Hibernation or ‘winter sleep’ how and why it happens is one of the mysteries of nature that has fascinated observers since the time of Aristotle. It occurs in some form, to some degree, in all vertebrates except fish, if we define hibernation, for the time being, as the act of resting in a dormant state in a protected burrow.

Hibernators are intensely active in summer. A part of that activity is building temporary burrows for summer use only. Ground squirrels, (Spermophilus citellus) for example, can build hundreds of such temporary shelters over a single summer, on average 15 shelters in a 10m2 area of open field. These shelters have only the one nest compartment, usally between 30 and 50cm below ground level. By contrast, their shelters for hibernation, sometimes as much as 3m below ground level, are intended for long, repeated use and contain a number of compartments-one for storing large quantites of food (usally dry seeds), one for use as a ‘toilet’, and a third for sleeping.

Before hibernation, animals prepare themselves for the hardship of the very long period of cold by storing up large amounts of fat in adipose tissue under the skin, up to some 40 % of total body weight. These fats are composed of fatty acids which typically have ten or fewer than ten carbon atoms and are consturcted with some double bonds between the carbon atoms of the chain and their esters with glycerides. These compounds provide the ideal energy source needed during hibernation because lipids yield twice the energy yielded by carbohydrates and proteins. Moreover, these fatty acids need very little oxygen in their degradation/conversion to energy.

Blood circulation and homeostasis during hibernation are not well understood. The animal steadies body temperature at around 2°C to 5°C: in mammals the body temperature remains about 1°C above environmental temperature. Usally, when body temperature falls to this level, metabolic rate is increased or the animal awakes, but during hibernation this does not happen. Metabolic rate at 5°C is usually 2-5% of the rate at normal body temperature. For example, the active heart rate of the genus Myotis of bats is between 500 and 700 beats per minute. During hibernation the rate goes down to 20 beats per minute at 5°C, and 8 beats at -7°C. Two consistent and characteristic changes are found in blood during hibernation: an increased production of herapin, which may be contributing to a reduction in the risk of blood clotting during very slow circulation, and an increase in serum magnesium, for which there is no explanation as yet.

Many different types of respiratory patterns have been observed during the state of hibernation. We will mention only one example here: The hedgehog, at a body temperature of 5°C, does not breathe at all for an amazing 56 minutes.

In controlled observations, it has been found that animals which hibernate show improved retention of learned behaviour compared to non-hibernating animals. Again, it is not at all clear why this should be so.

Hibernation is not a prolonged period of constant torpor. There are periodic arousals during winter caused by the accumulation of metabolic end products or having a full bladder. The awakening process is often assisted by shivering, especially when the body temperature is very low. Awakening is a costly process because it takes as much energy to wake up as it does to stay in hibernation for ten days. The ground squirrel, Spermophilus citellus gains around 150-200 g of fat before hibernation. That is more than enough for the energy being used up during sleep; the excess is needed for the wakening up intervals which occur fortnightly.

A number of hypotheses have sought to explain how hibernation is triggered-changes in weather and climate, temperature, humidity and change of diet are among the suggestions. Apart from these causes, a protein was isolated from the blood of a hibernator bear in the USA in the 1980s which, when injected into rats appeared to induce sleeping behaviour in summer. Today it remains uncertain if this protein is the only stimulator of hibernation. If it is, we still need to know what other conditions are related to the protein level and its effect and how the level of protein is maintained at the right level during the animal’s life cycle.

What is already securely known about hibernation establishes it as a truly amazing physiological phenomenon. It tells us that the body temperature of some hibernators will passively adjust ambient tempeature from between 2°C to 32°C without causing awakening. The inevitable question is what advantage such behaviour affords the hibernating animals. Some small animals, because of their high metabolic rates are faced with an acute need for a continuously available supply of food and water. Controlled investigations show that dormant animals at cool temperatures lose much less weight than the non-dormant ones. It has also been shown that small animals can survive for at least a hundred days on the energy derived from ten grams of fat. Hibernation is, in other words, a survival technique, an adaptation to the conditions of poor or non-existent food supply during the winter months.

That is, however, something of a mechanical explanation which, even as a mechanical explanation, is far from satisfactory. The secondary question immediately arises of why this particular adaptation and not another-why not migration, for example, to areas where winter does not affect food supply so drastically? There are many birds and other animals which take this option.

A more satisfying explanation must surely consider what adaptability itself is, how it relates to the variety of life-forms, to the individuation of species and kinds, and to the overwhelming intution (which must touch any truly objective observer) that, at levels of subtlety and intricacy which defy comprehension, the survival and provision of each and every living form is minutely arranged and co-ordinated to create a whole that is thoroughly interconnected. The value of that whole is manifested in many different aspects-beauty, variety, efficiency, the rich warmth of life. Is it not impossible to resist the impression of a wonderful generosity within and behind the world of living forms?