What Animals Hibernate And What Happens During Hibernation?

Hibernation is a remarkable physiological feat essential to an animal's survival. It is a reduced state of metabolic activities and bodily functions. It helps animals conserve energy and survive extended cold periods and food scarcity. If you are asking what animals hibernate, this post is for you.

Types of Hibernation  

There are various methods of dormancy animals use to escape harsh environmental conditions. They are: 

Brumation  

Brumation is a state of inactivity for reptiles. It usually occurs in late fall, when the days are shorter and the nights are longer. Brumation ends in early spring when the days are longer and warmer. Reptiles brumate because they can't maintain their temperature without relying on the environment. So, they enter a temporary sleeping state to survive the harsh conditions of nature.

Estivation  

Estivation is a form of hibernation animals use to survive hot and dry weather conditions. Animals enter a dormant state and conserve energy until the environment cools down.

Torpor 

Torpor is a short-term and temporary hibernation method that small mammals and birds use. It is less intense than hibernation and lasts for shorter amounts of time. 

Diapause

Diapause is a state of paused development in all insect life stages. It is a survival technique during adverse environmental conditions, like extreme cold drought.

What happens during hibernation? 

Animals hibernate to save energy. During hibernation, an animal’s metabolism and breathing slow, and temperature reduces. Most animals enter into a sleep so deep it’s like they are dead. They survive hibernation on the fat reserves from the food they eat before hibernating. 

19 Animals That Hibernate

Quick link to the different types of animals that hibernate:

1. Box turtles

Box turtles conserve energy during winter months through brumation, not hibernation. They spend warmer weather periods eating foods rich in Vitamin A and other nutrients, which increases their body weight.

Turtles brumate because food sources become scarce, and they can not maintain the body temperature required for regular activities. As the cold weather approaches, they stop eating and choose a spot to bury themselves.

Their bodily functions can barely keep them alive. Their heart rate slows down, and digestion stops. They are immobile; they can’t open their eyes or move their body voluntarily. 

Their preferred hibernation spot could be under the roots of a big tree covered in leaf piles, an abandoned rabbit burrow, or on a hill above water level and easy to dig. Most turtles and tortoises enter deep unconsciousness for 10 to 14 weeks and lose 6-7% of their weight. 

2. Bears

One of the most well-known animals that hibernate are bears. However, they are not true hibernators. Bears do not sleep all winter. Instead, they enter a light sleep state known as torpor to survive cold weather periods and food scarcity.

Bears hibernate when high-calorie food supplies are hard to find, the winter season arrives, and hormonal changes. They hibernate for 3 to 7 months, depending on the bear species. Unlike hibernation, animals enter torpor states involuntarily. It could be in short bursts or last for months.

Most animals drop their body temperature to drastically low levels, almost to a freezing point. However, a bear’s body temperature drops slightly lower than their regular temperature. Its temperature only goes down to 31° to 35°C.

Bears’ metabolism still works as they burn up to 4000 calories daily. Most bears come out of hibernation³, losing 15 to 25 percent of their weight. Lactating female bears have it worse because they lose the most weight. They do not eat, drink water, or pass excrement in hibernation. Instead, they rely on fat reserves.

Their metabolic bodily functions process the fat reserves to produce food and water while their kidneys shut down almost completely. Their bodies recycle urea, a major component of urine, to maintain their body mass and organ tissues. They must recycle their waste because it poisons them as it accumulates.  

3. European Fat Dormouse

The European fat dormouse is active for about six months, then hibernates roughly from October to May. They prepare dens in soft soil or caves, relying on fat reserves to weather the winter. 

Their body temperature and metabolic rate drop, and they stop breathing. This animal hibernates the longest, staying dormant for up to 11 months when food is scarce².

4. Marmots

Marmots are the largest living members of the squirrel family. They weigh up to 11 pounds and are two feet long. They live beneath high-elevation meadows and rocky fields. Because they live at high elevations, marmots experience harsh winter periods. So, marmots hibernate to conserve energy. 

Marmots bulk up for hibernation in spring and summer, sealing their burrows in October. Their body temperature plummets from 97°F to a mere 5°F, with breathing rates dropping to 2-3 per minute. Adult marmots safeguard their young's warmth by cuddling up to them during this period.

