A friend of mine once asked me, “Do muskrats hibernate?” The simple answer is that they don’t hibernate, although I certainly understand why the question was asked - we just don’t see muskrats very often during the winter months.
And even though muskrats are indeed rodents, some members of the rodent family hibernate while others, like muskrats, beavers, tree squirrels, and mice and voles, do not hibernate. Why some creatures evolved to hibernate while others didn’t is a mystery among mysteries.
There are many species of Minnesota wildlife that don’t migrate to warmer climates like waterfowl and most songbirds and bats do. Yet some species spend their entire lives in the Northland, but can only do so if they hibernate or adapt in some other way in order to survive cold nights and snow and ice.
Contrary to what is widely believed, black bears are not true hibernators. Mammals such as ground squirrels, woodchucks, and chipmunks are animals that enter into true states of hibernation. The heart rates, body temperatures, and metabolisms of these groups of rodents decrease to such extremely low levels that life itself is sometimes barely detectable.
Black bears, on the other hand, sleep in a less vegetative state - their vital signs decreasing much less drastically. Periodic arousal from their wintertime slumber, however, is actually quite common. In fact, female black bears give birth in January and February while “hibernating.”
Moreover, the newborn cubs remain active in the den, do not hibernate at all, and nurse and play while their mother slumbers.
During the months of rest, generally from late October to April, yearling and adult black bears survive on fat reserves and do not urinate or defecate. Because bears don’t eat or drink during their slumber, little nitrogenous waste accumulates. Those fat reserves, which can amount to several inches thick inside bears’ bodies, make up a substantial portion of their overall weight.
Unlike species of tree squirrels like fox, gray, and red squirrels, chipmunks, though certainly adept at climbing trees, are considered a species of ground squirrel. As such, chipmunks are inactive during the wintertime - at least above ground - and mostly spend the cold months in a deep, underground burrow fast sleep.
Chipmunks, like the other species of grounds squirrels, are true hibernators. But unlike black bears, chipmunks do not totally rely on fat reserves to carry them through the winter. This hibernating rodent stores lots of nuts, seeds and other foods below ground and will awake from time to time to eat.
Even some species of birds are able to adapt to cold temperatures in a physiological manner similar to hibernation. For instance, in order to survive bitter cold nights, black-capped chickadees have the ability to decrease their body temperature, thus their metabolic rate. It’s sort of like a mini hibernation, but is correctly called “torpor.”
Torpor enables the chickadee to live through cold nights without expending valuable excess energy. Keeping warm is hard work and the chickadee solves this problem by physiologically “shutting down” during harsh conditions. Surprisingly, ruby-throated hummingbirds do this as well, even though during a time most of us would deem unnecessary - the summertime.
Hummingbirds are birds well known to be very active and ravenous feeders but, like most birds, are active during the daytime. Thus, a hummingbird, since it has such a high metabolism and does not forage at night, enters into a state of torpor, just like the chickadee does in the winter, so as to survive the cooler nighttime hours when they cannot eat.
As well, reptiles, amphibians, and insects are unique in many ways from warm-blooded creatures, not the least of which include their ability to survive Minnesota winters. Some species of snakes spend the winter in underground dens and form entwined “snake balls” with other snakes to insulate themselves from the cold.
Wood frogs actually produce a natural anti-freeze to keep their body’s cells from totally freezing as they lie beneath forest debris in a near death-like state. Meanwhile, several species of insects survive the winter in larval forms by burrowing underground or by congregating together to form large nests of dormant masses.
Muskrats and beavers, rodents we commonly observe in lakes, wetlands, and streams during the ice-free months, are quite active throughout the winter beneath the ice and inside their lodges. They’ve adapted to the cold in ways similar to some hibernating rodents by storing extra fat on their bodies, growing thick fur, and caching food for later consumption.
Nevertheless, even though we usually don’t see them during the wintertime, beavers and muskrats do not hibernate and are actually very active throughout the coldest and iciest months.
Hibernation and torpor and other winter survival strategies are truly fascinating phenomenon in the animal kingdom. Though perplexing to us “non-hibernating” clan, such wonders of Nature are all about us as we get out and enjoy the great outdoors.
Klemek is a DNR Wildlife Manager. You can contact him at email@example.com.