Hibernation is a concept that everyone thinks they know. Woodchucks hibernate, chickadees don’t. Woodchucks cool down and go to sleep in winter. Chickadees don’t. So cooling down and sleeping in winter is hibernation, right? Simple.
Wish it were so!
It is much easier to identify animals that don’t hibernate than those that do. That’s because hibernation has many meanings, even if you differentiate the so called “true” hibernators from, well, the untrue (false?) hibernators. I’ll start off simply and identify animals that don’t hibernate: deer, moose, hares, grouse, and voles and shrews under the snow. These animals don’t migrate and can find food all winter.
Traditionally, hibernation has simply meant winter inactivity. Unfortunately, this definition applies equally well both to bears lying in their dens and to some insects and frogs that are frozen solid above the ground. And then there are “true” hibernators, such as the ground squirrels, which are usually inactive but periodically warm up for a day or more to – would you believe – sleep! (Sleep is necessary for mammalian brain repair and requires active metabolic processes which shut down in nearly frozen tissues.)
So the definition of hibernation has changed over time to mean torpor: a state of inactivity achieved primarily (but not exclusively) by a greatly lowered body temperature. But where should we draw the line on lowered body temperature, and the duration thereof? An Arctic ground squirrel cools itself down to -2˚C (but warms up to 37˚C every two weeks), a chipmunk to 6˚C, some hummingbirds to 10˚C, a chickadee to about 26˚C (nightly), and a bear to a balmy 34˚C. Worse yet, temperature-induced torpor has also been observed in some animals as a means for surviving not winter conditions but inhospitable summer conditions in the desert. (This phenomenon has since been named “aestivation” to distinguish it from hibernation.)
But what about cold-blooded amphibians and reptiles, who are clearly in cold torpor throughout the whole wintertime? The solution has been to invent yet another term, brumation, and to add the adjective “true” to the old term, hibernation. Coined in the 1970’s, brumation refers to winter sluggishness or torpor in cold-blooded amphibians and reptiles. True hibernators, therefore, are defined as warm blooded.
But what exactly does cold blooded mean? Many insects, presumed to be cold-blooded, at times maintain the same or even higher body temperature in the winter than the majority of birds and mammals. Like birds and mammals, these insects shiver (simultaneously contract opposing muscles otherwise used for locomotion to produce heat but little movement) so that they can become capable of rapid movement, in this case, flight. Some sphinx moths achieve body temperatures of up to 46˚C – high enough to kill any mammal. Other insects stay active without ever heating up, either by shivering or by basking (increasing body temperature by orienting to capture solar heat rather than by shivering), and a few (who live on glaciers) are even active with a body temperature at or slightly below the freezing point of water.
These cold-blooded overwintering insects with occasionally warm blood are covered by still another term: diapause, strictly defined as a state of arrested development. All hibernating insects are in diapause, in part because low temperature, if not freezing, retards or stops biochemical processes. But diapause can also occur in the absence of hibernation – some adult insects, for example, enter reproductive diapause in the summer when they migrate or search for host plants.
In the end, making an ever-more precise definition of hibernation based on body temperature or some other specific criterion does not increase our understanding of any animal. Animals are dynamic. Each animal’s choices fit in somewhere in a long continuum of almost anything that can be measured or imagined. Animals don’t follow rules or easily allow us to pigeonhole them into convenient intellectual boxes – especially when winter survival is at stake. They’ll use every trick imaginable to conserve energy.
But still, the term “hibernation” has to mean something, right? Without further delay, here it is, the latest and greatest definition of this oft-heard yet rarely understood phenomenon. Hibernation: a seasonal period of adaptive torpor, achieved by a variety of mechanisms, including lowered body temperature, that allow animals to survive a regularly occurring famine, provided it’s in winter and has evolved under winter’s selective pressure.