Fun Fact: Researchers discovered that marmots' hibernation contributes to their longevity by slowing down their aging.

5. Common Poorwill

Common poorwill is one of the few bird species that hibernates. Common poorwill is the smallest member of the Nightjar family. They are 7-8 inches long. You can rarely see the bird because it is active at night. They live in low tree branches or on the ground. 

Common Poorwill birds are not true hibernators because they enter a torpor state. This state occurs during cold temperatures and food scarcity. In hibernation, the bird's metabolism slows, and its body temperature drops, allowing it to survive long periods without food. 

6. Wood Frogs

Wood frogs are native to Alaska and the Northeast United States. These 3.5-inch-long frogs are easily recognizable by the black markings across their eyes and their varying body colors. Wood frogs can be brown, green, gray, or red.

They eat enough food during the hot weather to build up their reserves for long periods of freezing temperatures. As winter approaches, they burrow under a thin layer of fallen leaves and wait for their bodies to freeze. As they turn ice cold, little crystals form along their bodies. 

Wood frogs hibernate for 4 to 6 months yearly. Their bodies slow down their bodily functions; cellular activities function at a bare minimum, and their blood doesn’t flow. The liver of these frogs overproduces glucose and urea⁴, which moves to their cells, tissues, and organs. 

It prevents the cells and tissues from freezing up by binding the water molecules inside the cells. They emerge from their deep sleep as early spring arrives and the weather warms.

7. Groundhogs

Groundhogs, also known as woodchucks, are rodents related to marmots. These ground squirrels have a grizzly brownish-gray fur. They prefer to live in caves on fields, open farmlands, and bushy and wooded areas. 

Groundhogs experience true hibernation. They hibernate in late fall and emerge in late winter. The hibernation period lasts for three months. During this period, their body temperatures drop from 99F to 37F. Their heart rate slows to 5 beats per minute, with two breaths per minute. Also, groundhogs reduce their metabolic rate in hibernation. 

According to the National Wildlife Federation, they eat at least 1 pound of vegetation before entering deep hibernation and lose 25% of their weight when they emerge from hibernation.   

8. Fat-tailed Dwarf Lemurs

Next on our list of animals that hibernate are fat-tailed dwarf lemurs. They are the only hibernating species in the primate world⁶. These animals hibernate for up to 7 months. Fat-tailed dwarf lemurs prepare for hibernation by gorging on fruits and flowers during spring and fall. 

They store accumulated fat on their tails. The stored fat makes their tails reach 40% of their weight. In hibernation, their heart rate drops from 180 beats to 8 beats per minute. Their body temperature also reduces, matching with the environment’s ambient temperature. 

Scientists are trying to learn more about hibernation in fat-tailed dwarf lemurs to discover ways to induce hibernation in humans.

9. Land Snails

Land snails have two dormant periods: estivation and hibernation. Animals aestivate during summer and hibernate during winter. They gorge on food a month before entering a dormant period. Snails have no access to moisture in hibernation, so they seal the entrance of their shell with an epiphragm. 

The epiphragm is a dry layer of mucus that protects snails from outside elements until weather conditions become suitable. Reports show that snails’ physiological functions adapt to stressful situations to help them save energy.

10. Prairie Dogs

Prairie dogs are medium-sized burrowing rodents. They are native to the short-grass and mid-grass ecosystems of North America. They dig burrows 5 to 10 feet deep and about 35 feet long. These lairs have chambers with multiple openings. 

Prairie dogs are not true hibernating animals. They are known to enter a short state of dormancy at freezing temperatures. Their breathing and metabolism slow down. Unfortunately, they are prone to attacks from larger animals in hibernation. Reptiles, small mammals, eagles, and coyotes often attack them. 

11. Snakes

There are over 3000 snake species, which include anaconda, cobra, and garter snakes. Snakes are cold-blooded animals. They need external temperature to stay warm, especially during cold weather with freezing temperatures. However, they do not hibernate. They experience brumation.

The brumation state lasts up to five months. During this period, a snake becomes less active, and its metabolic rate slows. Snakes don’t have to eat because they don’t rely on their bodies to produce heat.

