Hibernation and Hibernating

There is hibernation and then there are other conditions that are similar to hibernation

• There appears to be a relationship between sleep and hibernation; available evidence suggests that hibernation is entered into from a condition of sleep.
• If hibernation is to be considered a form of sleep, then it must be considered as a complex one.
• Hibernation and sleep are somewhat similar in that essential body processes continue during both periods at a lowered level. In sleep, the heart beats less rapidly, and breathing is slower; the body produces less heat, making it necessary for a sleeping person to be protected from the cold."

The most common misapplication of the term hibernation is in relation to the bear, which is not a true hibernator

• Its body temperature, which normally averages 38° C (100° F), drops during its winter lethargy to about 34° C (93° F), seldom getting below 31.2° C (88.2° F); therefore, a bear’s temperature during the winter does not approximate that of the environment.
• This is indicative of winter rest rather than true hibernation.
• During this inactive period, the bear sleeps but it is, nonetheless, warm and capable of activity when stimulated, unlike a true hibernator.
• In addition, it is also during this period when females give birth to cubs that suckle and are maintained by maternal warmth until they emerge from the den in the spring.
• Such behavior is in contrast with that of the Arctic ground squirrel, whose normal temperature is the same as that of the bear but whose temperature during hibernation drops to near freezing and, in some cases, to a degree or two below 0° C (32° F).
• Although certain mammals are said to hibernate, they do not necessarily enter a state of deep hibernation during winter; instead, for weeks at a time they may be inactive and lethargic in behavior, with a slightly depressed body temperature.
• The chipmunk (Eutamias) is an example of what has been termed a shallow hibernator, as are bears and raccoons.
• Superficial hibernation, apparently a compromise between the minimum energy requirements of a deep hibernator and the high energy expended by an animal that remains active during the winter, saves energy without the stress of hibernation.
• The animal can thus conserve food, while still being able to escape from predators and such dangers as flooding of its burrow.
• The main energy source during the winter in this shallow hibernation state is food stored in the winter nest.
• There are instances, however, of shallow hibernators, such as the chipmunk, that enter a state of deep hibernation, particularly if it is without food."

True Mammalian Hibernation

• Omitting the thermally unstable mammals, the true mammalian hibernators are those whose lowered body temperatures approximate that of the environment and those who require extensive and complex physiological changes to turn from a warm-blooded animal into an essentially cold-blooded one for an appreciable length of time.
• Only three orders of placental mammals display such behavior: the Erinaceomorpha, as exemplified by the hedgehog; the Chiroptera, the bats; and the Rodentia, including the marmot, hamster, dormouse, hazel mouse, and ground squirrel.

Hibernation and sleep are somewhat similar in that essential body processes continue during both periods at a lowered level.

• A typical mammalian hibernator, such as the Arctic ground squirrel, finds a protected environmental niche—in this case, a burrow beneath the surface—and builds a nest of grass, hair, and other materials to provide still further insulation.
• The usual hibernating position is one of being curled up in a ball with the extremities tucked tightly against the body so there is a minimal surface-to-volume ratio.
• After the temperature of the animal has dropped near that of the surrounding temperature, it appears to be dead: its respiration is imperceptible, about three irregular breaths per minute; it does not react to outside stimuli in an observable fashion; nor does it react to being handled and uncurled, although such handling will trigger wakening mechanisms.
• The internal organs; such as, the digestive tract and the endocrine glands, are almost totally inactive and because the process of hibernation necessitates the mobilization of all body resources, it places great demands on the tissues, all of which are directed toward the problem of maintaining the animal’s metabolism at the minimal level necessary for life during the hibernating period.
• This means that all activity not immediately relevant to the process of living at the lowest possible metabolic level ceases or stops.

Even bones and teeth deteriorate during hibernation

• The hibernator apparently is balanced on a very narrow line between the maintenance of life at a level that makes recovery from hibernation possible and a reduction of metabolism to a level that will lead to death.
• Evidence obtained from tissues indicates that the process of hibernation is a dangerous method of survival at best and one from which many animals do not survive.
• As a mechanism of species survival, hibernation seems effective; for the survival of the individual; however, it is an uncertain and dangerous process.
• The hibernator does not remain in a continuous state of hibernation from the time it enters in the fall until it comes out in the spring.

One period of arousal and wakefulness consumes more heat and energy than many days in hibernation.

• Hibernating Arctic ground squirrels, for example, wake up at intervals of every three weeks or less.
• During this time the animals may move around and sometimes come out from the burrow.
• These periods of arousal are more frequent at the beginning and the end of a hibernation period than in mid-hibernation; and the lower the temperature at which an animal hibernates, the fewer the awakenings.
• During the period of hibernation about 40% of the total body weight is lost, an average of about 0.2–0.3% per day.
• One period of arousal and wakefulness consumes more heat and energy than many days in hibernation.
• About 90% of the total heat production and weight loss during hibernation takes place during the arousal periods; only 10% is required to maintain the animal in hibernation.
• In the case of an unusually long or hard winter, the animal may be called upon to use all of its available energy sources in periodic arousals; it then enters one final hibernation period from which it does not awaken.
• Animals that store food in the nest have a chance to renew their energy requirements by eating when they wake up every so often.

