Popular Science Monthly/Volume 77/October 1910/The Natural History and Physiology of Hibernation
|THE NATURAL HISTORY AND PHYSIOLOGY OF HIBERNATION|
LATE INSTRUCTOR IN COMPARATIVE PHYSIOLOGY, LATE ASSISTANT IN PRACTICAL PHYSIOLOGY, HARVARD VETERINARY AND MEDICAL SCHOOLS
IN this most important and interesting phenomenon of nature—a truly protective phase—let us first briefly consider the natural-history side of the subject, describing how hibernation is peculiar to, and differs, in the various animals possessing this function of protective winter sleep. Then we will consider the various physiological phenomena characteristic of this torpid state. Let us first make clear the meaning of some of the terms used in connection with this condition.
The term "hibernation" is used by scientists and naturalists to signify the peculiar somnolent or torpid state in which many animals, both warm-and cold-blooded, inhabiting cold and temperate climes, pass the winter. On the other hand, in hot and dry countries, various animals pass into a similar condition during the hottest and dryest season of the year, and this state is called "æstivation."
Many of the animals which hibernate during the winter are liable to fall into a similar state at intervals during milder seasons—Dr. Marshall Hall considers the day sleep of bats as a species of hibernation and has called it "diurnation."
In man it is a question if true hibernation ever occurs—a similar condition may exist, however, but it is very rare either for a prolonged or short period. Cases of hibernation (?) in man are most frequently found in India, where some religious ascetics are stated, upon unimpeachable authority to possess the power of throwing themselves into a state closely resembling hibernation (trance?) for an indefinite period. Many curious cases have been recorded by Mr. Braid in his treatise on "Human Hibernation"; the most celebrated and best known is that of a "fakir," who, in the presence of Sir Claude Wade, the English governor, was buried alive at Lahore in 1837, was exhumed and restored to consciousness after remaining in the ground for several months, the grave being guarded and every precaution taken, in the meantime, to prevent any interference or disturbance of the body.
One of the largest groups of hibernating animals we have is that of bats—Dr. Marshall Hall claims that the bat when hibernating, practically never wakes, but when disturbed will breathe naturally, for a few minutes, but will then return to its former state of quiescence.
Earlier or later in the autumn, according to the species, the bats retire to caves and hollow trees and similar places, where they cluster together, hanging by their hind claws, head downward, and clinging to one another as well as to the walls of their retreat. Such masses may comprise various species, yet all species do not retire at the same time.
The noctule is seldom seen abroad later than July—while the pipistrelle may be seen flying any warm evening during the spring, summer and autumn months.
Owing to the difference in the flying season of these different bats, it is only natural to suppose that the hibernation of the former species (noctule) is much deeper and longer than in the latter (pipistrelle), which may feed all the year round. Here in North America some bats may, and probably do, migrate southward to avoid the too intense cold, and among these is probably the noctule, which is the first to come and the first to go. These "flitter mice" are insectivorous, and before discussing some of the other mammalia, it may be advisable to call attention to some points of difference existing, during the hibernating state, between the carnivorous and herbivorous animals.
Bears come out of the hibernating condition as fat, or nearly so, as when they retired in the fall; marmots, ground squirrels, etc., emerge in poor condition, in some cases quite emaciated; this in spite of the fact that they are functionally torpid, while bears and skunks are not. Female bears even bring forth their young while hibernating and suckle them from about the beginning of February to April, while they obtain no nourishment themselves.
The bear and badger, in our northern and western climates, retire to their winter quarters and pass the greater part of their time in sleep, yet neither the brown bear nor the badger falls into a true state of hibernation. On the other hand, the black bear of North America is aroused with difficulty from his winter sleep, and this gives ground for the belief that he differs from the other species of bears in that he hibernates while the others do not.
