Little brown bats, native to Virginia, spend winters hibernating
in caves or similar shelter. Bats also go into embryonic diapause
in winter, breeding in the fall but delaying fertilization until
the spring. Photo by U.S. Fish and Wildlife Service.
Basically, animals have three strategies: fleeing to warmer climes (migration), evolving a physiology that tolerates cold, or hibernating. Since hibernation was most on my mind, I started my research there, but soon ran into terminology issues, which are underlain by lack of agreement within the scientific community about the biological processes surrounding this state.
“Hibernation,” “torpor” and “dormancy” are often used interchangeably. In a 2011 article for The Encyclopaedia Britannica Advocacy for Animals website, Kara Rogers, Britannica’s biomedical sciences editor, defined dormancy as “the slowing of an organism’s metabolism to facilitate energy conservation in times of environmental stress, which often are characterized by extremes in temperature and by the lack of food or water.”
Dormancy is marked by topor—a state in which animals appear sluggish or lethargic. Such torpor can be light or deep. Rogers writes that “true hibernators represent the extreme end of sustained torpor.”
“Hibernator” is the really tricky word here. “Hibernation” generally refers to an animal’s becoming dormant in winter, while summer dormancy—for example, due to drought—is generally referred to as “aestivation.” While an animal’s lowering its temperature was long thought to be the key determiner of hibernation, metabolism suppression is more of the focus in current research. Both processes are interrelated.
As BBC Nature put it:
“Attempts at defining or triggering hibernation have … proved elusive. For example, merely exposing animals that hibernate to cold does not always trigger hibernation. Instead it may trigger hypothermia, a life- threatening pathologic condition. In addition, the duration for which an animal must remain in torpor for it to be qualified as a hibernator remains debatable.”
BBC Nature adds that “considering that some animals exist in this state for the greater part of the year, it could just as easily be considered the default metabolic state and upregulation of metabolism merely a response to the availability of energy.”
In any case, the timing for going into dormancy differs among animals. Some become dormant in direct response to worsening external conditions (consequential dormancy); others enter the dormant state well before adverse conditions set in (predictive dormancy). In the latter, internal clocks or external cues, such as shortening of the days, can initiate the change.
Since ectotherms (“cold-blooded” animals), such as reptiles and insects, can’t regulate their internal temperature, the term “brumation” was added to the dormancy lexicon in the 1960s to apply to their state (particularly to reptiles) when they hibernate. The need for such differentiation has always been controversial.
Depending on the species and climate, hibernation can be intermittent or last through the entire cold season. Birds may just go into a light torpor at night when temperatures drop, while some hibernating mammal species vary the depth and length of their torpor. For example, ground squirrels (which include chipmunks and groundhogs) generally stay in deep torpor for the entire winter, while raccoons and skunks go in and out of torpor depending on availability of food.
One mark of hibernation that seems to be agreed upon is that animals do not eat, drink, defecate or urinate while in hibernation. This is mainly because of the lack of food and the amount of energy and water needed to find and metabolize what food might be available.
Rather than going into torpor, some animal species deal with winter cold and food shortages by halting only specific biological processes, known as “diapause.” Caterpillars, for example, take a break in their development cycle, entering into a chrysalis state in the fall and emerging as adult butterflies or moths in the spring. Reproduction can similarly be paused through “embryonic diapause,” in which some animals, such as bats, breed before winter but store the sperm until late winter or early spring, when fertilization is initiated.
The one thing to keep in mind when encountering animals that are hibernating is that it’s hard to be sure how deep into torpor they are. Animals brought out of torpor in the winter could die from burning off needed resources, and humans could be at the wrong end of a fully armed animal that is not happy at being roused. The best strategy, as with all encounters with wildlife, is to leave the animal as it was.
“To sleep, perchance to dream”:
The remarkable hibernation of the arctic ground squirrel
According to Bernd Heinrich, in his book Winter World: The ingenuity of animal survival, how the squirrel does this without any apparent antifreeze in its blood (an adaptation of some species, such as our native wood frogs) is not well understood but indicates that “supercooling” is likely involved. Supercooling is when the state of a gas or liquid is maintained when its temperature is lowered beyond its normal freezing point. It’s an unstable process—a mere stirring of an Arctic ground squirrel can start a crystallization reaction that can turn the rodent into an ice sculpture. However, taking this evolutionary gamble has enabled the squirrel to expand farther north than its cousins.
Another remarkable thing about the Arctic ground squirrel, according to Heinrich, is that, while it does maintain torpor during the winter, it also increases its metabolism periodically, raising its temperature “all the way up to when it is active” (98.6 degrees F.). Why does the squirrel do this? As Heinrich points out, research has shown that sleeping, dreaming, and oxygen
ating the brain—all of which need more elevated metabolism—have been shown to be essential to human health. Is it the same for this squirrel? According to Heinrich, this also remains a mystery.