For centuries, naturalists pondered how a warm-blooded creature could descend into a near-death winter state and surface unscathed the following spring. Ian Morton investigates the mysteries of how animals and insects hibernate, and ponders the biggest question it prompts: could humans follow suit? And what would happen if we could?

The Greeks wrote about it in the 3rd century BC. Medieval curiosity surfaced in the mid 16th century in the Swiss bestiary of Conrad Gessner. And, from the mid 1700s onwards, as folk remedy gave way to an emerging science, scores of men of medicine in Europe and the USA devoted themselves to experiment and theory concerning what they perceived to be the mystery of the living dead in the Animal Kingdom.

In 1916, Andrew T. Rasmussen of Cornell University trawled more than 80 academic papers covering the previous 170 years and sifted the varied opinions of some 60 long-forgotten physiologists – a bewildering display of groping for truth, guesswork, claim and counter-claim by reputable members of the new scientific age. Rasmussen dismissed it all as proof of ‘the most earnest and often tedious experimentation and observations of several generations having shed but little light on the factors and mechanism involved’.

A hibernating dormouse

A hibernating dormouse

In the midst of it all, the word hibernation is said to have appeared first in print in 1816, attributed to German naturalist and philosopher Lorenz Oken. It’s odd that he came to coin an English word lifted directly from the Latin hibernatum, when his own language described the condition most adequately as Winterschlaf, but hibernation was in good company in this vintage year for neology. Other terms given their literary baptism in 1816 included Double Gloucester, cleavage, obstetrics, juvenile delinquency, uptake and squiggle.

It’s now clear that certain warm-blooded animals hibernate because their food supply will no longer be available. Body temperature falls, breathing becomes slow and shallow, the heart rate reduces and the metabolism is automatically suppressed. Four bear species offer kingsize evidence in other climes, but, in the UK, we have three modest mammals that shut down for the winter.

The dormouse doubles its weight through the warm weather, weaves a small ground-level nest in which to sleep from October to April or May, slows its heartbeat, lowers its temperature to just above freezing and reduces its metabolism by some 90%, leaving predators little to go on.

The hedgehog likewise gorges, finds a secure nook in woodland litter or hedge bottom, reduces its heart rate from 190 to 20 beats a minute and allows its body heat to fall from the 35˚C norm to match the ambient temperature, surviving thus from November to mid March.

Bats find humid spaces in hollow trees, roof spaces, cellars and caves, managing on five breaths and 20 heartbeats a minute. They may wake and fly briefly; other hibernators may die if disturbed.

Many insects retreat to sheltered spaces to see out the winter, the flying varieties producing glycol to lower their fluid freezing point, notably butterflies – our peacock, small tortoiseshell, comma and brimstone, that happy harbinger of spring. They may respond to a warm winter’s day, but will return to hibernation.

Ladybirds creep into cracks – be that in trees, windowsills or door frames – and may huddle in groups. Fertile young bumblebee queens, fattened in the nest, wriggle away to hibernate alone, leaving the rest to die off.

Hibernating ladybirds in a hollow plant stem

Wasps and hornets follow the same pragmatic pattern. By contrast, hive bees stay active, sustained by honey. Clustering around the queen, they keep her temperature at about 35˚C, shuffling and rotating inward and outward like emperor penguins to give all a fair chance.

However, it’s the hibernation of mammals that engages physiologists today, especially the notion that the processes might be harnessed for human benefit. Experiments in Seattle with hydrogen sulphide have induced six hours of hibernation in mice without harm (‘Mice On Ice’, in one irreverent publication).

‘We are, in essence, temporarily converting mice from warm-blooded to cold-blooded creatures, which is exactly the same thing that happens naturally when mammals hibernate,’ explains cell biologist Mark Roth, lead researcher at the Fred Hutchinson Cancer Research Center. ‘We think this may be a latent ability that all mammals have, potentially even human beings.’ He has cited instances in which individuals frozen in accidents were revived without apparent ill effect.

Dr Roth foresees real medical significance here. In due course, seriously ill and badly injured people might be put into a state of induced hibernation until proper treatment reaches them, he suggests. He described to a recent USA neuroscience symposium how his team discovered a set of three adjoining elements on the periodic table, primordial antioxidants with the properties of causing reversible changes to reduce animation.

A hedgehog shelter on an RSPB reserve

 

Clinical application of induced metabolic hibernation could include treatment of severe blood-loss injury, hypothermia, malignant fever, cardiac arrest and stroke, he declared. Further, it could enhance cancer treatment by allowing tolerance of higher levels of radiation without damage to healthy tissue. It might even prolong the shelf life of organs for transplant and buy time for the patients awaiting them (there are some 6,400 in the UK alone and three die every day).

According to Dr Sanjay Gupta of the Department of Neurosurgery at Emory University School of Medicine, Atlanta, USA, there already exists the human equivalent of hibernation, which he calls ‘a grey zone, a faint no-man’s land where you are neither truly dead nor actually alive’. Induced by accidental or therapeutic hypothermia, it has saved individuals who would otherwise have died of a cardiac arrest on a gym treadmill or behind the wheel of their car.

The outstanding example is Swedish skier Anna Bågenholm. In 1999, she spent 80 minutes trapped in freezing water beneath 8in of ice, her body temperature falling to 13.7˚C, before she was freed and flown to Tromsø University Hospital. Staff worked on her survival for nine hours and her recovery over many months, the process a gradual re-warming to avoid cellular destruction.

A medical advance with monumental promise in trauma situations, therapeutic hypothermia doesn’t yet replicate the easy return of Nature’s hibernators to full activity. However, a longer-term vision was posed by Kelly Drew of the Institute of Arctic Biology at Fairbanks University, Alaska. She told symposium delegates that all mammals, even humans, might share the genes and hence the capacity to actually to hibernate: ‘Better understanding of the entire suite of autonomic nervous system control of hibernation will enhance the feasibility that humans will someday hibernate.’

The notion that astronauts and space travellers may, by this means, overcome the time taken to reach distant worlds excites visionaries and, if and when science fiction becomes space fact, mankind may have cause to salute those creatures that take a protracted winter snooze. If the human race must quit this wilting planet for a friendly star, the British contingent should take breeding stocks of dormice, hedgehogs, bats and butterflies. All hibernating, of course.