The most perfect hibernating animal we have in Canada is the ground-hog (woodchuck), which hibernates in a burrow of its own making. In Europe the hedge-hog (an insectivorous animal) is most complete. It retires to a hole among rocks, under a tangled mass of roots of trees or sometimes into an old disused drain, there it remains for the winter, seldom or never awakens until spring, and during this time it takes no food. In Canada it wakens to eat. If disturbed, it draws a deep sononous breath, followed by a few weak respiratory movements, returning almost immediately to its deep state of torpor and quiescence. The tuerec, an allied animal, found in Madagascar, sleeps similarly in its burrow for three months during the hottest period of the year.
None of the American (or European) squirrels truly hibernate except the chipmunks (Entamicæ) and the ground squirrels (spermophiles).
The dormouse hibernates in the strictest form and its torpid condition is much deeper than that of the squirrels, its favorite bed for its long slumber being an old bird's nest or a mass of dried moss or feathers. In captivity, this little animal has been known to sleep for several days during the summer. The myoxus, an allied animal, when brought to Europe from Africa, hibernated in the winter seasons as if it were its usual and natural habit, without sustaining any harm.
The hare will lie beneath deep snow for several weeks at a time, in a cavity just large enough to contain its body. In a similar manner, sheep, without any apparent injury, have been known to live for several weeks, buried in huge snow drifts or slides.
All the amphibia hibernate in cold and temperate climes. Land tortoises bury themselves in holes in the ground, while the fresh-water tortoises bury themselves in the mud at the bottom and sides of lakes and ponds. They obtain a deeper condition of torpidity than that of hibernating mammals, digestion and respiration (lung) being entirely suspended. Frogs hibernate in masses in the mud at the bottom of stagnant pools and if awakened from the hibernating state by warmth, can remain underwater, without drowning, considerably longer than they can during the breeding season. That frogs (toads?) can remain in a state of hibernation for an indefinite period remains to be proved, but too many circumstantial accounts have been reported of the discovery of live toads in masses of solid coal or rock formations to allow the idea to be dismissed as fabulous.
Reptiles, fish and batrachians exhibit little change from their usual condition (?). Lizards and snakes retire to holes in trees, under stones, dead leaves, and many species may congregate together in large numbers. They are in a quiescent and somnolent state, not true hibernation.
Fish of the temperate zone do not fall into a state of complete torpidity, but their vital functions are diminished and they retire to sheltered holes and cease to go abroad in search of food. In the tropics (Africa and India) large numbers of fish are known to survive long and severe droughts during which the streams and ponds are completely dried up. This, they do, by passing the dry season embedded in the mud.
Most of the species of molluscæ hibernate. The land snails bury themselves in the ground or conceal themselves under the bark of trees; in fact in almost any sort of a cavity to be found. They close the mouth of their shells with a calcareous plate technically known as the "epiphragm," and this is perforated by a minute hole to permit breathing. The substance forming this plate they secrete in their mouths. During the dry weather in summer, the snails bury themselves in the ground and cover the opening of their shells with this protective shield, but it is much thinner than the one used in winter. This they do to protect themselves from the drought, i. e., by checking evaporation. Slugs bury themselves but do not enter into a complete state of hibernation. Fresh-water molluscæ go into a state of hibernation in the fall, burying themselves in the mud until spring. It is believed that saltwater molluscæ. hibernate in a similar manner, but practically nothing is known concerning them.
Many butterflies and moths hibernate in the perfect state as well as in the form of imagos, but not in the larval state (?). Most insects which pass the winter in a state of larvæ hibernate during the period when they can not obtain any food. Insects which hibernate do not pair until spring and bees do not hibernate at all. It is well known to bee-keepers that these insects need plenty of food during the winter months.
In the seeds of plants and in the eggs of many of the lower animals, life may remain dormant for years in cold climates, until heat or moisture awakens them. Many plants die down, while their roots remain alive during the winter season (perennials and biennials), coming to foliage and blossom in the spring. In the same way trees shed their leaves in the autumn and the sap returns to the roots. Similar phenomena take place in tropical countries during the hot season, whenever the amount of humidity in the atmosphere is sufficient to maintain perennial vegetation during the entire year.
