For some species, hibernation makes winter easier

Northwoods Notebook

Small mammals that remain active through the winter months have to manage to stay hidden as much as possible to avoid attracting predators — like this vole, difficult to see as it forages among the shrubs at Six Mile Lake in north Dickinson County. (Betsy Bloom/Daily News photo)

Watching a small vole scurry around the collapsed remnants of last summer’s daylilies before popping into a tunnel under the snow raised a question: Why do some animals remain active in winter while so many others cope with this challenging time by sleeping in — for months?

It would seem to be much easier — and safer — to be tucked away somewhere, slumbering, rather than having to venture out in the cold for what scant food might be available, risking being picked off by a predator in the process.

Indeed, a fair number of animals that live in the north will opt for extended sleeping periods in wintertime. Raccoons, skunks and opossums all may go for days or weeks curled up, conserving energy and utilizing fat stores until it warms up enough to be worth foraging.

Even the tree squirrels will stay in bed rather than burn calories if conditions are poor.

While hibernation does have certain advantages in making winter easier, as with everything in nature, there are “evolutionary trade-offs,” said James Harding, an instructor and herpetology specialist with the Department of Integrative Biology at Michigan State University in East Lansing.

Red squirrels don’t hibernate like their chipmunk relatives, though they may spend more time sleeping if weather doesn’t allow for good foraging, such as freezing rain. They also cache extra food and have learned to take advantage of bird feeders to supplement their winter diets, rather than depend on the cone crop for seeds.

Take the woodland jumping mouse, a species that can be found in the Upper Peninsula by those with sharp eyes. Unlike the voles and deer mice, the jumping mouse does hibernate, avoiding the scramble for food. This is thought to be one reason why jumping mice are relatively longer-lived than other small mammals.

But that also can take the jumping mouse out of commission from late autumn through early spring, perhaps into May. It won’t start breeding and reproduction until almost summer. Deer mice, meanwhile, can begin cranking out the next generations in March and April.

That’s one reason why there are far fewer jumping mice than deer mice or voles, Harding said, adding, “If I’m out in the woods and I see a jumping mouse, I consider it a good day.”

A mouse or chipmunk in a hibernating state — low respirations, few heartbeats, can be handled without rousing — also is an easy kill should a weasel or other predator discover it, Harding added.

That’s hardly a concern, however, for the black bear, which in recent years has been recognized as a true hibernator, Harding said, rather than just sleeping a lot like raccoons. As with the woodland jumping mouse, a hibernating bear is believed to have a longer lifespan, on average, than the ones that dwell in the southern United States that remain active year-round.

In the north, some species have few other options but to hibernate. Woodchucks, for example, despite the name actually can’t switch from a diet of green vegetation to woody growth, as rabbits and deer do in winter. So they bulk up in summer and fall, packing on “brown fat” around back and shoulders, and spend the winter months underground — which is why it’s a rude move to drag poor Punxsutawney Phil out of a deep sleep every Feb. 2 to play groggy weather forecaster.

Some species take hibernation to an extreme level. Being cold-blooded, the reptiles and amphibians dwelling this far north can’t remain outdoors when winter set in, as it would quickly render them too chilled to move — and if they freeze enough to form ice crystals, they die, Harding said.

Snakes will seek out places to den — deep rock crevices, abandoned animal burrows, perhaps even a basement crawlspace in a home — where they will be cool enough to use little energy while dormant yet not freeze.

Turtles, more amazingly, take refuge from the elements underwater, though it can mean going without oxygen for months, according to research done by Donald Jackson of Brown University and Gordon Ultsch of the University of Florida.

“Indeed, as much as half of their (turtles’) lifetimes may be spent holding their breath during under-ice hibernation, accompanied by low levels of activity and greatly reduced metabolic rates,” their 2010 article published in the Journal of Experimental Zoology stated.

Baby painted turtles that hatch late in the season, too, will remain underground huddled with nestmates until the following spring, enduring “super cool” temperatures until it warms enough to emerge. This is why people living near water sometimes can find turtles no larger than a half-dollar coin in May, weeks before nesting for the year begins, Harding said.

And frogs … well, frogs are in a league of their own on hibernation. Wood frogs and spring peepers will hibernate “shallowly,” potentially putting them at risk for freezing in the winter if they didn’t produce a form of natural antifreeze as protection, Harding said. Staying close to the surface allows the spring peepers to emerge and breed early, sometimes setting up a chorus of calls even before ice has completely cleared from area lakes.

The wildlife that remains awake and active during the long U.P. winter usually can’t power down enough to go the hibernation route, such as the black-capped chickadee and shrews, or are simply ill-suited to that lifestyle. No white-tailed deer could find a secure place to sleep for months, Harding noted, though they, too, will have a lowered metabolism in winter and spend more time bedded down when possible to adjust to food sources that are tougher to digest and have less calorie content.


More reports are coming in from other Christmas Bird Counts in the Upper Peninsula and northern Wisconsin, and they match what’s been seen locally — which is, not much. This winter may well be one of the dullest this decade for birding, with many of the usual species at feeders or in the field, such as purple finches and redpolls, absent or with only a few individuals.

“It’s insanely quiet in Wisconsin’s north woods this year,” wrote Ryan Brady, the conservation biologist with the Wisconsin Department of Natural Resources in Ashland, Wis., after his CBC patrol in the Clam Lake, Wis., area turned up only nine species despite five hours walking plus three hours driving.

In addition to the predicted winter finch influx being a bust, snowy owl numbers are far reduced from last winter. It might be the region will have to wait until spring to see much change in the avian cast of characters.

Betsy Bloom can be reached at 906-774-2772, ext. 40, or