Hibernating mammals can be divided into four major groups according to the way they enter hibernation

• One group is exemplified by the golden hamster; it waits a variable time of from one to three months in the cold and then enters hibernation in one major temperature reduction.
• This is accomplished when the biochemical and physiological preparations have been sufficient to lower the animal to a level at which it is receptive to the hibernating stimulus, which causes the abandonment of the temperature differential between ambient and body temperatures.
• A second group, of which the pocket mouse (Perognathus) is an example, prepares for hibernation relatively rapidly, waiting only a few days before becoming torpid in one major temperature decline.
• The third group, which constitutes most of the mammalian hibernators, includes ground squirrels and marmots.
• These animals wait only a few days before entering hibernation and then go through a series of steps of torpor and arousal, each one at successively lower body temperatures, until the level dictated by the stage of preparation for hibernation is reached.
• The fourth group, which includes most of the bats, becomes inactive in the poikilothermous manner; that is, the body temperature follows the surrounding temperature.
• Even though the bat seems ready to hibernate at any season, survival during hibernation depends upon more adequate preparation than is necessary for the transitory periods of torpor.
• Bats not only exhibit true hibernation during the winter but also have natural periods of hypothermia (subnormal temperature), which are unrelated to hibernation, during the remainder of the year.
• The woodchuck, the dormouse, and the California ground squirrel enter hibernation in successive stages, with a complete or nearly complete awakening between each one.
• In the woodchuck, an initial decline in temperature is followed by an arousal; then, during the second decline there is a lower and more pronounced fall in body temperature, followed by a less pronounced rise. This process continues until the body temperature is essentially the same as that of the environment.

Physiological changes during mammalian hibernation

Heart rate and circulation

• The body temperature of a hibernating mammal is affected by changes in respiration, heart rate, and oxygen consumption; all are apparently mediated by a part of the nervous system.
• The heart rate decreases prior to a decline in body temperature. In the woodchuck, the rate may drop from 153 to 68 heartbeats per minute within 30 minutes.
• In the California ground squirrel, the heart may beat as slowly as once a minute at 5° C (41° F). In contrast, the hearts of non-hibernators generally will not beat at all at temperatures below 10°–20° C (50°–70° F).
• As an Arctic ground squirrel prepares for hibernation, its heart rate and its blood pressure decrease and both may be detected before a decrease in body temperature can be noted.

The body temperature of a hibernating mammal is affected by changes in respiration, heart rate, and oxygen consumption; all are apparently mediated by a part of the nervous system.

• When the animal enters hibernation, temperatures of both the heart and abdominal regions are identical, indicating an even blood flow between the anterior (front) and posterior (rear) parts of the body.
• As the body temperature drops, the resistance to blood flow in the peripheral parts of the circulatory system increases because of the increased viscosity (resistance to flow) of the chilled blood and the constriction of the distal arterioles (small arteries) of the body.
• This peripheral resistance maintains blood pressure at relatively high levels in the deeply hibernating squirrel, even when the heart beats only three or four times a minute.

Neural or nervous changes

• The nervous system of hibernators also is acclimated; certain specific structures and pathways are seemingly maintained to regulate and coordinate metabolism as temperatures drop.
• This adaptation of the nervous system enables changes in the environment to be perceived, even when the animal is torpid. In the Arctic ground squirrel, measurements of the general electrical activity of the brain indicate a 90 percent reduction when the animal is in hibernation, at which time brain temperatures approximate 6° C (43° F).
• During hibernation, both the peripheral nervous system (all the nerves outside the brain and spinal cord, which constitute the central nervous system) and the spinal cord have an increased sensitivity to certain stimuli; in addition, the areas of the brain that regulate temperature as well as cardiac (heart) and respiratory function remain active at ambient temperatures, below which the mammalian nervous system normally ceases to function.
• Changes in the circulatory system involving constriction (narrowing) of posterior vessels and the favoring of anterior circulation allow the brain temperature of hibernators to remain a few degrees warmer than the environmental level.
• This enables the temperature of the brain to remain constant despite fluctuations in the temperature of the skin.

Protection from diseases

• Hibernating organisms have a certain degree of resistance to infectious diseases that appears to be attributable to at least three factors, all of which are related to temperature.
• One is the fact that the lowered temperature of the host and the commensurate slowing of its metabolic processes prevent the multiplication of parasites to a greater extent than they affect the host’s defensive mechanisms.
• Second, lower temperatures are more harmful to the development of a disease organism than to the host, as has been shown with the parasite Trichinella spiralis. In bats hibernating at 5° C (41° F), only larvae have been recovered from the intestines; but mature adult worms have been recovered from the intestines of bats kept at 35° C (95° F).
• The third factor is that the influence of low temperature on the chemical composition of the host tissues may also affect infectious organisms.

Waking up from hibernation

• The process of waking up in the Arctic ground squirrel takes about three hours.
• There is a rapid rise in heartbeat and a decrease in peripheral circulatory resistance; the area around the head and heart warms more rapidly than the posterior (back) part of the animal.
• This differential vasodilatation (widening of the blood vessels) in the anterior (head or front) part of the body is a unique and vital part of the awakening process.
• The concentration of active circulation in this region results in a high blood pressure and an efficient and rapid warming.
• Later during the arousal process, after the anterior (front) part of the body has been warmed, the posterior (back) part of the animal warms rapidly.
• Despite the deterioration of glands and tissues and the drastic reduction of all metabolic activity during hibernation, within twenty-four hours after arousal, all the squirrel’s physiological processes are essentially normal.
• Hibernation is usually not possible in the Arctic, because there are no frost-free refuges; so, all the non-migrant, warm-blooded animals must remain active all winter or they won't survive.

There is more important information about "hibernating" word entries at this hiber- link.