Awakening from Hibernation

Groundhogs, squirrels, and other hibernating animals maintain an internal clock that tells them when it is time to wake up from their winter slumber. Instead of migrating long distances to a better climate or source of winter food, hibernating animals typically respond to local conditions by finding a safe, well-insulated dwelling and then altering their physiology and behavior. Hibernation is a well-regulated strategy to counter food shortages and increased energy demands during winter. In fact, hibernating animals can become so quiet they may appear to be dead!

Almost all hibernating animals prepare during the summer and fall by eating large amounts of food, which they convert to thick layers of fat. The fat keeps them warm and is a source of food during the winter. Although hibernating animals may occasionally arouse to forage or eliminate waste, they are totally inactive for long periods of time. Raccoons and skunks are not considered true hibernators, because they awaken during the winter to feed and have only slight changes in their body functions. Bears also are not considered true hibernators. Although their metabolism is slowed and their blood chemistry is radically altered, they maintain near-normal body temperatures and will awaken easily if disturbed. In winter, hummingbirds lapse into a state of torpor at night, similar to hibernation. However, for animals that hibernate for a period of months, the lack of food and water and the inability to eliminate waste mean that their physiology and blood biochemistry are dramatically transformed.

While birds are migrating in spring, animals emerge from hibernation. Animals stay inactive for shorter periods toward the end of hibernation season, and they come outside to assess the air temperature. They then decide whether to reenter torpor or terminate hibernation—think of Groundhog Day. Scientists think that the precise time hibernating animals finally emerge depends on both the outside temperature and the length of daylight, which normally increase in spring. These environmental signals are integrated in the brain, probably by the suprachiasmatic nucleus, which regulates the amount of melatonin released from the pineal gland. Melatonin seems to be the major hormone involved in controlling seasonal changes in physiology and behavior. Awakening reverses the surprising arrest of protein synthesis that occurs in the brain and other organs of hibernating animals—body temperature rises and the metabolic rate, respiratory rate, and heart rate all increase very quickly. The body temperature of the ground squirrel, for example, can rise from 4 °C to 25 °C in just three hours.

Internal factors also affect the rhythms of hibernation. For example, as the bodies of hibernating male squirrels return to normal in the spring, a sustained, increased secretion of sex hormones prevents a relapse into hibernation.

Because the end to hibernation is so sensitive to temperature, increased atmospheric warming is having a noticeable effect. Recent data show that the hibernation period for some animals is becoming shorter. Yellow-bellied marmots living in the Colorado Rockies, for example, are emerging from hibernation almost five weeks earlier than usual. Snowfalls have also increased with the changing climate. The ground is still covered with snow when the marmots finish hibernating, making food hard to find. If these trends continue, the marmot may experience problems.