Hibernation in Animals

What is Hibernation in Animals?

Hibernation in animals is a state of torpor where the animal conserves energy by reducing its metabolic functions during extreme temperatures.  A torpor, however, is just a state of physical or mental inactivity in the animal.

During hibernation, the animal’s heart rate, body temperature, and breathing rate are reduced to conserve energy. Their body temperature drops and their heart and breathing rates become just a fraction of what they use to be when the animal is in its active periods. This is an important adaptation process done by some animals for them to survive harsh climates like winter.

a picture showing ice on a flower

Surviving harsh winter weather and food shortages as an animal in the wild can be a very big challenge. This is why some of them have to adapt to some survival processes. One of these survival instincts is hibernation.

People feel when animals hibernate, they are just sleeping. They aren’t just sleeping.  Hibernation is very different even though it is often thought to be a long sleep. When the weather is cold and food is in short supply, animals like hedgehogs, bears, and bats become inactive just to save energy and reduce their need for food. According to National Geographic, they enter into a period of inactivity in which their metabolism is just 5 percent its normal rate. This enables them to survive the season.

A similar state, known as estivation (prolonged torpor or dormancy of an insect, fish, or amphibian during a hot or dry period.) occurs in some desert animals during the dry months of summer. Although hibernation is often seen as a seasonal behavior, it’s not only exclusive to cold-weather animals. There are tropical animals that may go into hibernation just to beat the heat.

Hibernation in Animals Examples

There is a misconception that when animals hibernate they do not wake up. The fact is they do wake up, but how often, depends on whether they are true hibernators or light sleep hibernators. For instance groundhogs, ground squirrels and many species of bats are true hibernators.

True hibernating animals can sleep so deep that to wake up is pretty difficult and takes a lot of time and energy. They may wake up every few weeks to eat and, like in the case of groundhogs, use the bathroom in their burrow.  Nevertheless, as spring approaches, they wake up more frequently.

Some other animals that hibernate are considered to be light sleep hibernators. They are not seen as true hibernators because their own hibernation is not as deep and long as a true hibernator. Bears, opossums, raccoons, and skunks are the most famous example of this kind of hibernation.

Unlike true hibernators, Light sleep hibernators wake up more often during the winter. Once they are awake, they carry on as usual. Their body temperature, breathing rate, and heart rate usually return to normal once they are awake. But once they begin to hibernate again, their body temperature, breathing rate, and heart rates drop back.

Animals that hibernate

Animals hibernate in lots of different ways. Let’s look at some common examples of animals that hibernate and how they go about it.

1. Fat-tailed Dwarf Lemurs

They are the only primate known to engage in a combined hibernation and torpor state for a long period of time. When dwarf lemurs hibernate, they reduce their heart rates from over 300 beats per minute to fewer than 6. And instead of breathing every second, they can go up to 10 minutes without taking a breath. Its body temperature drops, too, and becomes driven by the ambient temperature of the environment. Their brain activity however becomes undetectable.

a picture of a Fat-tailed dwarf lemur
Photo credit- Duke Lemur center

Before hibernating, dwarf lemurs prepare by accumulating fat in their tails by gorging on food during the wet season. They take huge advantage of the wet season when fruits and flowers are in abundance. Because during the dry season, these foods are more scarce. During their period of gorging, dwarf lemurs’ tails can reach up to 40% of their total body weight. So that when they enter a state of hibernation, they can live off on the fat stored in their tails.

They hibernate during the dry season when water and food are scarce. Their hibernation can last up to seven months. During this period, there is severely decreased metabolism, heart rate, and body temperature with intervals of metabolic active periods of rewarming called interbout arousals. Interbout arousals tend to occur every 6-12 days during hibernation.  And during interbout arousal, a dwarf lemur’s body temperature rises, its heart rate increases, and its breathing becomes more regular. But afterward, the animal drops back into torpor.

2. Snails

Snails do hibernate but not all snails. They usually hibernate when the weather is extreme. Both in extremely hot weather (estivation) and extremely cold temperatures. The advantage they have is they have a shell they can comfortably hide in during hibernation.

a picture showing an illustration of Hibernation in snail animal

Some snails hibernate during the winter (typically October through April in the Northern Hemisphere). During hibernation, snails attach themselves to a surface, cover themselves with their slime, and wait out winter.  Even in extremely hot weather, to stay moist, they use mucus to seal their shell and protect themselves from the harsh weather (this is known as estivation). They may also estivate in the summer in drought conditions. The dry layer of mucus snails use to seal their shell is called an epiphragm. Snails can hibernate for as long as several months, depending on the species.

