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Every year, Minnesota's harsh winters chase loons, monarch butterflies and other wildlife from the state. Critters that stay put are the true survivalists.

Lynn Keillor of Minneapolis wondered how the tiniest and seemingly most vulnerable animals — like chickadees, mice and squirrels — make it through the state's brutal winters. She submitted her question to Curious Minnesota, the Star Tribune's reader-generated reporting project.

"A bear in a den is child's play compared to what other small animals can survive," Keillor said. "They can say metabolism this, feathers that. But seriously: How does a chickadee not freeze solid in the winter? How do the bunnies survive under my deck? A nest of leaves can only do so much insulating."

The abbreviated answer is evolution and careful planning allow Minnesota's smallest winter holdovers to survive our notorious winters. In some cases, complex physiological changes are set in motion; in others, the simplicity of finding solid shelter could be the difference between life and death.

Those cottontails under Keillor's deck, for example, go through an autumn molt before developing a warmer coat and build up a layer of fat for energy when food sources are scarce.

And don't assume these small animals follow the bear's strategy for getting through the dark months.

"Hibernation is such a common word," said Lori Naumann, a specialist in the Nongame Wildlife Program at the Department of Natural Resources. "A lot of people think that our mammals hibernate. But there are actually very few mammals in Minnesota that go into a true hibernation state where their heart [and metabolic] rate slows down."

'Industrious' squirrels find insulation

Some small animals build elaborate nests to stay warm.

Gray squirrels seem ridiculously busy for parts of the morning as they gorge at feeders or stored sources in their territory to build up fat reserves. Later, they perform high-wire acts to reach their leafy nests, constructed to withstand the elements.

Combined, the (aggressively) resourceful critters are all but winter-proof.

Naumann recalled cleaning out a shed overhang at her home, where squirrels had organized different sections for areas like black walnut storage, nesting and the remains of their eating. Two 60-gallon garbage bags were filled with the aftermath, she said.

What's more, their nests contain more than meets the eye. Some include fur from themselves and other animals, along with feathers, dust and human waste like dryer lint. Naumann recalled even finding some seat cushion stuffing that disappeared from her patio set.

"They're industrious," she added, "and use what they can find."

Bird biology fights the chill

A number of bird species — including the loon, Minnesota's state bird — avoid the frigid winters by flying south. Those that stick around have some key biological advantages.

Even at a half-ounce or so, black-capped chickadees aren't weather lightweights. The ones that ride out winter are bigger than chickadees that live in warm weather areas. And that just begins to explain their winter plan.

The chickadees fatten up while sweeping up high-fat sunflower seeds at feeders and darting about for frozen insects, providing insulation and fuel for harsh conditions. What's more, when temperatures plummet, the birds go into a "state of torpor" as a protective measure, Naumann said. Many, too, will seek tree holes to roost on chilly nights.

Lowering their body temperature as much as 15 degrees (to a minimum of 86 degrees Fahrenheit) helps them conserve energy for heating. Like other avian holdovers, the diminutive birds fluff their dense plumage. They also shiver in bursts to thermoregulate their body temperature, even when it becomes time to warm up.

Humans, thankfully, have barriers to protect their skin from subfreezing temperatures. But waterfowl like some geese, ducks and swans are directly exposed to ice and near-freezing water.

They minimize heat loss several ways. One, called countercurrent heat exchange, relies on the bird's closely connected arteries and veins to moderate blood temperature. The bodies of birds standing on ice work hard to maintain core body temperatures, but the action to keep them warmer overall is cranking below.

Blood is supplied to the foot and as it returns to the core it "travels through veins grouped around arteries that are sending warm blood from the body to the foot," according to Cornell ornithologists. "Heat is transferred from the warm arteries to the cool veins."

Plus, their legs and feet have little nerve and muscle tissue, reducing the risk of frostbite, Naumann added.

Many waterfowl also stand on one leg, or even sit, to conserve heat.

Reptiles and amphibians have a cold-blooded strategy

Amphibians and reptiles are called ectotherms, meaning their body temperature adjusts to match their environment.

Some turtles in the winter dive to the cold, watery depths of lakes, where their body temperatures drop to about 39 degrees F.

Their blood-oxygen level will drop to near zero, but they breathe by drawing oxygen through their membranes around their mouths and their hind quarters (called cloacal breathing). Their heartbeats slow to a few per minute.

"Their whole metabolism is shut down, so they don't need a lot of oxygen," said John Moriarity, senior wildlife manager at Three Rivers Parks District and author, with Carol Hall, of "Amphibians and Reptiles in Minnesota."

To fully answer Keillor's question, though, some of the smallest Minnesota residents do freeze.

Some gray tree frogs, wood frogs and spring peepers will seek out leaf litter or wedge under or between logs, where their dormancy (no heartbeat or breathing) is spent partly frozen. Their bodies convert glucose into something called glycol — a form of antifreeze for their cells, even while ice builds between skin and muscles. The glycol protects the cells from rupturing when they partly freeze, Moriarty said.

His advice: You might come upon a frog that appears dead. Let it be.

"It is an interesting strategy that allows [these frog species] to go further north," he added. "The wood frog is the only amphibian found in the Arctic Circle."

Toads, meanwhile, will dig 2 to 3 feet into the soil to get below the frost line for sanctuary during winter. Bull snakes follow a similar strategy. Garter snakes in some places will tunnel through ant mounds to burrow into the earth.

"I just marvel at a lot of things," said Moriarity, of animals' adaptive powers. "That they manage to make it in the environments that they do."

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