis antiopa

Mourning cloaks, Nymphalis antiopa, overwinter as adults. (Fyn Kynd / Flickr; cc by-sa 2.0).

If they can prevent the formation of ice crystals — even if their body temperature drops below freezing — they can survive.  More about butterflies and moths

Pssst! Here’s the buzz!
Queen bumblebees rule over a busy nest of their children all summer long. But their offspring die in the winter, leaving them to endure the cold months alone. They leave their nests and bury themselves underground to survive off of their body fat.

Common Eastern Butterfly, Bombus impatiens.

Common Eastern Butterfly, Bombus impatiens. (Ryan Hodnett / Flickr; cc by-sa 2.0)

If they don’t put on enough fat (derived from nectar) to get them through the winter, they die. Like butterflies and moths (and other insects, as well), if their surrounding temperature drops to freezing, their body produces a natural antifreeze to prevent ice crystals from forming.

The little frog that can
You’ve heard of cryogenics, a process used by physicists to produce extremely low temperatures and sometimes suspend animation. We usually read about it concerning the preservation of human eggs, sperm and blood cells. Well, it so happens that nature does that naturally for some animals. 

Wood Frogs, Lithobates sylvatica.

Wood Frogs, Lithobates sylvatica, undergo cryogenic hibernation. (Judy Gallagher; cc by 2.0)

A good example is the North American Wood Frog (Lithobates sylvatica), the most widely distributed amphibian in North America. Only about 2 inches (5 cm) long, they’re found mainly in wooded areas, from the tree line of the Arctic Circle south through the eastern half of the U.S. to Georgia. Did you catch the part about the Arctic Circle? Yes, these little frogs survive winter up there! They’re the only frogs that spend winter with their body flooded with natural cryo-protectants.

Lying under leaf litter and other organic detritus on the forest floor, their body prepares for freezing weather in two critical ways: Urea (a highly-soluble, nearly non-toxic component of urine) collects in their body. And, their liver glycogen converts to glucose — at 50 times more concentrated than those in a human diabetic — as a response to ice forming in their body. (Commercial antifreeze is made of a sugar alcohol similar to glucose.) Together, the urea and glucose act as “cryoprotectants” to limit the amount of ice that forms and to reduce shrinkage of cells.

Amazingly, so long as no more than 65 percent of their body water freezes, they can survive many freeze/thaw cycles through the winter. Curiously, when the weather warms, instead of thawing from the skin inward, their heart and lungs thaw first.  More about frogs     Fascinating video about frozen wood frogs

Whoa there! Watch where you step!
Birds are noted for either migrating in winter or staying in their year-round habitat to meet its challenges head-on. This one, though, is decidedly different. It just might be on the ground at your feet!

Common Poorwill, Chordeiles minor.

Common Poorwill, Chordeiles minor, is the only bird known to hibernate. (Gavin Keefe Schaefer / Wiki; cc by 2.0)

Common Poorwills (Chordeiles minor) are the only birds known to hibernate. They stay put in winter and enter a state called “torpor,” not quite as deep as hibernation, but just as effective. Their metabolic rate slows down and body temperature drops. This allows them to endure long periods when their primary food, flying insects, isn’t available. Studies show they remain completely inactive for as much as 90 percent of the winter.

They go into torpor in warm weather, too, if their food supply is insufficient or temperatures drop. They live year-round in open, shrubby or grassy areas of the Southwest and spread out across the western half of the U.S. during their breeding season. Hopi Indians called them Hölchoko,”the sleeping one.” More about birds

Eastern Red Bat (Lasiurus borealis).

Eastern Red Bat (Lasiurus borealis) wrapped in his furry tail membrane. (Chris Harshaw / Wiki; cc by-sa 3.0)

I carry my blankie with me
The Eastern Red Bat (Lasiurus borealis) you see below is hard to distinguish because it’s partially wrapped in its long, furry tail membrane. Unlike many other bat species, this one doesn’t roost or hibernate in caves or mines. Instead, they spend their summer in trees and also hibernate there in winter, hanging in tree hollows or even on exposed tree trunks (there are some reports of finding them under leaf litter on the ground).


Roosting Eastern Red Bat (Lasiurus borealis) partially wrapped in his furry tail membrane. (Chris Henshaw / Wiki)

Their fur is long, silky and warm, and they can use their tail membrane like a blanket, wrapping themselves almost wholly within it. If temperatures drop too low, they’re able to raise their body temperature enough to keep from freezing to death. If despite these advantages they should get cold, they can survive body temperatures as low as 23 °F (-5 °C).

Red Bats are solitary except when joining together during migration and in the spring when mothers have pups. The bats live across the U.S. in the summer, and those in the north migrate southward to warmer areas in the fall.  More about bats

No admittance!
Many land snails carry their hibernaculum around with them — it’s their shell! They burrow into soil or under leaf litter for a safe place to hibernate and seal up the opening of their shell with layers of dried mucus called an epiphragm.

Epiphragm sealing the opening of a hibernating Roman Snail, Helix pomatia.

Epiphragm sealing the opening of a hibernating Roman Snail (Helix pomatia). (Hannes Grobe / Wiki; cc by-sa 2.5)

The epiphragm may protect them sometimes from predators, but more importantly, it seals in moisture. At any season of the year, snails must stay moist to survive.

Slugs, which are shell-less snails, often die in winter, but some species survive by hibernating deep underground. Of course, like snails with shells they must stay moist, a much harder job for them, so choosing a moist location is imperative. More about snails

*Bumblebees hibernate, honeybees do not. Honeybees stay active in their hive and live off their stored supply of honey.
Top photo: Sleeping bear. (20072 / Pixabay; PD)