Perchance to Hibernate

Can we tap a dormant capacity to downshift our metabolism?

This time of year, the wilds of North America are relatively quiet. The black bears that usually patrol the woods seem to have vanished. Many bat species are nowhere to be found, at least not by the causal observer. The same is true of ground squirrels and chipmunks. They are hidden away—hibernating.

COOL LITTLE FURBALL. In winter, the 13-lined ground squirrel drops its body temperature dramatically. Carey/Univ. of Wisconsin

COMING ON STRONG. A black bear emerges from hibernation with much of its muscle strength intact. P. Iaizzo/Univ. of Minnesota

Biologists have been intrigued for decades about how animals go dormant during the winter and survive physiological conditions that would kill them at other times of the year. Hibernators spend most of the winter in torpor, a state of self-induced reduction in body temperature and metabolic rate. Even some species that don’t contend with harsh winters by hunkering down for months at a stretch, such as mice, enter torpor daily when food is in limited supply and temperatures are chilly. Many small birds spend nights year-round in torpor.

In mammals, hibernation is so widespread that researchers reason that the ancestor of all mammals must have been a hibernator. People may be physiologically capable of tapping this dram of evolutionary heritage, says molecular biologist Sandra Martin of the University of Colorado School of Medicine in Aurora.

If people could mimic certain aspects of hibernation, they might benefit greatly. For instance, inducing a torporlike state in a wounded soldier or a bleeding-accident victim might give doctors precious extra time to stop and reverse the damage. Other patients would benefit if donated organs could be put in cold storage for prolonged shelf lives. And for astronauts, torpor, which some people call suspended animation, might facilitate travel to distant planets.

Such applications lie far in the future, Martin says. Researchers still don’t understand how natural hibernators put themselves into torpor or how they bring themselves out of it.

But new studies are peeling away the outer layers of that mystery. Far from succumbing to hypothermia, it seems, hibernators exploit it. Experiments are also revealing how animal tissues evade the damage that comes from inactivity and low blood flow, and suggesting that relatively few genes are involved in torpor and hibernation. That’s an auspicious sign for researchers who strive to manipulate the process. Other recent findings in animals point the way toward medical shortcuts that might mimic in people the effects of torpor, although these measures don’t exactly reproduce the biological state.

“These animals have got it right,” says physiologist Hannah V. Carey of the University of Wisconsin–Madison. “They know how to use hypothermia to their advantage.”

A distinctive state

Contrary to popular belief, hibernation is no “winter nap,” says biologist Steven Swoap of Williams College in Williamstown, Mass. In fact, hibernating animals spend relatively little of the winter asleep. Most of the time, they are simply in torpor.

In this state, an animal’s metabolic activity and body temperature drop. Mammals, being warm-blooded, generally maintain body temperatures close to 37°C when they’re active or asleep. When mice are asleep, they’re only about 1°C cooler than when they’re awake. But during torpor, their body temperature falls as much as 15°C, Swoap says.

Arctic ground squirrels, which hibernate in soil that’s permanently frozen, withstand even greater metabolic decreases. “Squirrels drop their body temperatures to that of an ice cube,” says biologist Brian M. Barnes of the University of Alaska-Fairbanks. “The brain receives very little blood. Very little oxygen enters the tissues.” A hibernating animal’s heart rate and blood pressure also fall significantly during torpor.

Some such metabolic changes are less dramatic in bears than in small hibernators, which is why some people maintain that “hibernation” shouldn’t apply to bears. But Hank Harlow of the University of Wyoming in Laramie argues that the word is appropriate. A black bear’s heart beats about 60 times per minute in the summer and as few as 5 times per minute during hibernation, he says.

Blood pressure in an active mouse fluctuates between about 80 and 120 millimeters of mercury (mm Hg) with each heartbeat. By contrast, in a mouse in torpor, blood pressure ranges between about 30 and 50 mm Hg. Such a drop in blood pressure would put a person on a gurney or in a grave, Swoap says.

In people, these changes in heart rate and blood pressure would deprive tissues of oxygen—a state called ischemia—and thus cause the tissue to die. Moreover, reperfusion—the return of blood flow to normal—can trigger inflammation that adds to the cellular injury. Yet hibernating animals somehow avoid ischemia-reperfusion injury as they enter and leave torpor, says Carey.

