杏吧原创

SO WHAT lucky creatures will manage to miss the nightmare of Christmas
shopping, office parties and family festivities this year? Hundreds of mammalian
species from six orders that鈥檚 who, not to mention thousands of different
cold-blooded insects, fish, amphibians and reptiles. Ensconced in burrows,
curled up in corners and at the bottom of ponds, animals all over the world drop
their body temperature and play dead. But how, oh how, do they chill out so
brilliantly鈥攁nd is there any chance that we humans might ever be able to
do it too?

It鈥檚 not just me who鈥檚 tempted. The oblivion of prolonged sleep, of
hibernation, is an enduring human fantasy from the suspended animation travel of
today鈥檚 space fiction heroes to the old legends of knights who slept under
mountains for centuries, awaiting calls to arise and save their nations.

The bad news for human wannabes is that there鈥檚 more to winter torpor than
the will to sleep. To begin with, no one knows exactly what triggers
hibernation. In the 1960s, scientists sought an elusive bloodborne factor,
transfusing the blood of hibernating bears into monkeys in the hope of sending
them into an instant sleep. But the research failed and was eventually
abandoned.

Then there is the little weight issue. Serious contenders for hibernation
need to be remarkably fat. Take the Arctic ground squirrel, an inhabitant of the
Alaskan tundra and the world鈥檚 most northerly hibernator. It鈥檚 a common,
unassuming rodent known locally as a sik-sik, after one of its alarm calls. And
it hibernates for eight months, sealed in its underground hibernaculum in the
pitch dark of the permafrost, eating nought. To survive that ordeal the sik-sik
is 50 per cent fat when it waddles off to its winter retreat in August, says
Brian Barnes, an animal physiologist at the Institute of Arctic Biology at the
University of Alaska in Fairbanks.

In North America, golden-mantled ground squirrels go on binges that last six
weeks, eating at twice their normal rate and boosting their body fat by more
than a third. Remember, you鈥檙e not going to eat a thing for months on end; your
body will survive solely on its fat stores. So if you鈥檙e fashionably thin to
start with, you won鈥檛 make it through the winter.

And quality, not just quantity, matters too. Saturated fats stay fluid only
at normal body temperatures鈥攎utton fat congeals on your plate even at room
temperature. And no mammal can make polyunsaturated fats from scratch. According
to Caroline Pond, a zoologist at the Open University in Milton Keynes, a
successful hibernator must tuck into a diet rich in polyunsaturated fats if it
is to keep its lipids mobile, and its stored energy accessible when body
temperature drops. In the autumn, a wise sik-sik gorges on seeds that contain
polyunsaturated fats.

Likewise, the human with aspirations to hibernate should avoid roast beef,
mutton, cheese, cream and mince pies made with suet, and feast on Yuletide nuts
instead, one of the most concentrated sources of the hibernator鈥檚 friend, the
polyunsaturated fat linoleic acid.

Prince Charming

Ecologist Craig Frank of the University of California at Irvine has recently
shown that the golden-mantled ground squirrel鈥檚 natural autumnal diet rich in
nuts and seeds provides the optimal level of linoleic acid for proper
hibernation. 鈥淎 major limitation on hibernation may be obtaining enough linoleic
acid in the diet,鈥� he says.

But size is important too, and our bulk goes against us. In a creature of
small mass, every point of the body is close to the exterior, and so the core
can cool or warm rapidly in just a few hours. Humans are just too big to be
efficient thermal manipulators. If Sleeping Beauty had been hibernating, Prince
Charming probably would have lost interest by the time she was warm enough to
function. True, brown bears become dormant after a fashion and may fast for long
periods over winter, yet their body temperature drops by only a few degrees.
Most 鈥渞eal鈥� mammalian hibernators are lightweights, ranging from the 20-gram
California pocket mouse to the 3-kilogram marmot.

Reheating a 70-kilogram human would eat up energy, and we aren鈥檛 naturally
equipped for the task. Chief among our failings is a paucity of brown fat,
nature鈥檚 hot water bottle. This peculiar kind of fat has specialised
mitochondria that break down fats and glucose to churn out heat rather than the
energy-rich chemical ATP. These cellular powerstations contain so-called
uncoupling proteins that shunt the energy released from body fuels into a
heat-generating pathway. Like many hibernators, European hedgehogs come fitted
with 鈥渁 vest-like arrangement of brown fat, strategically placed to dispense
heat to the heart and central nervous system,鈥� says zoologist Nigel Reeve of the
Roehampton Institute in London.

The intrepid sik-sik has yet another trick up its adipose tissue, as Barnes
and his colleagues have just discovered. They stuck thermal probes down
squirrels鈥� burrows and found that in the height of winter in northern Alaska,
the animals experience the coldest burrows of any hibernator, with temperatures
as low as 鈥�25 掳C. To prevent themselves freezing, these rodents must
generate heat at a substantial rate, and they have evolved a novel way of doing
just that.

This year, Barnes and molecular biologist Bert Boyer discovered that variants
of the mitochondrial 鈥渦ncoupling鈥� protein are active in the hibernating
squirrel鈥檚 white fat and skeletal muscles, as well as in its brown fat. 鈥淎ll
three uncoupling proteins may be activated to keep the pipes from freezing,鈥�
says Boyer.