These phenomena in the vegetable world are regarded generally as being analogous to those of hibernation in animals and therefore the terms "hibernation of plants" is sometimes applied to them.
In this article, so far, we have dealt with the natural-history side of hibernation, explained what is meant by the various terms used in connection with this state and in what respect the condition itself differs in the various animals subject to its peculiar manifestations: a condition provided by nature to tide an animal over a period when its very existence, owing to scarcity of food, becomes too difficult or even impossible to maintain; so by preserving the animal's life allows it to perpetuate its species.
We will now pass on to consider the purely physiological phases and phenomena of this state.
In hibernation all the activities of the body are greatly reduced, the temperature of the animal is lowered and even falls to a point slightly above that of the surrounding media. As it has been pointed out above, animals which hibernate do not belong to any one class, but examples are met with in mammals, reptiles (?), amphibians, insects, molluscæ, but curiously enough, no case is known among birds.
In some cases, previous to entering the hibernating state, the animal stores up food in its den or nest, on which it feeds when it wakes at intervals during its winter sleep. This is hardly pure hibernation, as in the true cases there is a special accumulation of fat in the animal's body before the commencement of the torpid state (the animal not waking to feed) and this serves as food during the hibernating period. A peculiar physiological change is here involved—a herbivorous animal becomes carnivorous, this being caused by the animal living on its own flesh, hence the excretions (small) of the animal become profoundly and completely altered in their chemical characters.
A low temperature is the cause generally assigned for the production of hibernation, but a more careful consideration of the facts long ago showed that cold could not be the sole cause of the phenomenon. Most observers who have worked on the subject have found that extreme cold will not cause an active animal to hibernate; although Saissy has observed that continued cold, and a limited amount of air for respiration caused a marmot to pass into a typical hibernating condition, even in summer. Against this we have Vernon Bailey's experiments with spermophiles (first cousins of the marmots or ground-hogs), which showed that in the case of hibernating animals a few degrees lower temperature changed the torpid state into one of death.
Mangili found that torpid marmots and bats were awakened by exposure to severe cold and that lessened or confined air would not cause hibernation. Dormice have been kept in a warm room throughout the winter and yet they hibernated and were not aroused when the extreme temperature was 20° C. The warmth, however, delayed the onset of torpidity by two months and made it less profound. Again, as has been mentioned before, hibernation may take place in the dry hot season. The supply of food may also be a factor producing this condition, as torpidity in dormice and groundhogs is delayed or prevented when the food supply is plentiful. I question this for the following reasons: Spermophiles and marmots retire for their winter sleep when their food supply is at its best; they only remain active until the full coating of fat is acquired. Here in British Columbia they will retire a month or more earlier in the low lands than they do at the timber line. In the latter regions they have not acquired enough fat until the end of September, as they come out of hibernation later in the spring.
In this connection it is interesting to note the influence of the food supply on man, protectively causing a condition closely resembling hibernation. For instance, there is in Russia a certain class of peasants who suffer from a chronic state of famine which becomes more acute at the end of the year and more or less severe according to circumstances. In these cases, when the head of the family sees, towards the end of autumn, that by a normal consumption of their supply of wheat it will not last the family through the winter, he makes arrangements to diminish the rations as much as possible. Knowing that it will be difficult to preserve their health and keep up the physical force necessary for their work in the spring, he and his family plunge themselves into a condition known as "lejka" which means that everybody simply goes to bed, lying down on the top of the flat stove, and there they stay during the four or five months of winter. They get up, during this time, only to replenish the fire, eat a small piece of black bread and take a small drink of water. The peasant and his family try to move as little as possible and sleep as much as they can—stretched out on the stove top, they preserve almost complete immobility. Their only care during the long winter is to keep down the body metabolism, to waste as little as possible of their animal heat, and for that reason they try to eat and drink less, move less, and to generally reduce the activities of their bodies. Their instinct commands them to sleep as much as possible—obscurity and silence reign in the hut where, in the warmest place, either singly or crowded, the occupants pass the winter season in a condition closely resembling hibernation.
