The animal most people first think of when the topic of hibernation comes up is the bear, but there is no species of bear that technically hibernates. Hibernation, also called winter dormancy, is defined as a period of deep torpor; bears only go into a shallow torpor during the winter months, an activity called winter sleep. Torpor is a body state in which the internal temperature is lowered, a few degrees below normal when shallow or down to only just above the ambient temperature of the animal’s surroundings when deep.
True hibernators among the mammals are predominantly from the taxonomic orders Rodentia, such as chipmunks (Tamias amoenus), Insectivora, like the European hedgehog (Erinaceus europaeus) and Chiroptera, the big brown bat (Eptesicus fuscus) being a prime example. They tend to be mid-range in size, allowing them to lower their body temperature to just above the ambient temperature of their surroundings, but still be able to raise their temperature again from internal fat reserves.
If bears or other large mammals did the same they would have to expend a prohibitively high amount of energy to return their body temperature to normal and take from 24 to 48 hours to do so. As their normal metabolic rate is low relative to their accumulated energy stores compared to smaller mammals and their large volume to surface area ratio results in comparatively slow heat loss, bears can more effectively get through the winter in a light rather than a deep torpor while tucked away snuggly in the microhabitat of their den. This is why they are able to awaken from their winter sleep quickly enough to be very dangerous to any humans that might be foolish enough to transgress their winter den.
Hibernation and winter sleep occur during periods of cold, typically autumn and winter, when food is scarce. There is a summer equivalent with very similar effects called estivation used by some mammals to see them through particularly hot and dry periods. These external environmental conditions stimulate the production of an opioid hormone called hibernation inducing trigger (HIT) that gets released into the bloodstream from endocrine glands. This causes a reduction in heart rate and respiration, lowering oxygen consumption and inducing a slow-wave sleep which altogether decreases heat generation, steadily lowering body temperature. For most mammals this process is automatic, polar bears (Ursus maritimus) and some bat species however, have some control over HIT production. Polar bears for example, when caught in inclemental weather that prevents them from hunting on the ice floes, can induce HIT production to put themselves into an energy saving torpor until the weather improves.
The true hibernators reduce their body temperature to just 0.5 to 2.0 degrees Celsius above the ambient temperature, their metabolic rate reduces thirty-fold. The animals primary fuel source for metabolic processes switches from glycogen (sugars) to lipids (fats). Breathing patterns vary between species, but most will include periods of apnea, when the animal ceases breathing, that may last up to two hours. Production of peroxiredoxin proteins, particularly in the brown adipose tissue and the heart, increases significantly. In the ground squirrel (Spermophilus tridecemlineatus) these antioxident isozymes in the heart tissue increased to a level 12.9 times that found at normal body temperature. These are needed to protect the hibernating animal from free radicals that might otherwise build up during the period of lowered metabolism as well as the large numbers produced during the thermogenesis (heat production) of arousal. Excessive numbers of free radicals can cause significant damage to cells, up to and including cell death. Unlike animals suffering from hypothermia, hibernators are able to awaken at any time and do so spontaneously periodically.
Some hibernating mammals like the woodchuck or groundhog (Marmota monax) build up a store of body fat prior to hibernating, while others like the chipmunks mentioned above stock their winter quarters with a supply of food. The later will need to rouse relatively frequently, usually weekly, to urinate, defecate and feed; the former can remain in a torpor longer but will still need to rouse, perhaps every two to four weeks, to excrete body wastes. Although it hasn’t been determined definitively, it is theorised that these arousals are triggered by the accumulative build-up of a metabolite or some other substance that can only be neutralised when the body is at a normal body temperature. After the brief period of arousal, if external conditions are still unfavourable, the animal sinks back into a hibernating state.
With the return of favourable environmental conditions as indicated by a rising temperature and with the increasing stimulation of hunger, HIT production tails off and the animal awakens from hibernation until the turn of the seasons starts the process all over again.
Sources:
Bruce, D., Darling, N., Seeland, K., Oeltgen, P., Nilekani, S. & Amstrup, S.(1990) Is the polar bear (Ursus maritimus) a Hibernator?: continued studies on opioids and hibernation. Pharmacology Biochemistry and Behaviour 35(3): p705.
Morin, P. & Storey, K. (2007) Antioxidant defense in hibernation: Cloning and expression of peroxiredoxins from hibernating ground squirrels, Spermophilus tridecemlineatus. Archives of Biochemistry and Biophysics 461(1): p59.
Randall, D., Burggren, W & French, K. (2002) Echart Animal Physiology Mechanisms and Adaptations 5th Edition. New York: W.H. Freeman and Company.
Sci-Tech Encyclopedia (2008) Hibernation. Retrieved from
http://www.answers.com/topic/hibernation?cat=health