3. Bears

There are a lot of mammals that hibernate. Bears might be the first that comes to mind. There is a strong evolutionary pressure for bears to stay in their dens during winter, once there is little or no availability of food. Although they leave their den occasionally if it gets damaged or flooded. If food is present, bears don’t need to hibernate except it is pregnant or winter conditions get severe ( like in Alaska). They hibernate during winter and don’t need to eat or drink, and rarely urinate or defecate during this time.

A picture of a bear sleping in winter

They adapt unique strategies to survive for long without food and water by lowering their body temperature and breaking down fat stored for energy. Some protein is used as well, but they largely conserve their muscle mass and therefore to a large extent do not become weaker during hibernation.

In the colder, northern parts of Alaska, bears can hibernate for 7 months. Whereas bears in the warmer, coastal regions of the state can hibernate for 2 to 5 months ( the bears raising newborn cubs are the ones with the long hibernation time). However, they do wake up and move around inside the den.  Bears would shift around to change their posture when they wake periodically. This aids to prevent them from getting pressure sore. They also shift positions to conserve heat better.

4. Hedgehogs

Hedgehogs tend to live in the outskirts of wooded areas and are often found living in hedgerows. A well-maintained hedgerow will shelter it and protect the hedgehog from harsh weather with a safe place to live, full of nice juicy bugs and critters. Hedgehogs often choose a nice compact hedge as somewhere to live throughout the summer and also as a good hibernaculum. Though there are some factors they consider when choosing a suitable hibernaculum. They search for a dry, sheltered, and compact place. To help insulate and keep the body heat and protect them from predators.

In hot, dry climates, creatures like the hedgehogs aestivate. This is when they find a cool, safe spot and become inactive. Aestivation usually lasts for a shorter period than most hibernation. During the months of winter, there is usually not enough food available and hedgehogs would starve to death if they don’t hibernate. Since there is no enough food they have to reduce their need to feed by going into hibernation.

a picture of a hedgehog

Towards the end of autumn, in preparation for hibernation, hedgehogs like to build nests out of grass, leaves, and straw, often under fallen logs, old buildings, or sheds. During the summer they eat as much as they can to build enough stored fat awaiting the dry season. They can hibernate from autumn to spring (October – March). Although this change is dependant on the availability of food and temperature.

During this period they use up most of the fat stores that they must have built over the summer. While in a state of torpor, hedgehogs are still able to move, but the movement is very limited for the six or seven months that they spend in their hibernaculum. They tend to wake up during hibernation, as often as every two to four days, or as infrequently as once per month. When they actually wake up during torpor, they may relocate to a new nest.

5. Squirrels

It’s been debated whether squirrels hibernate or not. Squirrels don’t hibernate per se because they can’t build up enough body fat. But during summer, they collect and stash food in various locations. Afterward in winter, they sleep for up to 20 hours a day. Then head out early in the morning to feed on the food they had stashed up.

a picture of a ground squirrel - Hibernation in these animals involves stashing up food and sleeping during the winter
A Ground Squirrel

Typical tree squirrels do not hibernate during the winter but they just sleep a lot. However, researchers have documented some species of Ground Squirrel rolled up like a ball in their underground burrow during hibernation from October to May.  Hence, Ground Squirrels actually do hibernate.

6. Reptiles

Reptiles are cold-blooded hence their body temperature is controlled by the environment (endothermic). Some reptiles find an unused burrow to stay till the cold season passes. This is called brumation, which will be discussed. Just like hibernation, brumation is a period of inactivity for reptiles during winter.

Box turtle

There’s a lot of debate about whether or not turtles hibernate. It all depends on the type of turtle. Turtles that live in warmer and sunnier climates do not hibernate. But, if it gets too hot they will estivate, or enter a torpor-like state after burying themselves to escape the extreme heat until a favorable weather returns.

a picture of a box turtle

However, the turtles that do hibernate mostly do so underground. To keep themselves from freezing to death, they will bury themselves in the dirt. While freshwater turtles will lodge themselves under the mud during the months of winter.

