Hibernation

As winter approaches, some animals enter a barely living state. Whereas the body temperature of warm-blooded animals is generally about 37 degrees Celsius (98.6 degrees Fahrenheit), some larger vertebrates, such as bears, enter a restful state for several of the colder months, during which body temperatures sink to about 30 degrees Celsius (88 degrees Fahrenheit).

In another class of smaller animals, notably the brown bat, the body temperature hovers just above freezing, usually between 1 and 3 degrees Celsius (34 to 38 degrees Fahrenheit). At these levels, the animal is barely alive. It does not bleed when cut. It breathes infrequently. Its heartbeat drops dramatically.

Groundhogs and chipmunks are found in areas with sporadic periods of warm weather during the winter, at which times these animals awaken from their torpor temporarily. Their body temperatures, which will have sunk to about 10 degrees Celsius (50 degrees Fahrenheit), increase as the temperature outside their underground lairs rises. When the temperature sinks again, these animals resume their sleep.

Animals such as bears, brown bats, some rodents, hummingbirds, whippoorwills, chipmunks, ground squirrels, skunks, and marmots have built-in mechanisms that prevent their temperatures from sinking below the levels their systems can withstand. When their temperatures approach life-threatening levels, they begin to shiver, thereby maintaining or raising their temperatures without wakening them from their slumbers.

This state of deep sleep is known as hibernation. While only a relatively small percentage of animals hibernate, for those that do, it is an important part of survival and growth.

The Why and How of Hibernation

During hibernation, the body shuts down and requires little energy. All of its systems, including endocrine, circulation, respiration, and elimination, are reduced to the barest essentials necessary to maintain life. Animals living in this state require almost no nutrition. They draw what nourishment they need from stores of fat that they accumulate by eating a great deal immediately before hibernating. They conserve their body heat by rolling themselves into balls.

The mechanisms that trigger hibernation mystify scientists. Some sort of internal clock, probably responding to light and temperature, clicks in at given times, determining the beginning of hibernation and its extent. This mechanism is far from precise. Some animals that have been maintained from year to year under consistent conditions may lose their synchronization, entering hibernation possibly in spring or summer.

The instinct of hibernation, though, is likely due to reduced resources and exposed habitats during winter months in cold climates. Smaller animals, particularly, risk exposure to predators topped with the inability to find food. Hibernation keeps the animal under cover while allowing their body to conserve energy and survive without external nourishment.

The Light Sleepers

On hearing the word “hibernation,” people think immediately of bears, who enter periods of dormancy as winter approaches. By the time they enter their lair for their winter’s sleep, they have gained considerable weight and have added to their bodies layers of fat to provide them with the nutritional reserves they will require to survive the next three or four months.

During this period of dormancy, the bear’s temperature drops less than 10 degrees Fahrenheit. Some scientists resist designating hibernation as the period during which such animals as bears, chipmunks, raccoons, and skunks sleep beneath the ground. This period is sometimes called “winter lethargy.”

Animals whose body temperatures do not drop dramatically during their dormant period may wake from their sleep several times during the winter. When the weather moderates, they often leave their lairs and scurry about seeking food. When cold weather resumes, they return to their lairs to continue their slumber.

The eastern chipmunk is among the light sleepers. Unlike bears, it does not accumulate excess body fat to see it through its three or four dormant months. Rather, it stores food in its burrow as winter approaches, building its nest on top of the food it has gathered. It frequently awkens in winter to eat and to defecate in a section of the burrow away from the food supply.

The male chipmunk usually ends its sleep late in February. Leaving the burrow, it first seeks food and water, then looks for a mate, who produces from two to five babies one month after mating.

Skunks are also among the light sleepers. When the outside temperature approaches 10 degrees Celsius (50 degrees Fahrenheit), skunks retreat to their dens for the winter. They may take over an abandoned woodchuck’s nest, but often they build their own dens below the frost line, at depths of between six and twelve feet. They line the den with dried leaves and grasses, creating a cozy nest. Although skunks are solitary in summer, they often live in groups in winter, huddling together to keep warm.

Raccoons enter a dormant state in cold climates but are active throughout the year in milder ones, undergoing fewer body changes in winter than other true hibernators. Their body temperature drops minimally. Their heartbeat, while decreasing slightly, remains close to normal. Raccoons sleep as long as cold weather persists. They stir during warm spells but sleep again when the temperature drops.

In winter, raccoons, usually solitary dwellers, change that pattern and share their dens with other raccoons for their body warmth. Regardless of the weather, male raccoons become active late in January, which is mating season. After mating, they return to their dens for more sleep before spring.

