Cottontail rabbits have coats capable of keeping them warm through a harsh Upper Peninsula winter, but they also need enough food to sufficiently fight cold weather. (Betsy Bloom/Daily News photo)

This past week’s weather definitely tested much of the wildlife in the region that isn’t tucked away in burrow or other shelter sleeping through these arctic conditions.

But most of the species that regularly spend the winter in the Upper Peninsula are fully capable of enduring even such extreme cold if it doesn’t last too long.

The secret to keeping warm in subzero temperatures isn’t just having fur or feathers but having it dense or interlocking enough to trap a layer of air close to the skin, so body heat doesn’t escape.

The same fluffy feathers that make down quilts such a great choice for sleeping snug and warm is why birds can withstand cold that might be lethal to us.

It’s why deer will have snow build up on their backs rather than melt, like it does on the roof of a well-insulated home. Their more northern relatives, reindeer and caribou, take this to the next level by having a double-layered coat with hollow guard hairs to further harness air as insulation.

It’s also how animals such as sea otters manage to survive in the northern Pacific Ocean without having blubber like the seals and whales — they instead have more hairs per square inch than any other mammal on earth. Add natural oil to make the coat waterproof and the sea otter is well-suited to life in the cold seas.

In the Upper Peninsula, river otters and mink, beaver and muskrat have much the same setup that allows them to take to the water even in midwinter — and makes their pelts desirable to hunters and trappers.

Fox, coyotes and bobcat grow much more luxurious coats, along with having long tufts between toe pads to protect the feet. Canines, too, have a higher fat content in the pads that doesn’t freeze as quickly as other tissue, according to a Japanese study published in 2011, and have a circulation system that makes sure blood from the extremities is warmed before going back to the core. The study states Antarctica penguins have the same ability and speculated it points to the domestic dog likely originating from cold-climate wolves.

In other words, much of what we see in this region has long dealt with harsh winters and are more than up to the task.

But such conditions can prove a challenge to species for which the U.P. is close to as far north as they get — such as cottontail rabbits and, surprisingly, white-tailed deer. Both have increased their range north along with human habitation but originally were more scarce in these climes.

Both do have coats capable of keeping them warm but likely will need extra calories to offset the energy expended fighting the cold, at a time when food supplies generally are low and more difficult to access. Some already taxed by winter — the young, the old or the unhealthy — may succumb to starvation during such times.

Opossums are another fairly recent arrival to the north and often will bear the signs of just how ill-suited it is to such winters by having ears and tail cropped by frostbite.

For some species, the cold snap will prove to be too much without help. A woman in southern Wisconsin has managed to have a male Baltimore oriole hang on by setting up a box with heating pad and making sure the grape jelly stays thawed.

The polar vortex was expected to lift overnight through today, with Sunday and Monday potentially reaching the upper 30s to perhaps 40.

But none of the U.P.’s animals are completely out of the woods in terms of weather. This warming trend, while a respite from the deep freeze, sets up a scenario that could be equally harsh for the animals.

If the top of the snowpack is melted or rain falls and then freezing temperatures return — pretty much a certainty in early February — it could leave a thick crust on the snow that will be treacherous for deer to move through and difficult for birds to punch through, be it an owl trying to hunt rodents or a ruffed grouse looking to plunge into a snowbank for shelter.

Every winter will thin a certain part of the wildlife population; this is normal. Let’s hope this season doesn’t take too much of a toll.

Betsy Bloom can be reached at 906-774-2772, ext. 40, or bbloom@ironmountaindailynews.com.