However Box turtles as reptiles, are endothermic and to save energy, it has to brumate once the atmospheric temperature starts dropping. Unlike other animals during hibernation, they don’t sleep. Generally, box turtles dig a hole and brumate for 3-4 months. Although it varies from one turtle species to another.

Snakes

The majority of snakes do experience brumation (hibernation for cold-blooded animals). Although the duration of their brumation depends on location. For instance, a snake in Minnesota might hibernate for months, while one in southern Texas might only go a few weeks. Snakes actually get signals from their surroundings. Once the daylight hours become shorter, they become aware that winter is coming.

a picture of a snake hibernating in its hideout

During the cold months of winter, snakes will look for a well-insulated hideout. When they find one they will dwell there using little or no energy until some warmth returns. During brumation, snakes experience periods of wakefulness when they’ll travel outside of their resting spot to hydrate. Sometimes, snakes will brumate in large piles.

7. Wood Frogs

Frogs are cold-blooded ( and precisely ectothermic), so their body temperature closely monitors the atmospheric temperature around them. Once it’s late winter or early spring, you’re likely to see a hibernating frog that’s not moving. In North America, a species of wood frog can survive being thawed and frozen a few times in a season. During hibernation, about 35 to 45 percent of its body becomes frozen.a picture showing a frog- hibernation in Wood frog animals involves them getting frozen.

The wood frog stops breathing and its heart stops beating entirely for days to weeks at a time. In fact, during its period of frozen winter hibernation, the frog’s physical processes (metabolic activity to waste production) come to a near halt. The ice begins to grow when ice crystals touch the frog’s skin. The temperature has to be slightly below 32 degrees Fahrenheit to freeze a frog. The wood frog can survive for weeks with two-thirds of its body water completely frozen. In spring, the frog thaw and begin the feeding and mating process all over again.

8. Echidnas

Echidnas are sometimes known as spiny anteaters. They hibernate in underground burrows or hollowed-out logs during bush fires in the hot, dry Australian outback. They stay carefully hidden away until it’s safe to emerge to find food.

Recently, researchers in Australia have found out that the echidna in the wild hibernates. They also display a pattern of classic hibernation. in this type of hibernation, the torpor is punctuated by periods of classic hibernation. In this type of hibernation, the torpor is punctuated by periods of so-called spontaneous arousal in which the body temperature shoots up towards the normal level.

a picture showing Echidnas- hibernation in these animals involves them staying in burrowsZoologists had previously believed that this type of hibernation is common in placental mammals such as rodents and insectivores, which is much further along the evolutionary path than the echidna. Scientists actually do not know why spontaneous arousal occurs, although some believe that it may be related to the need to urinate. Also, these periodic arousals appear to be triggered by rises in ambient temperature.

9. Common Poorwill

A picture of the common poorwill bird

The common poorwill is known as the first hibernating bird. As the winter cold deepens, these birds enter a hibernation state and can stay like that for hours or even weeks. They hibernate due to reduced food supply and harsh temperatures. While other birds migrate or enter brief states of torpor like the hummingbird, the poorwill can be in a torpid state for several months. During hibernation, it has a reduced heart rate, the body temperature lowers and the breathing rate reduces.

10. Dormice

They are one of the longest hibernators. they can hibernate for up to 11 months. Actually, the name dormouse comes from the French word ‘dormir’ meaning ‘to sleep’. Hazel dormice are one of the few animals in the UK that hibernate all winter. As to why dormice hibernate is really not understood. They come down from the safety of the trees and make a nest on the forest floor.

a picture of a Dormouse- hibernation in these dormice animals involves them making a nest on he forest ground and hibernating in it

Dormice hibernate on the ground. Probably, because the temperature is likely to be more stable and they need to stay damp and cold to keep them in hibernation. Although hibernation is an important period of their life, more dormice die during this period than at any other time. This is probably due to the variable winter temperatures in Britain that wake them up more often and cause them to simply run out of energy.

Most times,  people find hibernation nests in quite exposed locations. It is noticed that dormice in captivity would rather hibernate in soil with no cover than to soil covered by a roof. They can be in this torpid state for several months, in spots that seem rather vulnerable and open. Although hibernating on the ground would make them more vulnerable to predators, but evidence suggests they’re very hard to find.