The following observations are purely physiological phenomena, occurring in mammals only.
The frequency of respiration is greatly diminished, the rhythm is irregular and often of the Cheyne-Stokes type. What little respiration (i. e., interchange of gases in the lungs) there is, when the thoracic muscles and diaphragm are not acting (i. e., complete cessation of movement) is maintained principally by the "cardio-pneumatic" movement. A hibernating dormouse may not give a single respiration for ten minutes, then takes from ten to fifteen breaths, at the end of which it again lapses into a state of quiescence for a period of several minutes, when the spasmodic respiratory act again occurs. The same animal, when in a normal waking state, breathes at the rate of eighty or more respirations a minute. Similar results are obtained in other animals. It has been observed that hibernating bats and marmots could be kept for hours in an atmosphere of carbon dioxide gas, without suffering any ill effects, whereas a bird or rat placed in the same chamber died almost at once, thus showing that in the hibernating state the consumption of oxygen is extremely small or in other words very little oxygen is required by the hibernating animal (owing to the quiescence and lowered metabolism of the animal and naturally for the same reasons very little carbon dioxide is given off). Saissy has observed that the amount of oxygen taken in by a dormouse varied according to the activity of the animal, and so in true hibernation the amount of the intake of oxygen is naturally very small.
During hibernation, the force and frequency of the heart-beat is greatly reduced. In the case of the bat and dormouse, it is as low as fourteen or sixteen per minute, while in these animals in the active state it is one hundred and over. Hill and Pembry by applying ato the chest of a hibernating bat, make the observation that no sound of the heart-beat could be heard, I can confirm this observation as applying to the hibernating ground-hog in British Columbia, also, whereas with the animal awake and active, the sounds were so loud that they could be heard distinctly when the ear was an inch away from the animal.
I have found, and can confirm other observers, that the blood during hibernation has an arterial hue (bright red) in the veins, and, on the other hand, Marshall Hall states that it has a venous color in the arteries.
The activities of the digestive organs vary according to the habits of the different animals. Some, much as the dormouse, marmot (?) and hamster, store up food in the autumn which they consume during the winter in their waking intervals. Naturally, then, their digestive organs are intermittently active. In the black bear, however, digestion is completely suspended and his intestines become plugged up with an indigestible mass composed chiefly of pine leaves. This mass is not discharged until the bear wakes in the spring.
It may be well to note here the important part played by the liver during hibernation in maintaining the animal's life; it acts as a storehouse, storing up energy in the form of glycogen, often called animal starch—a substance derived principally from starchy or carbohydrate food. This glycogen is converted in the liver into sugar and poured into the circulation, which carries it to the tissues, where it is consumed during hibernation, as well as in the waking state. The presence of glycogen in the liver-cells of the frog and other animals may actually be seen by the aid of the microscope, immediately before hibernation, and its absence, more or less complete, at the end of this period, demonstrated in the same manner. (This is disputed by Weinland and Richl—they claim that the amount of glycogen in the body remains constant during hibernation.
The excitability of the whole nervous system is greatly depressed. Physiologically, it resembles, as do the other tissues, that of coldblooded animals in general, in that all the tissues (muscular, particularly) retain their excitability for a long time after they are removed from the body.
A warm-blooded animal during hibernation loses all control (reflex) over its temperature-regulating mechanism, and acquires all the characteristics of a cold-blooded organism, that is, instead of its body having a regular, normal and steady temperature, its temperature becomes about the same as the surrounding media, and as this rises or falls, so does that of the animal. On the other hand, by arousing a dormant animal from its stupor, it is possible to make it exercise enough to bring its temperature up to normal, i. e., what it would be in its ordinary walking and active circumstances, or in other words to bring it back to warm-blooded conditions again.
