How do critters survive Minnesota’s harsh winters?

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

Lynn Keillor of Minneapolis wants to know how the smallest and most vulnerable animals, including chickadees, mice and squirrels, survive the state’s harsh winters. She submitted questions to Curious Minnesota, the Star Tribune’s reader-generated reporting program.

“Compared to other small animals that can survive, bears are child’s play in their dens,” Keller said. “They can talk about metabolism this and feathers that. But seriously: How do chickadees not freeze in the winter? Solid? How do rabbits survive under my deck? A nest of leaves can only provide so much insulation.”

The short answer is evolution and careful planning to get Minnesota’s smallest winter remnants through our infamous winters. In some cases, complex physiological changes are induced. For others, the simplicity of finding sturdy shelter can be the difference between life and death.

For example, cottontail rabbits under Keller’s deck undergo a fall molt, then grow warmer fur and build up a layer of fat for energy when food sources are scarce.

Don’t think these critters will follow bear tactics to survive the dark months.

“Hibernation is a very common word,” said Lori Nauman, nongame wildlife program specialist for the Department of Natural Resources. “A lot of people think our mammals hibernate. But in fact, very few mammals in Minnesota go into true hibernation because their hearts stop working. [and metabolic] The rate slows down. “

“Diligent” squirrel finds insulator

Some small animals build elaborate nests to stay warm.

Gray squirrels appear to be extremely busy during part of the morning as they build up fat reserves by gobbling up food stored at feeders or territories. They then perform a high-wire act to reach their lush lairs, which are built to withstand the elements.

Taken together, these (aggressively) resourceful creatures are nearly winter-proof.

Norman recalls clearing out an overhang shed at her home where the squirrels had organized different areas, such as black walnut storage, nesting areas and eating wrecks. Two 60-gallon trash bags were filled with debris, she said.

What’s more, their lairs hold more than meets the eye. Some include their own and other animal fur, as well as feathers, dust and human waste such as dryer lint. Norman recalls she even discovered some seat cushion filling missing from her patio.

“They were diligent,” she added, “and took advantage of what they could find.”

Bird Biology Fights the Cold

Many birds, including Minnesota’s state bird, the loon, fly south to escape the cold winter weather. Those who persist have some key biological advantages.

Even at about half an ounce, black-capped chickadees are no lightweights. Overwintering chickadees are larger than those that live in warmer areas. Only then began to explain their winter plans.

Chickadees fatten up while clearing feeders of high-fat sunflower seeds while scurrying for frozen insects that provide insulation and fuel for the harsh conditions. What’s more, Norman said, when temperatures plummet, birds enter a “torpor” state as a protective measure. Many people also seek shelter in tree holes on cold nights.

Lowering body temperature by 15 degrees (minimum 86 degrees Fahrenheit) helps them conserve heating energy. Like other bird remnants, these small birds are fluffed with dense feathers. Even when it’s time to warm up, they will shiver suddenly to regulate their body temperature.

Thankfully, humans have a barrier to protect their skin from the effects of subzero temperatures. But waterfowl like some geese, ducks and swans are directly exposed to ice and near-freezing water.

They minimize heat loss in a variety of ways. One, called countercurrent heat exchange, relies on the birds’ closely connected arteries and veins to regulate blood temperature. The bodies of birds standing on ice work hard to maintain core body temperature, but the action that keeps them overall warm occurs underneath.

Blood is supplied to the feet, and when it returns to the core, it “travels through the veins surrounding the arteries, carrying warm blood from the body to the feet,” according to the Cornell University ornithologist. “Heat is transferred from warm arteries to cool veins.”

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

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

Reptiles and Amphibians Have Cold-Blooded Strategies

Amphibians and reptiles are known as ectotherms, which means their body temperatures adjust to their environment.

In winter, some turtles dive deep into cold, watery lakes, where their body temperatures drop to about 39 degrees Fahrenheit.

Their blood oxygen levels will drop to near zero, but they breathe by taking in oxygen through membranes around their mouths and hindquarters (called cloacal respiration). Their heartbeat slows to a few beats per minute.

“They are Your entire metabolism shuts down, so you don’t need a lot of oxygen.”

To fully answer Keller’s question, though, some of Minnesota’s smallest residents did freeze.

Some gray tree frogs, wood frogs, and spring peepers search for fallen leaves or wedges under or between logs, spending their dormancy (without a heartbeat or breathing) partially frozen. Even if ice forms between their skin and muscles, their bodies convert glucose into a substance called ethylene glycol, which is the cell’s antifreeze. Ethylene glycol protects cells from rupture when partially frozen, Moriarty said.

His advice: You might come across a frog that looks dead. Ignore them.

“This is an interesting strategy that allows [these frog species] Go north,” he added. “The wood frog is the only amphibian found within the Arctic Circle.”

Meanwhile, toads will dig 2 to 3 feet into the soil to get below the frost line for winter shelter. Bullsnakes follow a similar strategy. In some places, garter snakes will burrow through anthills and into the ground.

“I’m just surprised by a lot of things,” Moriarity said of the animals’ adaptability. “They manage to succeed in the circumstances they find themselves in.”

If you’d like to submit a curious question about Minnesota, please fill out the form below:

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