In spring they awaken more frequently. They normally wake up for longer periods so that by April, about half their population is awake. Basically, by May, they will all be active and awake.

11. Bats

Bats go into hibernation when the temperature gets cold and they need to conserve energy.  They can last in this state of torpor from a couple of hours to a month. During hibernation, bats need roosts that are cool and remain at a constant temperature. They often move into underground sites, such as caves.

A picture of a bat hibernating in a cave

Sometimes, they might move deeper and lower into trees to hibernate. During this period of torpor, their body temperature lowers and their metabolic rate slows. They use less energy. Rather than trying to forage for food, they can survive on the fat they have stored up. The bat’s heartbeat can go from 200 to 300 beats per minute down to as few as 10.

To rewarm themselves during the torpor state, they sometimes use energy from the sun. Bats mate during the autumn and sometimes into the winter when they hibernate. The females however store the sperm and do not get pregnant until spring.

 

Hibernation vs Brumation

Brumation is very similar to hibernation. It is a metabolic state similar to hibernation, although many people use the two terms interchangeable when talking of hibernation in reptiles. However, Brumation is basically just hibernation for cold-blooded animals like reptiles and amphibians.

These Ectotherms rely on their environment to regulate their body temperature. Some would say hibernation refers to the deep sleep that some warm-blooded animals engage in during the winter, while the term brumation refers to the deep sleep that some cold-blooded animals experience during the winter.

The key reason for the difference is that the body temperature of cold-blooded animals changes when the atmospheric temperatures change. For instance, If the temperature is 25 degrees outside, the body temperature of a snake, turtle, or frog will usually be about 25 degrees too. Whereas by contrast, warm-blooded animals don’t allow their body temperatures to fall until they begin hibernating.

Cold temperatures cause reptiles and amphibians to hide underground, in rock crevices, and in burrows to stay warm and safe. Their activity, body temperature, heart rate, and the respiratory rate actually drops like in hibernation.

In addition, brumating animals may wake up when the weather gets warm, even if this happens during the middle of winter. Cold-blooded animals will move on warmer winter days and find water, unlike hibernators who are in a deep sleep and do not move most times. One difference between brumation and true hibernation is longevity.

Hibernation vs Estivation

Estivation is when animals are dormant because weather conditions are very hot and dry. Their breathing rate, heart rate, and metabolic rate decrease to conserve energy under these harsh conditions. These animals will find a spot to stay cool and shaded. Many reptiles and amphibians estivate and some mollusks, insects, fish, and mammals will estivate as well.

The only contrast between hibernation and estivation is that hibernation is for a whole winter in which animals take rest in the warmer place. Their metabolic activity slows down, and they pass the tie in a dormant condition. Whereas estivation is of short duration, where animals search for a moist, shady, and cool place to sleep. This is done to protect themselves from the temperature dis-regulation in their bodies. Some common examples of estivating animals are bees, snails, earthworms, salamanders, frogs, crocodiles, tortoises. etc.

Hibernation vs Diapause

Ever wondered how insects survive the cold months of winter?

a picture of an insect in winter_ Hibernation in tiny animals like insects involves a process called Diapause

Many insects survive winter by entering a period of suspended development called diapause. Diapause is the way insects survive our freezing temperatures by going through a period of inactivity and stopping their growth. However, how these insects enter diapause varies. Some may slow their metabolism and survive on their stored fats. Others may produce alcohol that acts as an anti-freeze for their bodies. The alcohol stops ice crystals (which would puncture and destroy cell walls) from forming,

Not all insects enter diapause at the same stage of their life cycle. For example, woolly bear caterpillars enter diapause as larvae, while mourning cloak butterflies enter diapause as adults.  Also where they experience diapause varies as well. For instance, ladybugs form masses under plant debris, while the larvae of June bugs burrow underground and let the soil and mulch above serve as a protective insulating layer.

Advantages of Hibernation in Animals

1. Hibernation reduces the risk of predation on animals that hibernate.

For instance, most species hibernate for 4-6 months, and will not be exposed to their normal predators.

2. Hibernation can reduce the death rate amongst animals.

For species where predation is one of the major causes of death, hibernation is a huge advantage. Also instead of the animals starving to death or being killed due to the harsh climate, hibernation helps as a survival skill.

