Image: COURTESY OF MARK CHAPPELL/Stanford University

Although it might seem that escaping winter's wrath by hibernating would simply require an animal to find a comfortable place to lay its head, the process is much more complicated than that. Now new research suggests a potential addition to the list of tactics involved in settling down for a long winter's nap. According to a study in the current issue of the American Journal of Physiology: Regulatory, Integrative and Comparative Physiology, the immune systems of hibernating squirrels essentially shut down while the animals lie dormant.

Brian Prendergast of Ohio State University and colleagues studied 31 hibernating golden-mantled ground squirrels in the laboratory. These animals usually spend five to six months of the year in hibernation, during which time their body temperatures drop to within one or two degrees of the outside temperature. Each week or so, the squirrels awaken and stay up for 12 to 20 hours before returning to their hibernating state. The scientists tracked the animals' sleeping patterns through body temperature measurements. When they went into hibernation, the researchers injected some of them with dead bacteria, which normally triggers an immune reaction. But the injections elicited very little response from the hibernating critters--they did not wake up nor did they register a fever. When their regularly scheduled arousal time arrived, however, the animals' temperatures skyrocketed as if they had just been infected. "The immune system of these animals didn't seem to recognize a bacterial infection during a hibernation bout," Prendergast notes. "But when they came out of hibernation, the immune system reacted strongly."

To further investigate this association, the researchers next tested the effect of prostaglandin E2--a hormone that tells the brain to spike a fever when an animal is exposed to infection--on dormant squirrels exposed to bacteria. Animals injected simultaneously with both prostaglandin and bacteria immediately came out of hibernation and spiked a fever. "This shows the fever mechanism still works in the hibernating squirrels," study co-author Randy Nelson of Ohio State University says. "However, the animal's immune system does not activate a fever during hibernation."

In the wild, hibernating animals are often exposed to various parasites, bacteria and microorganisms. As a result, the squirrels probably still require some immune response. The scientists suggest that their findings may help explain why dormant creatures periodically awaken, despite the significant energy requirements for doing so. Although consensus on the issue is still elusive, Prendergast comments that "animals may arouse from hibernation to do a 'system check' for infections and parasites that they have picked up."