Hibernation in the Animal Kingdom
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When it comes to surviving the chill of winter, animals don’t have the luxury of four walls and a wood-burning stove. Instead, they have to come up with other ways to make it through the colder months.
While some may migrate to a warmer climate, others simply don’t have what it takes to travel thousands of miles. Instead, they curl up and hibernate. But this lengthy period of dormancy isn’t just a cozy sleep as it’s portrayed in Disney movies. There’s a lot of preparation involved and animals’ bodies have to be ready.
What is Hibernation?
As children, most of us are taught that hibernation is when animals go to sleep for the winter. That’s really all the explanation we are given, so if you imagine a cute little mouse or hedgehog snuggled away for winter then that’s probably why.
Of course, there is some truth to this description but hibernation is likely much more of a complex process than your school teachers would have had you believe.
Hibernation is a period of dormancy or inactivity. The depth of hibernation varies between animals, with some entering a very deep state of unconsciousness while others retain some function. The main issue with hibernation is that animals become vulnerable to predation but they have no choice. Without hibernation, they’d be exposed to extreme temperatures and a lack of food, meaning they’d be unlikely to survive.
When animals hibernate, they don’t simply close their eyes and go to sleep. This is a much deeper form of sleep whereby the animal will decrease its metabolism, therefore, reducing energy consumption and resulting in a reduced need for sustenance.
What are the Benefits of Hibernation?
When humans think of hibernation they usually think of a long sleep and, if we could do it, we wouldn’t have to worry about that pesky alarm going off in the morning. However, hibernation isn’t all about resting for the animals that do it. With that in mind, let’s take a look at some of the benefits of this process.
- Warm-blooded animals may find it extremely difficult to survive the cold temperatures of winter in the local climate. Instead, while they hibernate, they will keep their body temperature just above that of the surrounding air. This expends less energy compared to maintaining a higher body temperature.
- When food becomes scarce during the winter months, hibernation is a way to conserve energy.
- In line with this, hibernating ensures the animal’s survival and longevity when food sources are low. There’s even evidence that hibernating animals are long lived purely because of their seasonal dormancy. However, it’s thought that this is because hibernation also lowers the risk of predation since the animal is largely hidden away.
- Animals have circannual rhythms that act as an internal ‘hibernation clock’ telling them exactly when to go into and come out of hibernation. This allows them to tie their hibernation habits in with things like reproduction. The obvious main benefit of this is that the animals will emerge from hibernation to breed at a time when food sources are no longer scarce, which in turn benefits their young.
- The metabolic rate of an animal decreases significantly during hibernation. As a result of this, any bodily damage can be healed without using any extra energy.
How do Animals Prepare for Hibernation?
It might seem odd to prepare for sleep, but as we have already learned, hibernation is not the same as the regular sleep that humans and animals experience. Therefore, these creatures need to prepare their bodies for what otherwise might be something of a challenge. And the preparations they undertake aren’t just surface level; they are on a cellular and molecular level as well.
Building up Fat Reserves
While animals don’t use anywhere near as much energy when they’re hibernating, they still need to use some and this is where additional fat reserves are essential.
This is something most commonly seen in mammals who will eat more than they usually would in order to increase their fat reserves to keep them going during hibernation.
Changing Body Temperature
One of the most interesting things that an animal can do to prepare for hibernation is to lower its body temperature. Remaining just a few degrees above the air temperature, animals such as bears can ensure they’ll use up as little energy as possible.
Slow Down their Metabolism
Another way that animals reduce the amount of energy they use during hibernation is by slowing down their metabolism. Endotherms (animals that produce their own body heat internally) do things like shivering to warm themselves up but this is a metabolic act. Without the right amount of energy, they are unable to do this.
Lowering their Heart Rate
Bats are known to lower their heart rate during hibernation as again, this helps them to save energy. Amazingly, the dwarf lemur lowers its heart rate so much that it beats just six times per minute compared to its usual 300 beats per minute.
Changing Hormone Levels
The endocrine system is responsible for producing hormones, and this system comes into play when it’s time to hibernate. The glands in the animal’s body will react and produce varying levels of hormones, which in turn help to make the changes I have already discussed. For example, the thyroid gland helps to alter the metabolic rate. What’s more, the hormone insulin production can alter how much sugar an animal needs.
Types of Hibernation
The diversity of animals is so great and as such, there’s no one size fits all when it comes to hibernation. While some animals will enter into a deep state of unconsciousness, others might experience short bursts of inactivity.
Deep Hibernation
Deep hibernation is probably what most people imagine when they think of a hibernating animal. This can also be called true hibernation and is something that’s seen in many rodent species, although bears are also now considered to be deep hibernators.
By true definition, deep hibernation occurs when the animal enters into such a deep state that its body temperature drops to something similar to the outside temperature. However, in line with the broader definition, it can also relate to metabolic suppression.
One thing that is for sure is that deep hibernation is not a sleep but something much more profound. In some cases, the animal can be in such a deep state of dormancy that it may appear to be dead. Moreover, when it emerges from this state, there is a recovery period which is not the case with regular sleep.
Most deep hibernators will remain in this state for a few weeks at a time. They may then wake up to feed or excrete before returning to their hibernation. However, the dormouse is known to hibernate for as long as 11 months at a time!
