The Big Sleep
How and why Many Mammal Species Hibernate
by Sabrina Stangler, age 14
You may have noticed how squirrels, along with other mammals, seem to disappear in the winter in areas that have cold climates. They don’t actually vanish into thin air; instead, they go into hibernation. Hibernation is a state in which a mammal sleeps for several weeks or months to preserve body heat.
Before going into hibernation, squirrels must gather large amounts of nuts. They will need this supply of food to eat before, during and just after hibernation. When going into hibernation, squirrels find a cozy spot to curl up in before sleeping. Animals lose heat through their skin, and curling up exposes a smaller surface area to the cold.
If its body temperature drops too low, the squirrel will awaken and move to a warmer area. It also wakes up to defecate quite frequently, and then drifts back into a long and undisturbed sleep.
Hibernation is triggered by temperature changes in the animal’s environment. The animal becomes slumberous as the temperature begins to drop, and its heart rate and body temperature also fall. Since mammals are warm-blooded, they can withstand rather low temperatures. Because of this, they need more food than other animals to maintain their body weight. That’s why it’s important for hibernating animals to have a stash of food nearby.
Although animals hibernate for multiple weeks in winter, the exact time span varies. For example, the northern black bear may sleep for over 100 days without waking, while other mammals like the golden mantled ground squirrel will wake much more often to eat and defecate.
Some mammals go into what’s called deep hibernation. Like the hoary marmot, some mammals will spend half of their lives in deep hibernation. In this state, the mammal’s heart rate and body temperature drop significantly.
For instance, a bat’s heart rate drops from 700 beats per minute, all the way down to 12 beats per minute. When bats hibernate, they usually look for a moist cave. Not only does the cave provide protection from the elements and predators, but its moisture prevents the bat from dehydrating in its sleep. Often, bats will huddle together to stay warm and retain moisture.
Sometimes bats have to wake up to move or defecate, but this has to be kept to a minimum because their energy is limited and cannot be restored. Unfortunately, bats’ sources of food—bugs and other insects—are not around in the winter.
Although humans are mammals, they don’t hibernate. For one thing, humans’ evolutionary ancestors lived in a tropical environment; therefore, hibernation was unnecessary for human survival. Also, humans migrated to sub-arctic climates about 100,000 years ago, which is not long enough for the human species to evolve the necessary metabolic adaptations to hibernate. Along the way, humans have also made use of fire, clothes, shelter, hunting and agriculture.
Although some mammals are very heavy sleepers, others are known to be light sleepers, or torpor hibernators. Unlike their deep-sleeping companions, torpor hibernators’ body temperatures and heart rates remain unchanged throughout their slumber. The northern black bear is one such light sleeper. This mammal does not eat for approximately 100 days. Its body adapts to hibernation by forming a fecal plug, called a tappen, which prevents defecation. Interestingly, females in this condition can still give birth, and suckle their newborns.
Hibernation is critical to the survival of many mammals. This is particularly true if they live in harsh winter climates. Animals must do what it takes to survive, and for many mammals, hibernation is crucial.
[Sources: www.sciencefocus.com; Wildlife Explorer]