Hibernation isn't the cushy nap it's cracked up to be. Surviving the winter months means living in a hypothermic limbo that doesn't even qualify as real sleep. Plus, some hibernating animals wake up at regular intervals. Why they do this has been a persistent mystery. But now a group of researchers says that animals periodically rouse themselves from hibernation to rev up their immune systems and fight off infections.
The golden-mantled ground squirrel, found throughout much of the western United States, hibernates for 5 to 7 months of the year. After spending the spring and summer fattening up on seeds, nuts, forest fungus, and campers' handouts, the squirrels nest in shallow burrows. While hibernating, the animals throttle down many body systems--slowing their heartbeats, lowering their body temperatures, and shifting their immune systems into low gear. But even after they begin to hibernate, the squirrels still wake up every 7 to 30 days for periods of 8 to 20 hours.
A team of neuroscientists led by Brian Prendergast of Ohio State University, Columbus, and David Freeman of the University of California, Berkeley, reasoned that the squirrels might wake to fire up their immune systems. To test this hypothesis, the team simulated bacterial infection in the animals by injecting the dead outer cell coats of bacteria. Nonhibernating squirrels spiked a fever within 2 hours, but hibernating squirrels became feverish only when they awoke on schedule a few days later, the team reports in the April issue of the American Journal of Physiology: Regulatory Integrative and Comparative Physiology. These results suggest to the researchers that reviving the immune system could be the main reason for the hibernation interruptions.
But neuroscientist Norman Ruby of Stanford University isn't convinced. To him, the fact that the immune systems are so unresponsive during hibernation suggests that the animals don't need sensitive immune systems during hibernation. "It's safe to say we still don't conclusively know what the function of hibernation arousals are," he says.