Deep space travel seems a distant prospect, not least for a man being lowered by biologists head-first into a bear's den, armed with nothing but a syringe of tranquilliser on a pole. But thanks to that brave bear-catcher from the Alaska Department of Fish and Game, we know there's more to bear hibernation than we suspected – and further study of sleeping bears may help humans cope with long-haul trips to destinations beyond the moon.
American black bears (Ursus americanus) usually hibernate for five to seven months in winter and although they may rise once or twice a day to shift position or groom themselves, they never once eat, drink, urinate or defecate.
Keen to find out why the hibernating bears don't do the proverbial in the woods, Craig Heller of Stanford University, California, and colleagues enlisted the help of the Alaska Department of Fish and Game to catch some of the state's "nuisance bears" – those which have invaded one too many campgrounds. Then they took them to the Institute of Arctic Biology at the University of Alaska Fairbanks for study.
Let sleeping bears lie
The researchers surgically implanted radio devices to monitor the bears' core body temperatures, heartbeat patterns and brain activity as the animals hibernated in artificial dens. These dens also carried monitors to record the bears' oxygen intake, as a measure of metabolism.
As expected, the bears' metabolism plummeted, dropping 25 per cent below normal. But surprisingly, their body temperature – which is normally 37 or 38 °C – hardly changed: it dipped to an average of 33 °C.
"We expected that the reduction in metabolism would parallel a reduction in temperature, which is true for small hibernators," says Heller. "In this case it appeared that a large portion of the drop in metabolism is independent of the drop in temperature. What that means is there is a biochemical mechanism suppressing metabolism."
Exactly what biochemical spell bears cast to shut down their metabolism without going stone cold is still anyone's guess. Most obviously, bears are large and well-insulated animals with a low surface-to-volume ratio, so they simply do not lose as much heat as smaller hibernators, says Heller.
Peter Klopfer, a biologist at Duke University in Durham, North Carolina, adds that following the cycle of heating and cooling typical of smaller hibernating mammals is harder for the average bear.
"To take a crude metaphor: if you have a hot tub in a cold climate, in most cases it is cheaper to keep it warm, rather than cool it down and heat it up again," he says.
Analysis of the bears' brain activity – which has not yet been published – yielded two main findings. First, circadian rhythms were suppressed, which would allow the bears to slip out of their usual daily routines and into a more permanent slumber. Secondly, unlike small hibernators – in which REM sleep disappears when body temperature drops during hibernation – the bears maintained typical oscillations between REM and non-REM sleep.
Learning the secrets of hibernating animals could benefit people. Emergency doctors, for instance, might be able to safely induce a kind of suspended animation in seriously ill or injured people for whom medical treatment is not immediately available. Induced hibernation could also improve the health of people who must remain sedentary for long periods of time – during long periods of space travel, for instance.
"During their seven months of inactivity the bears have no muscle or bone loss, which is remarkable," says Heller. "People lose muscle or bone if we are incapacitated or involved in space travel. We've just got to figure out what the bears are doing."