The snake’s hiding spot during winter is called hibernaculum. It is a natural landscape like a dead tree, another animal’s burrow, or an old well. Snakes emerge from their hiding spot in spring when the sun is out. They spend some time near their hibernaculum, basking in the sun. Male snakes emerge when the temperatures rise to 15C and wait for their female counterparts to appear for the mating season.

12. Hedgehogs

Next on our list of animals that hibernate are hedgehogs. Hedgehogs are nocturnal mammals with over 5,000 spikes on their bodies. They are a spiky ball of cuteness. Hedgehogs roll into a ball whenever they feel threatened. 

In the United Kingdom, hedgehogs hibernate from November to April. They build a hibernaculum with leaves and twigs from broadleaved trees, which provides further insulation to help maintain their body heat. 

They wake up from hibernation when the temperature increases by a few degrees. Reduced food sources put them at risk of starvation. Scientists from Nottingham Trent University believe hedgehogs are going extinct because they do not have a proper hibernaculum during winter.

13. Bats

Bat species have two choices when winter arrives. They either migrate to warmer climate regions, hibernate, or do both. Bats are animals that use high energy to fly and hunt for food. In hibernation, their metabolism, heart rate, and breathing slow down drastically. Usually, a bat’s heart rate is 200-300 beats per minute but drops to 10 beats per minute. 

Their body temperatures also drop to near-freezing temperatures. Unlike other animals, bats can't store fats. These flying mammals conserve energy by being torpid⁵ for 99.6% of the hibernation period. They prefer to hibernate in caves, rock crevices, and mines. Bats hibernate in colonies, warming each other up with their own body heat.

14. Skunks

Skunks are nocturnal animals that release a pungent musk fluid whenever they feel threatened. They eat as much food as possible during fall to make up for the dormant period in winter. Eating a lot gives them layers of fat, which they rely on during their torpor state. 

Skunks do not build the dens they die in during hibernation. They prefer taking over other animals’ burrows or spaces beneath decks and porches. Skunks block the den’s entrance with leaves and grass to prevent unwanted visitors and disturbances. Once their den is secured, they enter into a deep sleep. 

Skunks' temperature reduces, and their metabolism slows down. By the end of winter, they lose 14 to 58 percent of their weight.

15. Bumble Bees

Bees can not survive intense cold temperatures, and they don’t have access to flowers and their nectar during the winter. Many species of bees exist, and they all have different survival tactics during winter. 

For instance, worker bees die in a bumble bee’s colony. Only the queen bees hibernate at the end of summer. The queens gorge on pollens and nectar to build fat reserves before hiding beneath loose soil. They come out of their resting state¹, called diapause, to replace the 80% fat body lipids they lost during dormancy. The queens lay their first set of eggs after rebuilding their energy reserves. 

16. Arctic Ground Squirrels

Hibernating arctic ground squirrels are native to the cold regions of Alaska. They are the largest species of North American squirrels, weighing up to 1,500 g. These ground squirrels hibernate for up to eight months in their chosen hibernacula. 

Ground squirrel hibernacula always has some vegetation coverage because it provides warmer soil temperatures. Their metabolic rates reduce drastically for three weeks. Their body temperature goes from 99F to 27F.

17. Hummingbirds

Hummingbirds are small, long, and narrow birds. There are 366 species of hummingbirds worldwide, with 17 species nesting in various regions across the United States.

They can enter deep and light torpor states. It depends on the weather conditions and their needs. The torpor state can last for a few hours or overnight, with a reduction of resting metabolic rate by up to tenfold. 

18. Ladybugs

Another animal on our list of hibernating animals is ladybugs. During winters, ladybugs in temperate regions may hibernate in clumps under vegetation, rocks, or ,atop hills. In hot summer areas, they enter dormancy or estivation.

19. Carpenter Ants

Carpenter ants also hibernate in the winter. You’ll find them nesting somewhere in the walls and floors of a structure. Carpenter ants are pests in homes across the northeastern United States but are beneficial to nature in their natural habitats. This animal goes into diapause during winter but they can be active only when its nesting area has adequate heat.

Conclusion: What animals hibernate?

Hibernation is a crucial energy conservation skill in animals, especially for animals that can’t regulate their body temperature. As animals like land snails, bats, and skunks enter a dormant state, their breathing slows, and they become brain-dead for a short period. Hibernation explains the absence of several animals during winter and other harsh environmental conditions.