So, what is this thing called hibernation? First, hibernation is essentially a strategy for surviving a scarcity of food, not cold. Food is the fuel that runs animal metabolism. Hibernation is a type of adaptive dormancy (hypothermia) in which heart rate, body temperature, and respiration are reduced. And, as body temperature decreases, so does the required amount of fuel. In fact, for every 18°F drop in body temperature, the rate of energy used is halved. So, hibernation is a critical practice that enables animals to extend their range into lands with seasonal food shortages.
Many animals practice some form of hibernation including reptiles, amphibians, insects, birds and mammals. But let’s keep things simple and restrict our discussion to mammals. To start with, everyone is in agreement that the woodchuck is the prototypical hibernator. During this three to four month dormancy, the heartbeat of a woodchuck drops from 100 to 15 beats per minute, body temperatures are reduced from 96°F to 47°F, and respiration rate drops to one breath per three or four minutes. All other species are compared to the woodchuck with respect to hibernating. Nine of our twelve species of bats and our two species of jumping mice are also accepted as mammalian hibernators. But, when I read about chipmunks and black bear, that’s when the fur really begins to fly. The quick answer is commonly that they are not ‘true’ hibernators, since chipmunks rely on winter food middens to supply them with energy throughout the winter (true hibernators store their winter fuel as brown fat). Nor do they spend prolonged periods in this dormant state (or so most publications say). They usually spend only a few days at a time in this ‘shallow’ dormancy, or torpor. And the body temperatures of black bears drop only a few degrees from its summer norm, enabling them to return to an alert state in seconds, as many bear biologists well know. A significant drop in body temperature is the hallmark of true hibernators. Yet, some say black bear can’t drop their body temperatures very much since, due to their large size, if they dropped it very much, it would take too long (or take too much stored energy) to get the bear back to an active state. On the other hand, some say they are hibernators, since the pulse rate drops by nearly half. Also, it is known a little blood from a hibernating bear will drop a woodchuck in an instant.
But, it gets worse. Even in the case of several of these ‘true’ hibernators, such as the woodchuck and several of the bats, hibernation tends to be longer in more northerly latitudes and will not occur at all in some individuals occupying southerly latitudes. This is also true for some black bear in our own Shenandoah National Park and many male black bear to our south. (However, it has been found that females hibernate even in deep southern states. The suggested reason for this winter hibernation in the south, where food is abundant year-round, is due to caring for the newborns. It is believed that, due to the substantial body heat lost by the tiny cubs at birth, the body heat produced by the dormant lactating mother is vital in ensuring survival of her young over winter.) Food supplies also affect black bear hibernation. In seasons of plenty, black bears enter the hibernaculum weeks later than in seasons of scarcity. It’s been reasoned that black bear will enter hibernation when the energy they expend foraging exceeds that which they can gain by continuing to forage.
Whether chipmunks hibernate or not depends on your source and definition. They do not develop a layer of brown fat, as does a true hibernator. Instead, they go through periods of torpor of variable length (rarely more than several days at a time) and frequency, normally starting in October or early November. Not all animals in a population will become torpid. During these periods of torpor, studies reveal chipmunk respiration rates drop from 60 to less than 20 breaths per minute, and temperature drops from 100 to 42 - 45 degrees. Chipmunks must wake frequently (sources range from almost daily to about every two weeks) to eat from their caches and defecate. And some studies have shown up to 30% of chipmunk populations may not enter winter torpor. In this case, it is apparently a function of food reserves. If food reserves are scarce, the chipmunk will extend its periods of dormancy, whereas, large winter middens of stored foods will support much greater activity throughout the winter season. Additionally, like the species mentioned above, those of more northern habitat tend to spend more time “hibernating” while those of the south enter a torpid state only during severe winter weather.
Raccoons are another species subject to interpretation. According to my resources, they do not hibernate, but will stay in a den for several weeks of severe cold, living on its brown fat layer (up to four months north of the mid-Atlantic area, only a few days at a time south of this area).
Hibernation allows the animal to skip over the cold, stressful seasons when food (i.e., fuel for food and heat) is scarce, and only expend itself fully in those months of abundant food and moderate climatic conditions. While most mammals don’t migrate (bats, caribou, and marine mammals the noted exceptions), some ecologists refer to hibernation as "time migration". Hibernation traditionally has meant winter inactivity. It has been applied to a prolonged period of time of sustaining a significantly lowered body temperature. When it was learned that some mammals undergo this lowered state of physiology for shorter periods of time, such as weeks, days, or even hours, the concept of torpor was introduced. Hibernation became just a longer, or seasonal, period of torpor. Subsequently, this adaptive torpor was observed in some animals that survive not only winter food scarcity, but also inhospitable conditions caused by seasonal (normally summer) drought. In this context, hibernation has come to be called aestivation. The strict use of a significant drop in body temperature to define hibernation became a problem when it was applied to cold-blooded animals, which never regulate a high body temperature, but engage in adaptive winter inactivity. Thus, was introduced the term brumation; referring to the winter sluggishness, or torpor, of reptiles and amphibians. Finally, it was found that some mammals and birds undergo nightly torpor, to save energy, not only in winter, but every summer night. And bats and hummingbirds are known to exhibit torpor daily on a diurnal cycle. Thus, the whole concept of hibernation, at this point, breaks down totally.
We like to think in terms of pigeonholing through definitions, yet, trying to fit nature into these categories just doesn’t work. Not only are there exceptions to the rule, but, as is often the case, there are no clear rules at all for nature. The issue is simple. How does an animal deal with a scarcity of food? And the answer is just as simple: whatever way is best for a given species in a given situation.
What is important to remember is that each species has defined it’s own set of rules that allow it to best adapt to it’s environment. There exists a continuum of adaptive measures that have been addressed through evolution, which vary, not just by species, but also within each species by environmental habitats.
So, what is hibernation? Perhaps we can say it is a long-term, winter form of torpor; a type of adaptive dormancy (hypothermia) in which heart rate, body temperature, and respiration are reduced. Such a simple concept with so many complex adaptations.