How do small animals survive Minnesota’s brutal winters?

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Every year, Minnesota’s harsh winters drive loons, monarch butterflies and other wildlife out of the state. The critters that stay put are the true survivalists.

Lynn Keillor of Minneapolis wonders how the smallest and most vulnerable animals like chickadees, mice and squirrels make it through the state’s brutal winter. He submitted his 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 tell its metabolism, that’s feathers. But seriously: How does a chickadee not freeze solid in the winter? How can rabbits survive under my deck? A nest of leaves can just a lot of insulating.”

The short answer is evolution and careful planning allows Minnesota’s smallest winter storage to survive our notorious winters. In some cases, complex physiological changes are activated; for others, the ease of finding a stable shelter can be the difference between life and death.

Those cottontails below Keillor’s deck, for example, go through a fall molt before putting on a warmer coat and building a layer of fat for energy when food sources are scarce.

And don’t think that these little animals follow the strategy of the bear to get through the dark months.

“Hibernation is a common word,” said Lori Naumann, a specialist in the Department of Natural Resources’ Nongame Wildlife Program. “Many people think that our mammals hibernate. [and metabolic] the rate will slow down.”

‘Industrious’ squirrels find insulation

Some small animals build nests to stay warm.

Gray squirrels seem ridiculously busy in the mornings as they flood feeders or stored sources in their territory to increase fat reserves. Later, they build high-wire works to reach their leafy nests, which are built to withstand the elements.

Combined, the (aggressive) daring animals are anything but winter-proof.

Naumann remembers cleaning out a shed in his house, where the squirrels organized different sections for places like black walnut storage, nesting and the remains of their food. Two 60-gallon trash bags were filled in the aftermath, he said.

Plus, their nests have more than meets the eye. Some include fur from themselves and other animals, along with hair, dust and human waste such as dryer lint. Naumann recalls that he even found some chair cushions missing from his patio set.

“They are industrious,” he added, “and use what they find.”

Bird biology combats the cold

Many species of birds including the loon, Minnesota’s state bird avoid the cold winter by flying south. Those that stick around have some significant biological advantages.

Even at half an ounce or more, black-capped chickadees are no lightweights for the season. Winter riders are larger than chickadees that live in warm weather areas. And that begins to explain their winter plan.

Chickadees grow fat while sweeping high-fat sunflower seeds at feeders and run for frozen insects, which provide insulation and fuel for tough conditions. What’s more, when the temperature drops, the birds go into a “state of torpor” as a protective measure, Naumann said. Many, too, will seek out tree holes to roost on cold nights.

Lowering their body temperature by as much as 15 degrees (to a minimum of 86 degrees Fahrenheit) helps them conserve energy for heating. Like other avian holdovers, small birds fluff their dense feathers. They also shiver in bursts to regulate their body temperature, even when it’s time to warm up.

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

They reduce heat loss in several ways. One, called countercurrent heat exchange, relies on the closely connected arteries and veins of the bird to moderate the temperature of the blood. The bodies of birds standing on ice work hard to maintain a core body temperature, but the action of warming it up overall is to crank it down.

Blood is supplied to the foot and as it returns to the core it “travels through veins grouped around arteries that send warm blood from the body to the foot,” according to Cornell’s ornithologists. “Heat is transferred from warm veins to cold veins.”

In addition, their legs and feet have less nerve and muscle tissue, which reduces 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 that their body temperature adjusts to match their environment.

Some winter turtles dive into the cold, watery depths of lakes, where their body temperature drops to about 39 degrees F.

Blood oxygen levels drop to near zero, but they breathe by drawing oxygen through their membranes around their mouths and their backs (called cloacal breathing). Their heartbeat slows down to a few per minute.

“Their entire metabolism is shut down, so they don’t need a lot of oxygen,” said John Moriarity, senior wildlife manager for the Three Rivers Parks District and author, with Carol Hall, of “Amphibians and Reptiles of Minnesota.”

To fully answer Keillor’s question, however, some of Minnesota’s smallest residents freeze.

Some gray tree frogs, wood frogs and spring watchers look for leaf litter or twigs under or between logs, where their hibernation (without a heartbeat or breathing) is spent it’s a bit frozen. Their bodies convert glucose into a substance called glycol which is a type of antifreeze for their cells, even as ice builds up between skin and muscle. Glycol protects cells from destruction when they are partially frozen, Moriarty said.

His advice: You might come across a frog that looks like it’s dead. Leave them alone.

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

Frogs, on the other hand, dig 2 to 3 feet into the ground to get below the frost line for sanctuary during the winter. Bull snakes follow a similar strategy. Garter snakes in some areas pass through ant mounds to burrow into the ground.

“I’m just amazed by so many things,” Moriarity said, about the adaptive powers of animals. “That they were able to do it in the environments that they did.”

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Read more Curious Minnesota stories:

Are many wild animals moving to Minnesota cities?

Does Minnesota have the coldest and longest winters of any of the US states?

When did the wild bison disappear from Minnesota?

Why do wild turkeys seem to thrive in the Twin Cities?

Why is Minnesota the only mainland state with large numbers of wolves?

Are grizzly bears native to Minnesota?

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