Do Bears Really Hibernate?

Bears might not regarded as true hibernators. Their body temperature is reduced by about 12 degrees Fahrenheit but stays above 88 degrees Fahrenheit. Bears also wake up several times during such periods of dormancy, which makes that state similar to torpor. Before the 1980s, the term “hibernator,” used to describe bears, was shunned by some people who preferred torpor while describing the bears’ winter sleep. The term hibernation was redefined in 1981 as “a specialized, seasonal reduction of metabolism concurrent with scarce food and cold weather.” The new definition was used to describe the survival techniques exhibited in many animals, not just those with a drop in temperature to near freezing point. Today many biologists and physiologists say that bears hibernate while others call it extended torpor. Typically hibernation is used to describe longer sleeps while torpor is characterized by shorter periods of sleep.   

Hibernation

Hibernation is a state that animals enter to conserve energy and survive periods when food is scarce. During hibernation, organisms go into a deep sleep that lasts a number of days or months. The breathing rate also reduces significantly. In smaller mammals such as the hedgehog and ground squirrel, their body temperature also drops to near freezing point (37 degrees Fahrenheit). In bats, the heart rate can reduce from 600 beats per minute to about 20 beats per minute. The state is triggered by hormonal changes in the animals and day length changes that indicate when it is essential to conserve energy. Before animals go into hibernation, they store fat, which helps them survive the long winters. During hibernation, some animals may wake up for brief periods to eat drink or defecate. Arousal from hibernation can take several hours and uses up a significant amount of the animal’s conserved energy reserve. The term “true hibernators” is used to describe a smaller group of animals, including deer, mice, snakes, woodchucks, ground squirrels, and some bats. Today the word hibernation is used to describe even other lighter states such as torpor.

Torpor

 Many animals, including the chipmunk, are known to go into torpor on cold winter days as a survival mechanism. Torpor is similar to hibernation in that the heart rate goes down, breathing rates slow down, and body temperature lowers significantly. But unlike deep hibernation, animals only go into a period of dormancy or sleep for about a day or two and then wake up to forage for food. The body temperature also lowers only a few degrees compared to animals in deep hibernation. Animals that go into torpor can be said to be taking a break from the harsh cold winter day while conserving energy for foraging. During periods when the animals are active, they maintain normal body temperature and physiological rates and only go into torpor to conserve energy. Arousal from torpor takes about one hour and involves violent shaking and muscle contractions. Arousal from the torpor state consumes a significant amount of energy out of the amount conserved during the torpid state. Torpor is triggered by changes in the availability of food and ambient temperature. Raccoons, skunks, and bears are considered “light hibernators” and use the torpor state to survive winter. 

Types Of Bears

There are eight bear species around the world that occupy various habitats. They include the North American Black Bear, Brown Bear, Polar bear, Asiatic black bear, Andean bear, Panda bear, and the sloth bear. Some species such as the Panda bear do not go into hibernation at all while others such as the Brown bear and the North American black bear go into a period of light hibernation or extended torpor. Pandas typically cope with harsh winters by moving to a lower elevation to keep warm and then traveling back to a higher elevation when summer sets-in to keep cool.

Brown Bears

Brown bears typically go into a winter resting period between October and December. The bears dig dens, which are then used for several years to escape harsh environmental conditions or take advantage of natural caves and rock fissures. During light hibernation, the bear’s body temperature and respiratory rate drop significantly. The heart rate also drops 8 to 10 beats per minute. Blood is also redistributed to major organs such as the heart, brain, and lungs. During such a state, brown bears do not eat drink, urinate, or defecate. Fat accumulated during summer is broken down to provide energy and metabolic water. During winter hibernation, males lose around 22% of their autumn body mass, while females lose about 40% of their mass due factors such as reproduction.

Black Bears

Studies have shown that black bears go into a period of extended torpor during winter as a strategy for surviving the harsh conditions of the period. During such periods, black bears display physiological patterns such as significant decreases in metabolic rate, heart rate, and body temperature. Black Bears typically expend about 4,000 kcal (calculations based on body fat utilization rates), every day during such periods of extended torpor. They do not eat, drink, defecate, or urinate during such times. They are, however, easily driven into a mobile and reactive state when the need arises. They also display an awareness of their surroundings and can defend themselves. Pregnant females are also able to give birth to offspring and nurse them under such conditions.

Polar Bears

Polar bears are among the largest carnivores in the world. Adult males weigh about 1100 pounds while females are much smaller and weigh about 660 pounds. Polar bears are found around the arctic. They rely on body fat reserves during summer when there is little to no sea ice and, therefore, a lack of access to seals. During summer, polar bears living on land lose about 2.2 pounds and have to rely on foods such as berries, eggs, and kelp, which lack sufficient nutrition as they wait for winter. Polar bears go into true hibernation. Pregnant polar bears, however, go into dens for months (October to March) to give birth to cubs and nurse them while others are hunting seals during winter. In the dens, the female lowers her heart rate, metabolism, and breathing rate, but are not considered to be in true hibernation. The state can, however, be described as light hibernation. Some scientists believe that polar bears go into “walking hibernation,” a form of light hibernation that allows the polar bear to save energy during lean summer months while still being able to be moderately active. Various studies have found that polar bears can go in and out of this state with relative ease. Polar bears go in the fasting state (walking hibernation) during spring months when sea ice conditions are getting worse. Research has also found that despite being in walking hibernation, polar bears still lose a significant amount of weight. Some experts caution against the use of the term “walking hibernation” while describing summer-fall fasting, as it implies more significant energy conservation than it happens.

Effect Of Climate Change On Hibernation Cycles In Bears

Experts believe that as our planet warms up, bears are likely to experience significant changes in their extended torpor cycles. The hypothesis is supported by observations made in countries such as Spain where brown bears were spotted roaming the Pyrenees for food during a period when they should be in hibernation. Experts believe that the warmer than usual winter prevented the bears from starting their hibernation cycle. Gemma Rodrigez of the World Wildlife Fund (WWF) believes that if climate change continues to provide warmer winters, brown bears in Spain may eventually learn to resist their sleepy instincts.