Hibernation: Catching a Few Z's

Critters in the Garden

Carol Lerew
RSABG Volunteer, Nature Interpreter

Wouldn’t it be great to curl up for a long winter’s nap and awaken when the spring flowers are blooming? Not humanly possible, of course, but there are animals who do take advantage of such abilities.

Hibernation is a resting state in which the mammal’s body temperature has dropped, and breathing, heart rate and metabolism have all slowed. This occurs during hostile seasons when the animal is less able to find adequate food to maintain energy. When this type of condition happens during the summer it is called estivation. Both are survival adaptations.

We know the story about bears snoozing in their dens while snow covers the countryside. Just what exactly happens to produce this state? Beginning in the early summer bears begins to eat heavily, chowing down great amounts of vegetable matter like ripe berries, insects, small rodents and fish. Thick layers of body fat accumulate in preparation for winter. At some point, perhaps coinciding with colder weather, precipitation, declining daylight length and the animal’s internal rhythm, its body signals it’s time to sleep.

In the den, the bear’s metabolism, breathing and heart rate slows, and its body temperature drops slightly. Proteins and urine are recycled allowing it to remain in torpor for most of the winter as it lives on body fat. Cubs are born in January while the mother is hibernating, suckling and nestling against her warm body as she remains dormant. Occasionally the mother rouses to check on her cubs. Some authorities assert that bears do not really hibernate because their body temperatures do not drop as low as that of the environment, one of the guidelines to determining hibernation.

Hibernation among rodents has been studied for decades, particularly for the golden-mantled ground squirrel (Spermophilus lateralis). Curled in its underground nest, its breathing, heart rate and metabolism slow. Its body temperature drops to a point just slightly above that of the surrounding air. Periodically it will rouse briefly but, like the bear, does not eat until it emerges from hibernation in the early spring. The chipmunk, however, stores large amounts of food in its nest and does eat during periodic arousals.

Many small mammals lose body mass during the winter thus lowering metabolic needs and reducing the need for hibernation. Some species of shrews and voles actually lose brain mass causing significantly fewer metabolic requirements, for the brain needs a large amount of energy. This shrinkage is thought to be controlled by the declining length of daylight.

Among closely related groups of California mammals there are both hibernating and non-hibernating species. Why this occurs is not known. For example, kangaroo rats are not known to hibernate, but several species of pocket mice are. Most species of ground squirrels and chipmunks hibernate, but tree squirrels do not. Some carnivores, such as the black bear, have a deep winter sleep, but various kinds of weasels do not.

Almost all California bats hibernate, usually moving into caves or mine shafts in the fall. Their seasonally acquired layer of fat enables the bat’s body temperature to drop close to the ambient temperature of the cave in a complex metabolic action not fully understood. During hibernation the fat serves as an energy supply and their bodies seem to lose body water. They feel cold to the touch and are sometimes covered in frozen dew droplets. Upon their periodic arousals they frequently ingest these droplets from their fur.

Hibernation as a survival adaptation is one of the wonders of evolution in the natural world. Researchers continue to study this phenomenon trying to find human applications. Perhaps induced hibernation in people with diseases or injuries could be a key to the healing process. Would periodic torpor help to lengthen human life? The questions are many, and significant discoveries may be just around the corner.