She and her colleagues studied livers from rats, which don’t hibernate, and ground squirrels, which may spend 6 months or more in their burrows during winter. The researchers stored the livers at 4°C—about the temperature at which doctors preserve donated organs prior to transplantation—for up to 72 hours. Then, the team warmed the organs back to 37°C and measured the concentration of lactate dehydrogenase in a solution that had passed through the liver tissue. The presence of this enzyme signals tissue breakdown.

After rewarming, rat livers contained 37 times their original concentrations of the enzyme. By comparison, livers from ground squirrels showed smaller increases in the enzyme, suggesting that the hibernator’s organs are more resistant to damage from ischemia and reperfusion. The enzyme’s concentration was 10-fold the original in livers from summer-active squirrels and just three fold in livers from squirrels that had been in torpor.

The researchers reported their results in the March 2005 American Journal of Physiology: Gastrointestinal and Liver Physiology. They have since done similar experiments, with similar results, in intestinal tissues in live rats and ground squirrels. Those data appeared in the November 2006 issue of the same journal.

Awake in the cold

In both sets of experiments, the researchers examined hibernators’ resistance to ischemia-reperfusion injury during different stages of hibernation. For example, most hibernators periodically warm their bodies to nearly normal summertime temperatures, sometimes as many as a dozen times over the course of the winter. After less than a day of this so-called interbout arousal, they cool off and return to torpor.

“Hibernation is a very dynamic period,” Carey says. “While most of the time is spent in torpor … mammalian hibernators periodically turn back on their engines.”

In her experiments, “livers from torpid squirrels were very protected [against ischemia-reperfusion injury], but the livers from aroused hibernators were just as good,” Carey says.

Hibernators must have compelling adaptive reasons for interrupting torpor with bouts of arousal, says Martin, because warming a chilly body burns considerable energy. Researchers suspect that the animals either need to replenish metabolic substances that can be produced only at warm temperatures or that they need to dispose of substances that can’t be broken down in the cold.

Periodically restoring circulation to normal may also combat muscle atrophy, which in people is a harmful consequence of prolonged inactivity. Harlow has observed that at intervals throughout the winter, hibernating bears momentarily increase blood flow to their extremities and shiver. Bear lose 22 percent of their muscle strength during their foodless, 4-month hibernation season, Harlow says. By comparison, even with adequate nutrition, people lose about 70 percent of their strength if bedridden for the same period, he noted at a meeting of the American Physiological Society in Virginia Beach, Va., last October.

Colder ambient temperatures make arousals more frequent. This is true even in the only known hibernating primate, Madagascar’s fat-tailed dwarf lemur. In 2004, Kathrin H. Dausmann of Phillips University in Marburg, Germany, and her colleagues found that the lemurs tend not to arouse if ambient temperatures remain above 30°C throughout their hibernation season.

Shifting gears

A few genes toggle their activity up or down during the switch between chilly torpor and normal activity, Martin says. At the October physiology meeting, she and her colleagues reported some preliminary results from gene-expression studies of ground squirrels.

The researchers measured the abundance of 961 specific proteins in ground squirrels during the summer and during the winter as the animals were entering torpor. Concentrations of 84 proteins differed significantly between the two seasons. Some of those proteins are involved in blood coagulation, fat metabolism, and energy production.

But few studies have found specific chemicals to be essential for hibernation. Last year, researchers in Japan identified a hormone that is necessary for hibernation in Siberian chipmunks. When the team treated hibernating animals to block the activity of the hormone, some animals ended their hibernation prematurely. The researchers dubbed the hormone hibernation-protein complex (SN: 4/15/06, p. 229: Available to subscribers at

These molecular changes probably make possible the dramatic behavioral changes seen during hibernation, such as long-term fasting, says physiologist Gregory Florant of Colorado State University in Fort Collins. “These animals turn off their appetite for 6 months a year.”

Research by Matthew T. Andrews of the University of Minnesota–Duluth offers a possible mechanism for how ground squirrels and other animals manage to go without eating: Chemicals in their bodies prepare them for their long fasts. For example, in the fall, these animals overproduce a molecule called MCT1. During an extended fast, that molecule is needed to transport fat-derived fuel packets called ketones to brain cells. When animals eat frequently, they use few ketones.

As winter approaches, Andrews says, “the animals poise themselves to use ketones.” He and his colleagues will report their finding in an upcoming Journal of Neurochemistry.

