Groundhogs and other hibernators take a very sensible approach to winter: They slip into a state of suspended animation and let the worst of the cold weather pass.

The cold prompts profound physiological changes in these animals, causing their normally fast metabolism to come almost to a stop during winter. With metabolism slowed to a crawl, the animal draws on its fat stores sparingly to make it through the winter.

Hibernation has become the focus of interesting physiological research. A hibernating squirrel's heart may fall from 300 beats per minute to just three per minute. Its oxygen consumption can drop to just 2% of normal. Its core body temperature can drop from 37° C (98.6° F) to about 2° C (35.6°F).

The hibernating animal may be a frog frozen in a winter pond, a turtle buried in the mud of a swamp, a bear in its den or a squirrel curled in a tight ball in its underground burrow. There is even a primate, the lemur, which hibernates in tree trunks. The range of hibernating species suggests that many animals, including humans, possess the genes necessary to hibernate.

Hibernation research has implications for medical advances, including for victims of stroke, hemorrhagic bleeding and hypothermia. The research may also lead to better ways to preserve organs for transplant and to help control obesity.

For example, when a person suffers a stroke, heart attack or severe hypothermia, it is not the loss of blood flow (ischemia) that causes the greatest damage to the organs and tissues. Instead, the greatest damage occurs when the blood flow is restored (reperfusion). Hibernating animals have significantly reduced blood flow when they hibernate, but reperfuse without injury when emerging from hibernation. If physiologists can figure out the mechanisms that allow ischemia and reperfusion without injury, it might help the victims of these conditions.

Current Research On Physiology Of Hibernation

• Matthew T. Andrews, a professor at the University of Minnesota in Duluth tells school children that they can find out what a hibernating squirrel feels like by putting a tennis ball in the refrigerator overnight. The next morning, the cool, fuzzy ball, feels much like the hibernating squirrel. Andrews is looking for the genes responsible for inducing and maintaining hibernation in mammals. In particular, his laboratory is identifying the genes responsible for regulating the physiology in the heart of the hibernating 13-lined ground squirrel.

• Hannah V. Carey, a professor at the University of Wisconsin School of Veterinary Medicine in Madison, has found that ground squirrels that suffered a loss of 60% of their blood volume survive much longer compared to other rodents and other non-hibernating ground squirrels. The ground squirrel's liver and heart remain viable outside the body for much longer during the hibernation season. These findings could one day extend the time that organs for transplant can be preserved and give doctors time to find a suitable recipient.

• Gregory L. Florant, a professor at Colorado State University in Fort Collins, studies changes in metabolism during hibernation and investigates how this relates to the regulation of food intake and obesity. He studies the role that insulin and leptin play in the regulation of energy balance and examines the role nutrients play in regulating food intake.

Warm Winter Could Be A problem

The warm winter that has settled over parts of the globe this year could mean hibernating animals wake up sooner, possibly before there's an adequate supply of food. In the Midwest, for example, warmer temperatures are expected to raise the metabolic rate of the 13-lined ground squirrel, a hibernator which burrows underground. A higher metabolic rate demands more energy, causing the squirrels to use fat stores more quickly. This may cause them to emerge from hibernation sooner to seek food -- perhaps before food is available.

Leptin, which plays an important part in the cessation of food intake during hibernation, decreases with warmer temperatures during hibernation. Decreased levels of leptin stimulate food intake. Research has found that ground squirrels exposed to normal winter cold have high fat depots, higher leptin concentrations and do not feed.

The warmer winter will not affect hibernators everywhere. Golden-mantled ground squirrels in the Rocky Mountains, where the temperatures are still cold, aren't expected to have a problem. But in past dry winters, the lack of snow pack resulted in the ground freezing farther down, killing some of the hibernating squirrels.

The American Physiological Society was founded in 1887 to foster basic and applied research. The Society, which has 10,500 members, provides a wide range of research, education and career support and publishes 11 peer-reviewed journals containing 4,000 studies annually.

Note: This article has been adapted from a news release issued by American Physiological Society.