Hibernation is a dormant condition in which many warm-blooded animals pass through a period of harsh environmental conditions, usually winter. Although a common biological occurrence, many aspects of it are still not understood.
A full understanding of animal hibernation might help scientists develop methods of suspended animation for people during future deep-space travel, or, more mundanely, to allow human organs remain well-preserved in long-term storage while awaiting use in transplantation.
Hibernation has been noted in hundreds of species of mammals - for example squirrels, woodchucks, hamsters, hedgehogs, bats, and bears - primarily in the arctic and temperate zones.
Some desert-living species disappear during the driest, hottest months, probably to hibernate in their cool burrows.
The practice evolved as a strategy that allows animals to avoid the very harsh conditions they would otherwise face if they had to remain active and predatory.
The problems that hibernation usually allows an animal to circumnavigate are winter famine and the high-energy costs of maintaining normal body temperature during freezing periods.
During hibernation the animal's general rate of metabolism drops, as does its temperature, heart rate and breathing rate.
The ground squirrel is a well-known hibernator in temperate zones. During hibernation its metabolic (body chemistry) rate drops about 10 per cent below normal; its heart rate drops to 10-20 beats per minute from 200-300 beats per minute; and it breathes only four times per minute instead of 100-200 times.
Hibernating animals store enough food in their bodies to fuel their metabolism over the hibernation period.
Hibernation is always accompanied by a very significant drop in body temperature. In this respect, the record is held by the arctic ground squirrel. The average squirrel-burrow temperature during hibernation is 14 Fahrenheit, and it can drop as low as - 13 Fahrenheit.
During hibernation the squirrel body temperature cools to 26.8 Fahrenheit, below the freezing point of water (32 Fahrenheit). This is the lowest body temperature naturally attained by any mammal or bird.
The arctic ground squirrel performs this impressive cooling feat by exploiting a capability displayed by water called supercooling. If you take some water, cool it slowly and keep it very still, it may remain in the liquid state as its temperature drops below its normal freezing temperature. But ice will instantly form if you contaminate or disturb the water.
For example, a contaminating speck of dust acts as a nucleating site on which ice can form, and this ice in turn acts as a template for more ice to form and spread throughout the bulk of the liquid.
Experiments at the Institute of Arctic Biology, University of Alaska, carried out by Brian Barnes and co-workers, have shown that blood plasma taken from arctic ground squirrels during their active summer months freezes at a temperature about 7 higher than the plasma taken from winter-hibernating squirrels.
Evidently the summer blood contains a substance that nucleates ice-crystal formation on cooling, a substance that is not present in the winter blood.
Cold-blooded animals that cannot generate their own body heat use special mechanisms to prevent themselves from freezing solid during very cold winters.
For example, certain Antarctic fish make a special biological anti-freeze to protect themselves. Many insects concentrate glycerol, a normal body chemical, in their body fluids, up to a concentration of 30 per cent, which prevents them freezing solid in winter.
Hibernation is not one long uninterrupted deep, cold sleep (indeed, it may not be sleep at all). For example, some species may awaken on warmer days and feed. Bats awaken and copulate, and bears sometimes give birth.
Apart from the examples of some species occasionally getting themselves into full activity, all hibernating species periodically warm up for a short while over the hibernation period and then sink back into torpor. This periodic warming uses up a lot of precious energy reserves, so it must fulfil a vital function, but its precise nature is unknown.
The periodic warming of hibernating animals may be necessary to excrete some toxic chemical that has built up or to carry out other occasional necessary metabolic readjustments.
Barnes and co-workers make the interesting proposal that the animals warm up to sleep. They claim very cold brains are unable to sleep, and sleep is probably so important it is essential for the very cold hibernating animal to warm up now and then to grab a nap. When hibernating ground squirrels periodically rewarm and rouse themselves, they spend most of the time asleep before sinking back again into torpor.
IF HUMANS are ever to travel significant distances into space we will have to solve the travel-time problem. Our closest star-neighbour is 4.3 light years away, i.e., even travelling at the speed of light, which is held impossible, it would take an astronaut 4.3 years to get there. The nearest interesting location might be hundreds of light-years away, which is still very close.
Obviously, humans could not travel such distances unless they could avail of a hibernation mechanism that suspends the normal ageing process.
A full understanding of animal hibernation may allow us to develop a method of suspended animation for humans.
A more immediate application of studying animal hibernation is that it may allow us to greatly prolong the storage life of organs awaiting transplantation.
At the moment such organs often go bad in storage while awaiting a suitably-matched recipient. Experiments have shown that the storage life of isolated lungs, kidneys and hearts can be extended three-fold compared to standard preservation methods by treating them with blood plasma drawn from deeply hibernating animals.
William J. Reville is a senior lecturer in Biochemistry and Director of Microscopy at UCC.