On studying the changes in external temperature, we find that the output of carbon dioxide and the temperature vary with the activity of the animal. If the animal is very active, it responds to a fall in external temperature by more muscular activity, and by this way maintains the normal heat of its body. If, however, this animal is in a sleeping condition and there is a sudden fall in temperature, it somewhat arouses at first, becomes active, and this causes an increase in the output of carbon dioxide, but after a few minutes it coils itself up again and returns to its former somnolent condition, and from which it is not so readily aroused. It has also been observed that when the surrounding temperature has been raised, the temperature of the hibernating animal does not keep much above it, until a point is reached when the animal wakens. Then its temperature rushes up many degrees in a few minutes and at the same time the excretion of carbon dioxide becomes enormously increased.
Immunity and Formation of Antitoxine
Hansmann describes the influence of temperature on the incubation period and the formation of antitoxine. He found much greater resistance to infection and lengthened incubation time and no production of the various antibodies during hibernation. Blanchard and Blatin made the observation that in the hibernating condition the marmot was immune to parasitic maladies.
It may be stated and accepted that when hibernation has been fully investigated, all degrees of cessation of functional activity of the various organs and tissues will be found represented, from the normal sleep of man and other animals to the lowest degree of activity manifested in life. Though some observers claim that in true hibernation there is complete cessation of function in some organs, as, for example, the lungs and movements of respiration, this is extremely doubtful. The awakening of an animal from its winter sleep is never sudden, but slow and gradual, often lasting for hours. This gradation from a passive to an active condition is no doubt protective to the vital machinery, as it has been noticed that when bats have been awakened suddenly they have quickly died.
We have spoken of hibernation in man, and by some authorities, sleep in man is closely allied to a state of hibernation. Natural daily sleep is favored by moderate exhaustion, the cravings of hunger being satisfied, and the absence of all peripheral stimuli. Sleep is a rhythmic diminution of the activities of all the tissues, but especially of the nervous system, which has control of all the others. As we have mentioned before, Marshall Hall and others have shown that the gaseous interchange in a hibernating animal is greatly lessened and so too it is in sleep. It has also been shown by experiments that hibernation, like daily sleep, is not a series of fixed and rigid phenomena, but is varied in depth and in season and its main use is that of protecting and conserving life.
All forms of profound winter and summer sleep are protective, both of the individual and of the species. If it were not for this act of hibernation, many of the mammalia, amphibia, as well as some other groups of animals, would be utterly destroyed from the face of the earth.
- I. e., as in a fully wakened state, quicker and deeper.
- These names—noctule and pipistrelle—are the common names of two species of European bats; the genus Pipistrelle is represented in North America, but only in the extreme southern portion of the United States, by three species and two subspecies. The common bats of eastern North America are the hoary, brown, silvery, red and the little brown bat, all of which are found here in British Columbia except the red variety.
- Bats undoubtedly migrate; see Miller, "North American Fauna," No. 13.
- Old term.
- The "brown bear" is a very loose term—the cinnamon bear is only a color phase of the common black bear. The grizzly may be any color and are subgenerically distinct from the black bear. It is doubtful if any bear is completely torpid during hibernation; the grizzly is much less so than the black bear, the period of hibernation being shorter.
- Hedge-hog; this term hedgehog is confusing; it is the term applied in Maine and New Brunswick to the common porcupine.
- In Canada, both the porcupine and squirrel store up food for winter consumption, rousing themselves at irregular intervals to eat.
- "Our common toad hibernates in the ground, as does the wood frog (Rana sylvaticus) and the small jumping frogs (Hyla). Our newts also hibernate in the ground."—John Burroughs (private communication).
- In the Cheyne-Stokes type of respiration, there is a pause in the respiratory act, then a small respiration occurs, to be followed by a deeper one; then a still deeper act and so on until the maximum is reached, when the respirations begin to gradually diminish until they die away altogether. This is followed by a prolonged pause, then they gradually begin again.
- Cardio-pneumatic movement. Here the visible movements of respiration, dilation and contraction of the thorax, have ceased, but still air (a very small amount) is drawn into and expelled from the lungs. This is due to the heart's action, it also being contained in the thoracic cavity, hence its contraction and dilation so alters the pressure in the thorax that an interchange of respiratory gases is produced in the lungs.