3. Hibernation in animals helps to save the animal’s energy.

Oxygen consumption and breathing rate will slow during hibernation. Hence due to this low energy intake, most hibernators can survive for a few months on fat reserves. This is of enormous value for animals living in climates where the environmental conditions or temperature are too harsh to be active in, or in areas where food availability in winter is too low to sustain the organism. Without the energy-saving aspects of hibernation, existence in these regions will almost be impossible for those small mammal species.

Disadvantages of Hibernation in Animals

1. Hibernating animals have decreased vigilance during hibernation and therefore cannot quickly respond to threats.

Most times awakening from hibernation can take several minutes to several hours. This makes them more exposed to predation or danger.

2. Hibernation in animals may have some harmful physiological effects on the animal.

It has been discovered that hibernation has a bad effect on the animal’s memory.

3. In male animals, their sperm cell production is inhibited during hibernation.

This is because of low body temperatures and low metabolic reactions.

4. Immunity is also reduced when an animal hibernates.

This makes the animal vulnerable to infections and parasites during and shortly after hibernation.

5. Hibernation in reproductive female animals can cause a slowed growth of their young ones

 

Frequently Asked Questions

How do animals prepare for hibernation?

To prepare for a long hibernation in which they don’t eat or drink, animals stuff their body with enough food when food is abundant against the upcoming drought. For example, dormice, bear, and some others eat so much that by the end of summer, they can be double their normal size because they can lose up to half their body weight during hibernation.

Fat-tailed Dwarf Lemurs too also prepare by accumulating fat in their tails by gorging on food during the wet season. During this period their tails can reach up to 40% of their total body weight. So that when they enter a state of hibernation, they can live off on the fat stored in their tails.

Some animals also prepare by finding a safe shelter, cave, or burrow where they can hide and hibernate through the winter.

Is hibernation just sleeping?

We like to think of animals that hibernate as sleeping the winter away, but sleeping and hibernating are not the same. Hibernating animals get ready for their winter sleep by eating extra food and storing it as body fat which they then use as energy while sleeping. However, there are two types of fat: Regular white fat and Brown fat.

The brown fat forms patches near the animal’s brain, heart, and lungs. It sends a quick burst of energy to warm these organs first when it is time to wake up. This is very different from sleep (which is just a gentle resting state) where unconscious functions are still performed. Unlike sleeping, during hibernation, the animal’s metabolic activities reduce drastically or come to a near halt.

How long does an animal hibernate?

Hibernation can last as long as the determining factor or weather lasts. Animals can hibernate from a period of days to weeks to even months, depending on the animal species. Some animals, like groundhogs, hibernate for as long as 150 days.

What happens when animals hibernate?

During hibernation, an animal’s heart rate, body temperature, and breathing rate are reduced to conserve energy. Their body temperature drops and their heart and breathing rates are just a fraction of what they use to be when the animal is in its active periods. This is an important adaptation process done by some animals in order for them to survive harsh climates like winter.

What animal hibernate in summer?

We commonly hear of animals hibernating during the winter, but many animals have adapted this practice too during the hotter months. This hibernation-like practice is actually called estivation. Like hibernation, these animals will lower their metabolism and survive for extended periods of time in this state. Unlike hibernation, estivation is generally much shorter, but both serve to protect animals from extreme temperatures and food shortages.  Hence some animals do hibernate in summer too. Common examples are the salamanders, snails, beetle, mouse, frog, lungfish, desert tortoise, crocodiles, hedgehogs, Gila monster, earthworms, ladybugs, and many others.

What animal hibernates in winter?

The majority of animals that hibernate do it during the winter. We have a variety of them. Chipmunks, bears, bats, box turtles, bumblebees, garter snakes, hedgehogs, land snail, fat-tailed dwarf lemurs, wood frogs, skunks, deer mice, ground squirrels, prairie dogs, and many others are all common examples.

Which animals are true hibernators?

How often an animal wakes up depends on whether they are true hibernators or light sleep hibernators. True hibernating animals can sleep so deep that to wake up is usually difficult and takes a lot of time and energy. They may wake up every few weeks to eat and, like in the case of groundhogs, use the bathroom in their burrow.  Nevertheless, as spring approaches, they wake up more frequently. Woodchucks, groundhogs, ground squirrels, and many species of bats are true hibernators.

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