Hiber nation: understanding how creatures put their lives on hold could yield therapies for a variety of human ills

Natural History, Dec, 2007 by Olivia Judson

As the nights close in and winter takes hold, I get a hankering to burrow into my duvet, curl up, and hibernate until spring. But alas, humans are not among the large and diverse group of animals that can put themselves on hold for a few months. At least, not yet.

Human hibernation is a hot topic, as I discovered one afternoon when I happened across the Journal of British Interplanetary Science. Space agencies are interested because the ability to hibernate on demand would come in handy on long-haul space flights. The immortality crowd is interested too: if you've got an incurable disease or simply won't settle for an 80-year-life span, wouldn't it be great if you could put your head down, catch forty million winks, and wake up when medical science catches up? But the applications aren't all so futuristic; some are much closer to home. For instance, Matthew T. Andrews, a biologist at the University of Minnesota in Duluth, foresees that discoveries from hibernation biology will be useful in treating everything from heart conditions and hypothermia to obesity. Indeed, writing earlier this year in the journal BioEssays, he argued that "there is tremendous potential for applying hibernation strategies to improve the human condition." Gosh. I threw off my duvet and went to investigate further.

First off, I discovered that hibernation takes different forms in different animals. A black bear, for example, drops its body temperature by only a few degrees, and spends the winter in a kind of deep and continuous sleep. During that time, it neither urinates nor defecates. For small mammals such as bats and ground squirrels, in contrast, hibernation typically features profound drops in body temperature, during which the animal is inactive, punctuated by regular bouts of warming up to normal and rousing into activity for some hours. The Arctic ground squirrel in particular may be the most extreme case. During regular life, its core body temperature, like ours, hovers around 98.6 degrees Fahrenheit. But during hibernation, its core body temperature can actually fall below freezing, to as little as 26.8 degrees, for days at a time. How do Arctic ground squirrels manage that? They supercool.

Supercooling is what happens when the temperature of a liquid falls below its freezing point yet doesn't freeze. That can happen if a liquid has no nucleating agents--no particles around which crystals can form. But add a particle--a piece of ice, say--to a cup of supercooled water, and pow! The entire cup of water will freeze instantly. Being able to super-cool is rare among mammals--but popular among insects. In Alaska, yellowjacket queens of the species Vespula vulgaris survive the cold, hanging by their mandibles for nine months in a dry, snow-free cavity, by allowing the fluids in their bodies to supercool. A supercooled yellow-jacket can drop her temperature to as low as 3 degrees, but rather miraculously avoids turning into a waspsicle. Bring her into contact with snow, however, and again, pow! She freezes solid--which kills her. Other insects can get even cooler: the beetle Rhagium inquisitor, otherwise known as the ribbed pine borer, can supercool to -24 degrees. Is a supercooled Arctic ground squirrel at risk of freezing solid? It's possible, but unlikely. Its skin would have to be pierced by an icicle or something like that.

HIBERNATION, ON MANY LEVELS, looks passive, almost like a temporary death. In a small mammal, the body is cold, the heart rarely beats, the animal scarcely breathes. A hibernating little brown bat, for example, may take a breath less often than once an hour. The cellular machinery shuts down, too: little DNA is copied, few proteins are made. But hibernation is far from being a full suspended animation. For one thing, many hibernating animals remain alert to unusual noises or disturbances. Cough loudly in a cave full of hibernating bats, and they will start to rouse. Stride into a bear's den, and the bear will wake up--and it probably won't be pleased to see you. Moreover, hibernation is tightly regulated. Ground squirrels move in and out of hibernation with clockwork precision--somehow, they measure time--and their body temperatures are always kept a bit higher than the temperature of their dens.

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Hibernation seems to have evolved as a way to save energy when food is scarce, rather than a way to bypass winter months. That is why it isn't found only in cold climates. For instance, the fat-tailed dwarf lemur--a small primate that lives in Madagascar--hibernates to escape not cold, but drought. The animal beds down in a treehole, often snuggled with its mate and perhaps a couple of their offspring, for as long as seven months, even though the outside air can reach a balmy 85 degrees.

But cold climates do encourage energy-saving methods. Mammals maintain their high body temperatures by burning fuel, and it costs more to stay warm when the difference between the usual body temperature and the outside air is large. It costs more for a small mammal to adjust to dropping temperatures than for a big one (smaller animals lose heat faster). In short, it is hard for an animal the size of a mouse to stay warm when the weather is below freezing for weeks on end; thus, lowering the body's thermostat saves on heating bills. So it's all the more mysterious that ground squirrels bother to warm up every ten days or so. Warming up is expensive. Indeed, that's the main energetic drain of hibernating.