Our planet is home to many unique animals, each with its quirky behaviors and unusual lifestyles. A common theme among many species is hibernation, but what is hibernation, and why do animals hibernate?
Animals hibernate to survive harsh winters where the weather is freezing and food sources are scarce. Their bodies slow down, allowing them to conserve energy. Their heart rates, breathing, and body temperatures reduce, allowing them to survive through winters with less food.
Hibernation is a state similar to sleep but lasts much longer. Some animals similarly slow down their activities, eating and moving less, whereas other species will enter a deep state of hibernation where they sleep for weeks or months.
During this time, an animal’s heart rate will slow, and its body temperature will drop. This helps to conserve energy and protect their vital organs. Most animals dig themselves a burrow or den to hibernate in, while others will find a small cave or hollow in a tree.
While many animals living in cold climates use hibernation during the winter months, animals in hot climates have their hibernating form, called aestivation. Animals will adopt aestivation to prevent their bodies from drying out due to their hot, dry environment. Many species of reptiles, amphibians, and insects aestivate to conserve water and energy.
Why Do Animals Hibernate?
Hibernation is an adaptation to a harsh environment. During the winter months, food is scarce, which can be dangerous for endothermic animals who use their energy stores to regulate their body temperature. Hibernation can help warm-blooded animals to survive the harsh winter months when there is very little food available.
The Latin translation of the word “hibernation” is “to pass the winter,” and many animals will enter a state of hibernation lasting many weeks or months. Black bears, for example, have a hibernation season that begins in November and ends in March.
A black bear’s heart rate slows during this time, and its body temperature drops by around 7°F (or 4-5 measuring in Celsius). This drop in body temperature slows the bear’s metabolism by 75%, meaning they use less energy. Female black bears even give birth during hibernation. Before birth, a female will shiver to raise her body temperature, which scientists believe is vital for the cub to survive until the mother wakes.
In the months before hibernation, warm-blooded animals will often feed to build up their fat reserves. They can then use this fat as a source of energy during hibernation. Animals adapted to hibernate will store a type of fat called “brown fat.” This is useful as animals utilize brown fat at a slow rate to maintain their low body temperature, and their fat reserves will last far longer.
Some animals, particularly smaller mammals, will not enter total hibernation but will enter a state known as torpor. This is a form of hibernation, but the animal will wake for around 24 hours every few weeks before falling back into torpor again.
The hummingbird enters a state of torpor every night to conserve energy. Due to their tiny size, they would have to wake every couple of hours through the night to feed to survive. Instead, they use torpor to lower their body temperature, meaning they can enter a deep sleep during the night and not need to feed.
Ectothermic animals – those that use the environment to regulate their body temperature – also enter a form of hibernation but with a slight difference. Known as brumation, amphibians and reptiles will enter a state where their heart and metabolic rates are lower, but unlike mammals, amphibians and reptiles remain alert and active.
As their metabolic rates are naturally slow, brumation does not alter the animals’ awareness or energy levels. It does, however, enable them to use less energy, just like mammalian hibernation.
5 North American Animals That Hibernate
North America is home to thousands of animal species, and hundreds are known to hibernate or use short periods of torpor to survive cold winters or hot summers. Winter in northern states can be very harsh, and summers in the south can be dry and hot. Being able to hibernate is a significant advantage for survival. Some animals even girth birth during hibernation!
Here are 5 North American animals known to hibernate or use torpor during winter.
All bears can hibernate, but some choose not to. If there are mild winter seasons in temperature and food is still plentiful, bears may not hibernate. The presence of available food sources triggers the need for hibernation in bears. When these food sources decline, hibernation becomes necessary.
In preparation for their hibernation period, bears will dig out a den. This could be dug from the ground around tree roots, a space in a rock crevice, a hollow tree, or under fallen trees. Black bears can be found across North America, with brown bears present in Yellowstone, Glacier, Grand Teton, and North Cascades National Parks.
Like many small mammals, Chipmunks do not hibernate fully but will enter a state of torpor for several days or up to two weeks. They will wake to eat, relieve themselves, and then go back into inactivity. This period of waking and sleeping begins in late fall and lasts until early spring.
Instead of eating food to store up fat before fall arrives, chipmunks are hoarders. Like squirrels, they will spend late summer gathering nuts, seeds, and berries they hide in their burrows. This food is what they eat during winter on the days they wake from torpor.
If another chipmunk steals another chipmunk’s food store, they will not starve, as chipmunks will have at least one emergency stash of food away from their main burrow. Sometimes chipmunks will forget where their food is, and scientists believe this nourishes the soil and allows new trees such as oak and maple to grow.
Bats are unique creatures as they are the only mammal capable of true flight. Some mammals can achieve temporary flight by gliding, but bats are the only mammalian species capable of staying aloft like birds.
Bats rely heavily on insects for nourishment. During winter, the availability of insects decreases significantly. Bat species that do not migrate to warmer climates must hibernate or enter a state of torpor to survive.
Some bat species will hibernate for up to six months, whereas others will enter torpor for several days or weeks, waking briefly to drink water and then returning to their sleep. Bats typically choose warm, humid places to hibernate, such as rocky crevices, caves, or abandoned mines.
Of all the North American hibernators, bumblebees are the most fascinating. Only the queen hibernates in a colony that may number as many as 500 individuals. The remaining bees will die before winter arrives.
A newly matured queen will leave the nest for the first time by participating in a mating flight. She will mate with a male drone bee and store enough sperm to fertilize all the eggs she will lay in her lifetime. The queen then finds a nest just large enough for herself, and she will hibernate until early spring. Her heart rate slows, and her body temperature drops so that her body is only performing the most basic functions to preserve life. In early spring, the queen wakes from hibernation, lays her eggs, and begins a new colony for the year.
While many birds are known to enter a light torpor during the winter months, the common poorwill is the only known bird species capable of total hibernation. Native to Southern and Mid-western states, the common poorwill is a nocturnal bird with similar plumage to an owl.
While the most common method for poorwills is to utilize torpor for a few days or weeks, common poorwills have been known to hibernate for months when the weather is icy or insects are scarce. No other bird species are known to hibernate truly. Most migrate to warmer climates and return in spring. Common poorwills are also known to aestivate (summer hibernation) during scorching weather.
Common poorwills typically live in open, grassy areas. During cold weather or periods of food scarcity, these birds will choose a sheltered grassy patch or hollow log to hibernate until the weather warms and insects return.
These animals have adapted to extreme weather, whether it is hibernation, aestivation, or torpor. Their ability to sleep through harsh conditions gives them a considerable advantage compared to those animals who cannot hibernate.
Bryan Harding is a member of the American Society of Mammalogists and a member of the American Birding Association. Bryan is especially fond of mammals and has studied and worked with them around the world. Bryan serves as owner, writer, and publisher of North American Nature.