The Heavy Sleepers

Some species undergo significant changes during dormancy that qualify them as true hibernators. In these animals, heartbeat, temperature, and respiration drop so dramatically during winter that life is barely sustained. These heavy sleepers do not respond to temporary increases in outside temperature, rather sleeping soundly through the months of their hibernation.

The most renowned of the heavy sleepers is the woodchuck, also known as the groundhog. People watch groundhogs’ burrows every February 2 to determine whether the groundhog will see his shadow. If he does, legend has it, he will return to his den, and there will be six more months of winter. Though this has become a holiday in many states with unique groundhogs attached to local traditions, the groundhog actually leaves its burrow in February to seek a mate.

Woodchucks accumulate as much body fat as possible before hibernation, sometimes reaching ten pounds. During their deep sleep, they take nourishment from their layers of fat, although they may wake at times to nibble seeds stored in their dens. By the time they emerge from hibernation, most have lost between 35 to 50 percent of their weight.

The den into which woodchucks retreat when winter arrives may have a forty-foot tunnel, camouflaged at its entry, that leads into the den. The winter den is below the frost line, usually about five feet deep. The winter rooms are high in the tunnel so that they cannot flood.

When the woodchuck enters its den for hibernation, it seals off the tunnel leading into it. The outer tunnel may become the winter dwelling of skunks or rabbits. During hibernation, the woodchuck, which normally breathes about thirty-five times a minute, breathes about once in five minutes. Its body temperature drops from about 36.7 degrees Celsius (98 degrees Fahrenheit) to 3.3 degrees Celsius (38 degrees Fahrenheit), only slightly above freezing. Its heart, beating in warm weather at about eighty times a minute, now beats four times a minute.

Woodchucks have a layer of brown fat that builds up around their vital organs. When it is time for hibernation to end, the animal receives an instant jolt of energy from this brown fat. It begins to shiver, which gradually warms it up. Its supply of oxygen increases steadily, resulting in increased blood flow, which warms the woodchuck. It takes a few hours for it to move from its dormant state to its springtime state.

In mild climates, ground squirrels are active all year. The bodies of those that do hibernate change slowly from their summer to their winter phase. Their body temperature, around 32.2 degrees Celsius (90 degrees Fahrenheit) in summer, drops a couple of degrees daily until it approaches 4.4 degrees Celsius (40 degrees Fahrenheit). An internal mechanism keeps the squirrel’s body temperature from dropping below that number. Brown bats enter hibernation when the air temperature stabilizes around 10 degrees Celsius (50 degrees Fahrenheit), entering their winter quarters, or hibernacula, in swarms. In these caves, the temperature remains constant and is above freezing, which is crucial to the bats’ survival because bats subjected to lower temperatures develop fatal ice crystals in their blood.

Brown bats hang upside down in their caves in winter. Their bodies become stiff and appear to be dead. Their heartbeats drop from over five hundred beats a minute to between seven and ten beats a minute. They hibernate for three to four months, waking occasionally for water and any insects they can find to devour. In dormancy, brown bats’ bodies assume the temperature of the atmosphere surrounding them.

The jumping mouse is another true hibernator. Late in October, it seals itself into its den, having gorged on all the food it can find in the weeks before hibernation. It curls up into a ball, placing its head between its hind legs. During hibernation, it breathes just once every fifteen minutes.

Effects of Climate Change

Because of climate changes, scientists have documented changes in the ways animals hibernate, durations, and physiology. They have also noted changes in seasonal life-expectancy because of changing environments. For example, while summer life expectancy is rising due to longer breeding seasons, winter life expectancy for many species is expected to decrease due to a lack of available resources, changes in snowfall, and an increase in predators.

A study by the British Ecological Society in 2018, for example, found that for every one degree Celsius that temperatures increase in winter, black bears hibernated for six fewer days. The study found that warmer temperatures and human development reduced hibernation in black bears, therefore extending the active bear season. In regions where bears venture into human residential areas, this could increase instances of bear-human interactions. Other species will likely be impacted in a similar way.

Principal Terms

Anesthesiologist: a physician who administers anesthetics during surgical procedures

Celsius: a scale for measuring temperature in which freezing is zero degrees, and boiling is one hundred degrees, abbreviated C

Fahrenheit: a scale for measuring temperature in which freezing is 32 degrees, and boiling is 212 degrees, abbreviated F

Hibernacula: the winter habitats of brown bats

Synchronization: causing events to occur simultaneously

Vertebrates: animals with brains and spinal cords

Bibliography

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