To hibernate or not to hibernate?
A friend of mine recently asked me, "Do muskrats hibernate?"
The simple answer is they don't, although I certainly understand why the question was asked -- we just don't see muskrats during the winter months. As such, rodents they are, some members of the rodent family hibernate while others, like muskrats, beavers, tree squirrels and mice and voles, do not.
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, for example, spend their entire lives in the Northland, but can only do so if they hibernate or adapt in some other way to survive cold nights, snow and ice.
Contrary to popular belief, black bears are not true hibernators. Mammals such as ground squirrels, woodchucks and chipmunks enter into true states of hibernation. These particular rodents' heart rates, body temperatures and metabolisms 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 winter 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 sleeps.
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 winter -- at least above ground -- and mostly spend the cold months in a deep, underground 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 awaken occasionally to feed.
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," so to speak, during harsh conditions. Surprisingly, ruby-throated hummingbirds do this as well, even though during a time most of us would deem unnecessary -- summer.
Hummingbirds -- birds well known to be insanely active, ravenous feeders -- are, like most birds, diurnal. 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, 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 antifreeze to keep their body's cells from totally freezing as they lie beneath forest debris in a near deathlike state. Meanwhile, several species of insects survive the winter in larval forms by burrowing underground or congregating together to form large nests of dormant masses.
Muskrats and beavers, rodents we commonly observe on 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 -- or signs of them, for that matter -- during the winter, beavers and muskrats do not hibernate.
Lastly, you've probably heard the expression, "Never wake a sleeping bear" (or is it, "sleeping baby")? Whatever it is, if you want a special treat, you can now visit an active bear den in the comfort of your own home or office by simply visiting the Web site of the North American Bear Center. Once there, you can watch in real time a 3-year-old female adult black bear who gave birth Friday in her northern Minnesota den.
It will give observers a rare glimpse into the secret world of a slumbering black bear and her newborn cub.
You can view the black bear (she's been given the name Lily by her research team of biologists) and her offspring on the "Den Cam" at www.bear.org/website. Videos of the birth are on YouTube.
Hibernation and torpor and other survival strategies are truly fascinating phenomena in the animal kingdom. Indeed, such occurrences, as strange as they seem, are there for our discovery as we get out and enjoy the great outdoors.
Blane Klemek is the Bemidji area assistant wildlife manager, DNR Division of Fish & Wildlife. He can be reached at firstname.lastname@example.org