Yet even with these intriguing physiological extras, 鈥渢hese squirrels cool to
the lowest body temperature ever measured for a mammal,鈥� says Barnes. The
sik-sik鈥檚 body temperature drops as low as 鈥�3 掳C in the depth of
winter. At this point, the animal becomes supercooled鈥攂elow freezing point
but not frozen. It鈥檚 a brilliant but risky strategy.

鈥淚f you break the skin and poke them with an icicle, they will freeze
catastrophically and die,鈥� says Barnes. In fact, if any solid particle makes
contact with their blood it suddenly 鈥渘ucleates鈥�, an ice embryo forms and
spreads rapidly, crystallising the rest of the water in the body. 鈥淲e think they
supercool themselves by purging something from their blood that would otherwise
cause nucleation,鈥� says Barnes. 鈥淏ut we don鈥檛 know what that something is.鈥�

There are alternative ways to stop the pipes freezing and the cells bursting.
Rather than go to the trouble of supercooling, cold-blooded hibernators such as
tree frogs and Antarctic fishes use an anti-freeze. They accumulate small
molecules such as glucose in their blood, depressing their freezing point. And
some overwintering insects allow themselves to freeze, but not before
accumulating cryoprotectants such as glycerol in their bodies, which help to
protect cells from damage during freezing.

But other insects鈥攏otably yellowjack wasps in Alaska鈥攐pt for
supercooling. Their temperature drops to 鈥�16 掳 C, and once again they
must avoid contact with nucleating particles. If scientists artificially cool
them in contact with snow crystals, the insects can cool only to 鈥�5 掳C
before they freeze and perish. So for the wild wasp looking for somewhere to
hibernate, it is crucial to find a place that is dry, says Barnes. In rare
sightings, he has found supercooled hibernating yellowjack queens hanging by
their mouths in tiny underground chambers, or beneath fallen leaves in a birch
forest, carefully suspended so as to surround themselves with dry air.

Brain dead

Why do these animals make themselves vulnerable to death by ice crystal?
Energy economy is the name of the game. 鈥淭he fuel required just to stay alive is
not trivial,鈥� says Bernd Heinrich, professor of zoology at the University of
Vermont in Burlington. Every 10 掳C drop in body temperature halves the rate
at which energy is used simply to keep things ticking over, and supercooling
allows an animal to drop its temperature lower than an antifreeze or a
cryoprotectant alone. If the pickings are slim, best to shut down and wait for
better times. Hibernation is essentially 鈥渁 strategy for surviving famine, not
cold鈥�, says Barnes.

Yet like all other mammalian hibernators, arctic ground squirrels cannot
remain in hibernation for more than about three weeks at a time. They repeatedly
use substantial amounts of energy to warm all the way back up to 3 掳C, for
just half a day or so. But they don鈥檛 go out, and they don鈥檛 eat. Then, after a
few hours, they recool themselves. They repeat this pattern over and over again
during their eight-month stint down the burrow, and consume as much as 80 per
cent of their fat reserves in the process. Why do they do it?

鈥淢ost hibernation biologists assume that animals rewarm because they
must鈥攕omething goes wrong at low body temperatures that takes rewarming to
fix,鈥� says Barnes. No one yet knows what that something is, but Barnes has a
theory: 鈥淢ost people think animals are asleep when hibernating, but we think
their brains are too cool to sleep.鈥� Electrical recordings of the brains of
hibernating squirrels, carried out with Serge Daan of the University of
Groningen in the Netherlands, back up the idea. 鈥淚n deep hibernation, the EEG is
essentially flat, a condition in which a human would be considered brain dead,鈥�
says Barnes.

鈥淗ibernating animals become sleep deprived,鈥� he suspects. 鈥淲e think this
somehow generates a pressure to bring the brain back up to functioning
temperatures, so the animals can sleep it off. Once rested, they recool.鈥� No
natural hibernator has evolved a way of avoiding this need to rewarm every few
weeks.

So much for human space travellers in suspended animation鈥攏ot much good
if you鈥檇 have to get up every three weeks. But wait, three weeks鈥�that would
just see me through Christmas and New Year鈥�

* * *

Help a hibernator

FANCY providing a home for a hibernator this winter? A small Germany company
has become the world鈥檚 estate agents for wildlife. More than 8 million Schwegler
homes are now hanging in gardens, woods and orchards all over Europe and North
America. Purpose-built accommodation is available for hibernating bats and
hedgehogs as well as beneficial insects such as lacewings.

Made of 鈥渨oodcrete鈥�, a rot-proof mixture of pine sawdust, burnt clay and
concrete invented as wood and steel shortages hit Germany after the Second World
War, these wildlife refuges are warmer than wooden houses and less prone to
condensation. 鈥淭his makes them particularly good for hibernating animals,鈥� says
Chris Mead of the British Trust for Ornithology, who has tested a range of
Schwegler homes in his own garden.

While Americans, closely followed by Britons, delight in feeding wildlife
visitors in their gardens, Germans have long preferred to provide housing. 鈥淲e
may now be witnessing a crossover, as feeders become more popular on the
continent and Britons take to nestboxes,鈥� says Graham Evans of Jacobi Jayne, the
British importers of Schwegler nature products.

For details contact Jacobi Jayne & Co, +44 (0) 01227 860388, fax +44 (0)
1227 860521, e-mail enquires@jacobijayne.com http://birdcare.com