All You Need To Know About Hibernation

European hedgehog curled up into a ball, hibernating in colourful autumn leaves.
European hedgehog curled up into a ball, hibernating in colourful autumn leaves.

Hibernation refers to the state of reduced metabolic activity of an organism, which is associated with lowered body temperature, and certain types of animals assume hibernation as a way of adapting to adverse and severe winter conditions. Hibernation is typically used to refer to all forms of winter dormancy witnessed among the vertebrate animals. Animals that exhibit hibernation include different species of fishes, reptiles, and amphibians whose body temperature reaches to almost freezing point during winter. It also includes bears and some mammals that burrow themselves in dens during winter. However, bears do not experience an excessive lowering of body temperature, and they are easily awakened during hibernation. Occasionally, bears are not considered as true hibernating animals. True hibernating animals spend much of the winter in a state that is close to death, and they occasionally appear as dead. Their body temperatures fall to almost 32 degrees F, and the respiration drops to only a few breaths every minute. Their heartbeats also slow down to virtually imperceptible levels. During this stage, when moderate warmth is provided to the animal, it gradually awakens and may require up to an hour or more to attain a full alert state.

True Hibernators

The true hibernators among the mammals are found in Chiroptera order (bats), Rodentia (such as marmots and ground squirrels among others), Insectivora (allies, and hedgehogs). Ordinarily, the hibernators rely on reserves of body fat, food store supplies as in the case of rodents and dens that are protected to survive through the winter. The body temperature of hibernating animals would occasionally increase after intervals of several weeks and they would awaken, move about, feed, and returns to a state of torpor. In other regions such as deserts, the animals avert the extreme summer drought through a process of estivation that in many ways is similar to hibernation

Hibernation Is A Mechanism Of Energy Conservation

Hibernation, together with estivation, is a mechanism used by different organisms to avoid stress as a result of extreme temperatures, unavailability of food, lessened photoperiod, and lack of water. As a result, they serve as energy conservation mechanisms. Even the shortest periods of torpor can conserve significant amounts of energy. This efficient mechanism of conserving energy is best illustrated with a hummingbird, which is the smallest bird that exhibits circadian torpor as compared to the smallest mammal, shrew that always remains active in 24 hours. Metabolic rate is a function of oxygen consumption, and in an environment with temperatures of approximately 75°F in the day, a resting but awake hummingbird consumes about 14 ml of oxygen per gram every hour. The rate drops at dusk to sleeping level and finally plummets to a torpid level of approximately 0.8 ml of oxygen per gram every hour. At daybreak, the hummingbird awakens to another period of activity. Hummingbirds have among the highest metabolic rates as well as the highest range of metabolic of all vertebrates. On the other hand, the shrew consumes almost the same levels of oxygen as the hummingbird consumes during the day. However, at night it increases the consumption even higher.   

Entering Into Hibernation

Mammals that hibernate can be categorized into four groups depending on their way of entering hibernation. The first group is represented by the golden hamster. This group waits in the cold for about one to three months before entering hibernation following one significant reduction in temperature. This is achieved through a mechanism of physiological and biochemical preparations that reach sufficient levels to make the animal receptive to hibernating stimulus and cause the animal to abandon the differences in body temperature and ambient temperature. The pocket mouse represents the second group, and they prepare for hibernation much quicker. They become torpid after waiting for only a few days following a single significant decline in temperature. 

The third group accounts for most of the hibernating mammals, and marmots and ground squirrels represent the group. This group enters into hibernation after waiting for a few days, and they go through a series of torpor and arousal, with each successive step being of lower body temperature than the previous one until they achieve a stage of hibernation. The 4th group includes the majority of bats, and their body temperatures follow the ambient temperature. Bats do not only exhibit characteristics of true hibernators during the winter but the remaining part of the year, they exhibit hypothermia, which is not related to hibernation.  

Heart Rate And Circulation

Changes in oxygen consumption, heart rate, and respiration affect the body temperature of hibernating animals. Part of the central nervous system mediates these changes. The heart rate slows down before the fall in body temperature. For instance, the heart rate in a woodchuck could fall to 68 from 153 heartbeats per minute in about 30 minutes. The heartbeat of a California ground squirrel may slow dramatically as low as one heartbeat per minute at a temperature of 41°F. On the other hand, non-hibernating mammals at temperatures of between 50°F and 70°F, their hearts would instantly stop beating.   

Neural Changes

Hibernating animals have special adaptations on both the central nervous system and peripheral nervous system that perceive changes in the environment when the animal is torpid. For instance, when the Arctic ground squirrel is hibernating, there is a 90% reduction of electrical activity in the brain, when the brain temperature is about 43°F. During this time, there is an increased sensitivity to certain stimuli of both the central and peripheral nervous systems. Besides, parts of the brain responsible for the regulation of temperature, heart (cardiac), and respiratory are always active at temperatures below which the nervous system of mammals would stop functioning.   

The Activity Of The Endocrine System

It has been established that hibernation cannot take place without adrenal glands, and it seems that some minimal adrenal activity is crucial for hibernation as well as survival. Testosterone, which is a male sex hormone, is responsible for stimulating reproductive activity. If more than 5mgs of the hormone preparation is injected into a golden hamster, it will not hibernate. Similarly, animals will not hibernate if they are fed or injected with thyroid stimulants or thyroid hormones. There is a seasonal variation in thyroid activity among the hibernating animals, with maximum activity during spring and minimum activity during fall.    

Protection Against Diseases And Radiation

All hibernators have some degree of protection from infectious diseases that could be attributed to about three different factors, which are all related to temperature. The low body temperature, together with the slow metabolic process, prevents the growth and multiplication of agents that cause diseases. Similarly, low temperatures are more harmful to disease-causing organisms as opposed to the host. For instance, bats hibernating at 41°F were found having only larvae of worms in their intestines and mature worms in bats kept at temperatures of 95°F. Besides, low temperature influences the chemical composition of the tissues of the host, which is believed to affect organisms that cause diseases. Radiation also seems not to affect hibernating animals. For instance, when the ground squirrel was exposed to radioactive cobalt during hibernation, they were found to be more resistant to the effects of radiation compared to exposure when they are active and not hibernating.

Awakening After Hibernating

After hibernation, the arctic ground squirrel takes only three hours to awaken. During the process of awakening, there is a quick increase in heartbeat, a sharp decline in peripheral circulatory resistance, and rapid warming of regions around the heart and the head compared to the animal’s posterior parts. The vasodilation differential in the animal’s anterior region is critical and a unique way of awakening from hibernation. Active circulation is concentrated in this region leads to high blood pressure as well as rapid and efficient warming. If a drug that causes vasodilation in the whole body is administered, there would be a sharp decline in blood pressure even when the heart is almost doubling its rate. The heart would not be able to sustain high blood pressure if all blood vessels in the body are dilated. However, during the process of arousal following the warming up of the anterior region of the body, the posterior region of the body warms up rapidly. 

Animals That Hibernate The Longest

According to National Geographic, the animal with the longest hibernating time is the edible dormice (Glis glis). The animal can remain dormant in hibernation for 11 months in their wild habitats. To achieve this feat, edible dormice typically have to double their body weight or even triple when they are active. Another experiment found that the big brown bat (Eptesicus fuscus)hibernated for a total of 344 days in a refrigerator. Although the big brown bat could easily be the longest hibernating of all animals, the bat did not choose to hibernate naturally, and it did not manage to survive the ordeal. In a natural environment, big brown bats begin their hibernation in November and often awaken temporarily to look for water and locate a warmer shelter. Finally, they come out of hibernation during spring.


More in Environment