Dormancy is 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. The stress may be mild enough that only brief spans of time each day are devoted to conserving energy. This occurs, for example, when birds allow their body temperatures to drop at night when air temperatures are cool. The birds warm again to their active body temperatures during the day. This type of short-lived dormancy is known as daily torpor. Torpor becomes hibernation when decreases in body temperature and activity are sustained over long periods of time during winter.
True hibernators represent the extreme end of sustained torpor. These champions of deep sleep include many small mammals, such as chipmunks, woodchucks, and ground squirrels. Hibernation of the Arctic ground squirrel (Spermophilus parryii) serves as a truly amazing example of dormancy. During hibernation, this small creature, a year-round resident of the tundra, undergoes supercooling, with its core body temperature dropping from 98 °F (36.7 °C) to just below freezing. The squirrel's hibernation period lasts seven or eight months, and about every two to three weeks, it shivers (without waking) for more than a dozen hours to reheat its body.
Some true-hibernating animals that shiver actually awaken during the warming period and take the opportunity to tend to routine tasks, such as eating and urinating. But they soon settle down and resume their hibernation. Many small hibernating mammals also experience precipitous declines in heart rate and respiration rate as they again fall into a deep sleep. The heart rate of a woodchuck, for example, drops from 80 or 100 beats per minute when active to rates in the single digits when hibernating. Its rate of respiration may decline to just one breath every six minutes.
Black bears and grizzly bears are famous for their ability to sleep through long winters, but there are several important differences between bear dormancy and dormancy in true hibernators. With the exception of pregnant females, who awaken in January or February to give birth and nurse their cubs, black bears and grizzlies do not engage in any activities, including eating, during their hibernation period, which may last anywhere from four to seven months. In addition, the body temperature of a bear drops only by about 12 °F (6.7 °C) when hibernating. Because of this, bears are not generally considered to be true hibernators. The state they enter is often described as winter lethargy or simply dormancy. Some researchers believe, however, that bears are actually “super hibernators,” due to their complete inactivity and unusual winter adaptations.
The onset of dormancy in animals or plants varies. In autumn, decreasing day length (short-day photoperiod), for example, sends some species into a dormant state. This is most obvious in deciduous trees, which drop their leaves and stop growing for the winter, thereby conserving energy for an explosion of growth in spring. For many animals, dormancy is triggered by cooling temperatures, by a lack of food, or by a combination thereof.
As for humans, as much as we may feel like hibernating in our homes to pass the short dark days of winter, the closest it appears that we can ever come to attaining a “dormant” state is sleep. And, rather than a shortage of food, an overabundance of tasty treats seems to be the trigger for this not uncommon form of human “winter lethargy.”
Kara Rogers is the senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is also a member of the National Association of Science Writers and a contributor to the Britannica Blog, where she runs a series called Science Up Front. She holds a Ph.D. in Pharmacology/Toxicology, but enjoys reading and writing about all things science. You can follow her on Twitter at @karaerogers.
This post also appears on the Britannica Blog.