Settling into one of the harshest winters in memory, most of us would probably rather escape to the tropics than hibernate in a makeshift den. But our hibernating bear brethren stand to teach us quite a lot about the physiology of super-low metabolism.
Although hibernating bears hardly conjure visions of sci-fi starships on their way to warp speed, hibernation may be a way to preserve both crew and resources during long space voyages.
In fact, hibernation research can offer both real-time benefits to the medical community while building on the skill set necessary to enable human hibernation on our journey to the stars.
But first, we have to figure out the fine points of just how bears hibernate.
Bears are thought to have evolved largely in Europe, Russia and Asia. But some 15,000 years ago, they crossed the Bering Land Bridge into what is now North America. As the ice receded from the last major ice age, they gradually expanded their range east and south across the North American continent.
Somewhere along the way, however, their internal clocks told them it was better to sleep than try to find food all winter. And, to their credit, various species of bears began negotiating winter by going into a hypo-metabolic torpor, or as we call it, hibernation.
Now, researchers have turned their sights on various species of North American bears to try and learn how these animals survive winter by hibernating — sometimes as long as eight months — without defecating, urinating, eating or drinking. They rely on their existing collected body fat as their sole source of food and water.
A large part of the current research is aimed at understanding the timetable for the onset of hibernation.
In summer, bears are diurnal; rising at dawn and bedding down at dusk. And while some bears begin hibernating as early as the beginning of October, more commonly they don't "turn in" until November.
"Bears come out of hibernation in April, maybe as late as May," explained Washington State University wildlife biologist Charlie Robbins, part of a WSU group in Pullman, Wash., that's studying how bears adapt to hibernation. "They breed in May and June; then starting in August, they become food focused. If they're on a salmon stream they can accumulate fat real quickly."
Hibernating bears may also provide clues to reversing the scourge of human obesity. For instance, bears go into hibernation obese, notes Heiko Jansen, a neuroscientist at WSU, but during this sedentary phase, they don't appear to suffer from diabetes, muscle loss or bone loss the way humans do.
Hibernation could also be used to prevent muscle and bone atrophy in convalescents who are subject to long bed rest. Or it might be used to avoid excessive internal bleeding in stroke victims or in patients suffering from brain injuries.
And because bears hibernate at higher temperatures than smaller mammals, reproducing "bear-type" dormancy in humans should be much safer than attempting to induce human hibernation at temperatures just above freezing.
However, WSU veterinary cardiologist Lynne Nelson is interested in how the bears are able to slow their heart rates down during hibernation. For humans, heart rates under 50 beats per minute (bpm) qualify as dangerously abnormal. However, during hibernation, bears slow their hearts to as low as 16 to 17 beats per minute, down from 70 to 90 bpm in summer.
During hibernation, two of their four heart chambers more or less go to sleep, says Nelson, whose own work is based on a WSU research colony of brown bears (and which was featured on TV's "Animal Planet."). Their atrial heart chambers go to sleep while their ventricular heart chambers continue to function, she notes. And, amazingly, these particular WSU bears don't seem to suffer any ill effects from running on only two cylinders.
"They don't have any cardiac enlargement or overt muscle changes that we can see with ultrasound," said Nelson. "They do lose some of their heart muscle mass in hibernation, but they regain it before they start to eat in the spring."
Contrary to popular belief, bears do not sleep continuously during hibernation. They are active almost every day, says Jansen, although not to the same extent as at other times of the year.
Yet as Jansen notes, the bears do get up and move around on their den beds before laying back down. Female bears even give birth and nurse cubs during the hibernation period. But their body temperatures are typically three to five degrees cooler than normal and when they do move, they tend to do so as if they are still in a somnolent torpor.
Researchers are still debating whether or not there is a chemical trigger that induces hibernation or whether the onset of hibernation is related to length of day, diminished food supplies and/or cooler autumn temperatures.
Wildlife ecologist Eric Hellgren, at Southern Illinois University in Carbondale, personally believes that length of day is the primary hibernation trigger. If the bears' food were to be shut off during the middle of summer, he says, they still would not hibernate.
But thus far, no one has isolated the chemical inducer of hibernation in bears or any other species. In fact, Jansen says that their WSU veterinary research lab remains among the few actively trying to characterize bears' endocrine (hormonal) profiles.
"We're interested in looking at the peptide leptin [an amino acid hormone compound]," he said. "In most of us, leptin levels are proportional to the amount of fat we have. This molecule may act as a trigger to tell the bear when to stop eating and enter hibernation. Perhaps it has other roles as well."
Jansen says leptin may also be a way to keep a hibernating human heart beating in a way that is physiologically normal.
People who are bedridden or in space for long periods suffer both muscle atrophy and bone loss, said Hellgren; hibernating bears' aren't as susceptible to such losses.
Officially, neither NASA nor the European Space Agency is actively pursuing hibernation technology for spaceflight. But that doesn't mean they wouldn't appropriate the technology should it become available.
"Right now, NASA is not studying hibernation," said planetary scientist Wendell Mendell, chief of the Office for Lunar and Planetary Exploration at NASA's Johnson Space Center in Houston. "NASA has had no need to hibernate anybody, because we don't have a ghost of a chance to create a mission of that duration."
Even with NASA's current budget woes, Jansen says that if we're serious about space travel then he could see hibernation technology for space being adapted within the next three decades.
A one-way trip to Mars takes six months with current propulsion technology. During that time, Jansen envisions rotating crewmembers in and out of hibernation using a slow leptin drip.
If you could put people in hibernation (remember the opening scenes of the movie "Alien"?), even en route to Mars, he said, that would lessen space travel's known effects of bone loss and muscle atrophy.
Thus, hibernation technology would be expected to at least partially alleviate the need for starships to use a "generational crew structure" — crewmembers born en route would succeed their older colleagues.
Relying solely on such generational crew scenarios for travel outside the solar system would be onerous, both in terms of cost and allotted resources. Thus, Harry Jones, a systems engineer at California's NASA Ames Research Center, says such hibernation scenarios would definitely be enabling technology for interstellar colonization.
In theory, humans might be able to hibernate more than eight months at a time. But human hibernators would have to be fed intravenously as well as monitored by other waking crewmembers. Unlike the bear, humans cannot live on their own fat for months at a time.
Would lack of gravity affect hibernation?
Nobody knows, says Hellgren. So after isolating a safe chemical trigger, the first step might be sending a small mammal into orbital microgravity to see just how they fared under hibernation.
Despite skepticism over its feasibility for space travel, even Mendell agrees that when humans do go into the distant reaches of the solar system, mission length will become a serious issue.
"Technology today is not sufficient to support a mission of that length," said Mendell, "so hibernation might be a radical solution."
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