Metabolic poison

While much remains to be understood about hibernation and torpor, researchers have made progress toward inducing similar states in laboratory studies.

Some of the earliest studies have involved not true hibernators but lab mice, which enter torpor under certain conditions.

Nearly 2 years ago, researchers in Seattle reported in Science that exposure to hydrogen sulfide gas can lower heart rate, metabolism, and body temperature in lab mice (SN: 4/23/05, p. 261: Mice in the study revived and appeared healthy when exposure to the gas ended. Hydrogen sulfide, the compound that gives rotten eggs their odor, can be lethal at high concentrations.

Anesthesiologists Gian Paolo Volpato and Fumito Ichinose and their colleagues at Massachusetts General Hospital in Boston recently confirmed the Seattle findings: Both heart rate and respiration rate in lab mice fell by more than half, and body temperature plunged to barely above ambient temperature when the animals were exposed to hydrogen sulfide. Oxygen and energy use decreased by 90 percent, indicating a metabolic slowdown.

However, the Boston team unexpectedly discovered that hydrogen sulfide exposure doesn’t alter the animals’ blood pressure.

Hydrogen sulfide “certainly lowers metabolism, but it by no means mimics a real torpor bout,” comments Swoap. He and others suggest that hydrogen sulfide acts as a temporary “metabolic poison.” Poison or not, the gas may have therapeutic potential if it’s found to have similar effects in humans, Ichinose says. Hydrogen sulfide treatment might improve the safety of operations, such as coronary artery–bypass surgery, that can temporarily reduce oxygen supply to the heart and brain, he suggests. Lowering the body’s metabolic rate prior to such procedures might protect those tissues during an oxygen deficit.

Hydrogen sulfide might also reduce ischemic damage caused by strokes, heart attacks, or major bleeding injuries. Battlefield medics might someday carry hydrogen sulfide in portable tanks and use it to stabilize wounded soldiers before evacuating them, Ichinose suggests. He and Volpato say that they hope to conduct experiments in mammals larger than mice, such as pigs, to see whether hydrogen sulfide can safely turn down metabolism in animals that don’t naturally undergo torpor.

Other chemicals may also induce torporlike states. Recently, Cheng Chi Lee of the University of Texas Health Science Center in Houston and his colleagues found that constant darkness elevated the concentration of a compound called 5′-adenosine monophosphate (5’AMP) in the blood of lab mice. Blood concentrations of that chemical were also elevated in mice during torpor, which the researchers induced by temporarily depriving the animals of food.

To see whether 5’AMP could induce torpor, the researchers injected a synthetic version of the compound into animals on normal feeding and activity schedules.

Injections of small amounts of the compound caused body temperature in the animals to plunge from 37°C to 27°C or less within an hour. The animals also became inactive, suggesting that they were in torpor, said Lee and his colleagues in the Jan. 19, 2006 Nature. Temperature and activity returned to normal after 3 to 12 hours, depending on the dose of the chemical administered, they reported.

Martin, of the University of Colorado, and Williams College’s Swoap express skepticism that the state induced by 5’AMP was akin to natural torpor.

“AMP induced hypothermia,” Swoap says. That “is not equivalent to a daily torpor bout.”

In separate experiments, Swoap injected mice with the same compounds that Lee’s team used. At the American Physiological Society meeting last fall, he confirmed that mice injected with 5’AMP enter a temporary state of lowered body temperature and lowered metabolism.

However, Swoap says, the animals responded differently to the chemical than they do to natural torpor. For example, body temperatures fell more rapidly in chemically treated mice than in mice that were entering torpor induced by the absence of food and warmth.

In contrast to Lee’s results, the newer experiments indicate that similar concentrations of related adenosine phosphate chemicals also lower core body temperature and metabolic rate in mice, Swoap says.

He calls his results a “rebuttal” of Lee’s study. “AMP induces a dose-dependent, reversible, hypothermic state,” he says. “But it’s not torpor.”

Nevertheless, both studies confirm that experimenters can lower metabolism in mammals by chemical means, Andrews notes. And 5’AMP could have an advantage over hydrogen sulfide as a drug for people, he says. The former could be put in solution and then injected on a battlefield or in an ambulance. Hydrogen sulfide would have to be administered as a gas.

While neither chemical may reproduce all features of torpor, Andrews says, “it gets the job done” by putting an animal, and perhaps a person, into a state of lowered metabolism. For the medical applications that researchers envision, that might be sufficient.