Torpor
Torpor is a different kind of hibernation that typically lasts for a much shorter period, maybe only a day. Animals such as squirrels and badgers may display this behavior and will use torpor as a way of conserving energy that regular sleep cannot achieve. They’ll still lower their body temperature and metabolic rate and this allows them to get through a cold spell without having to build up huge fat reserves. This is why we typically only see this in smaller animals such as the hummingbird.
Aestivation
Many animals will enter into a period of dormancy in order to survive cold conditions. However, on the other end of the spectrum, there are creatures like crocodiles and snails that need to get some relief from very dry or hot conditions. In this case, they use a process called aestivation.
Aestivation is a way that the animal can prevent itself from drying out and is usually done in the summer months.
Brumation
Certain species of snakes, lizards and other reptiles also need to protect themselves during the winter. But being ectothermic, they are unable to regulate their body temperature and so need to find somewhere that is warm enough for them to hibernate. Only in the case of reptiles, this process is known as brumation.
Because reptiles are unable to control their body temperature, lower environmental temperatures mean that their bodies would slow down making it almost impossible for them to hunt and survive.
Animals that Hibernate
Hibernation certainly has its benefits, and many animals make the most of this. While there are thousands of species that hibernate, let’s get to know a few of the most common and interesting hibernators.
1. Fat-Tailed Dwarf Lemur
The fat-tailed dwarf lemur is the only primate known to hibernate; well, aestivate at least. These animals are endemic to Madagascar which has a tropical climate. What’s unusual about it is that no other tropical animal aestivates or hibernates, making this lemur even more unique.
Fat-tailed dwarf lemurs may hibernate for up to seven months in order to avoid droughts during the winter which is common where it lives. While they may spend some time hibernating in hollow trees, these lemurs are also known to spend up to 6 months at a time under the ground.
What’s more, while most hibernating animals don’t enter a typical period of sleep but rather dormancy, the fat-tailed lemur is the only animal that fully sleeps during hibernation.
2. Brown & Black Bears
Bears are often seen as the best hibernators but they don’t actually enter into true hibernation since they do not drop their body temperature quite as low as some other animals. However, their metabolic rate does drop by as much as 25%.
However, just like many species, brown and black bears will go through a period of hyperphagia or excessive eating to build up their fat reserves. When they do this, they may eat up to 20,000 calories a day! Not only does this ensure they will survive the winter, but also makes reproduction possible.
When bears start denning depends on the local climate and this also affects how long they take their winter nap for. For example, in southern areas like Mexico, bears may only hibernate for a few days whereas further north, some bears may not emerge for up to six months.
3. Arctic Ground Squirrel
The Arctic ground squirrel is a fascinating hibernator that is able to drop its body temperature to below freezing during hibernation (27°F (-3°C)).
But the key seems to be waking up every few weeks to shiver themselves back to a warmer temperature before continuing with their hibernation. On top of this, it’s been noted that while the rest of the body gets very cold, the head stays slightly warmer, enabling the brain to function. What’s more, certain modified proteins within the brain may help scientists in their research for a cure for Alzheimer’s Disease.
They typically hibernate in areas with a lot of vegetation cover and may remain in this state for as many as eight months of the year. Their heart rates drop to around one beat per minute, and their bodies are even able to recycle nutrients to survive.
4. European Edible Dormouse
The word dormouse comes from the French verb dormir which means to sleep. Knowing this, it will then come as no surprise that these creatures are among some of the longest hibernators on the planet. In fact, they’re known to hibernate for 11 months or more!
The European edible dormouse may wake up to feed during hibernation but in most cases, they will build up their fat reserves before their big sleep. However, over the course of hibernation, they may lose up to half of their body weight. Their nests are usually along the bases of hedgerows but they’ll sometimes sleep in abandoned bird nests.
While scientists believe that one of the main reasons for hibernation in dormice is to avoid predation, there is a suggestion that as many as 80% of hibernating dormice are taken during this time.
5. Spadefoot Toads
When we think of hibernation, we usually imagine mammals, but the spadefoot toad is an example of an amphibian hibernator. During their hibernation, their increased fat reserves ensure their survival and allow them to remain in an underground burrow for up to ten months at a time!
While they will hibernate to avoid dry, cold weather, these toads are rarely seen even when they’re active as they are largely nocturnal.
What’s interesting is that while the spadefoot toad will hibernate over winter, it may also enter into a period of aestivation in summer when the weather is particularly dry.
6. Hedgehogs
Hedgehogs are obligate hibernators that typically start their winter ‘sleep’ between October and November. They’ll continue hibernating until around April although if there is a mild spell, they might venture out in search of a new nest.
Being true hibernators, hedgehogs not only decrease their metabolism, but also drop their body temperature. What’s more, while a hedgehog’s heart might usually beat at around 190 bpm, during hibernation, this drops to around 20 bpm. Their rate of breathing slows so that they may only take a breath every few minutes.
So, why do hedgehogs put themselves through this? Well, their main food sources are insects and bugs. But in winter, there aren’t as many of these around so it makes sense for the hedgehog to hunker down under a log pile, a garden shed or among the hedges and wait for winter to pass.
7. Chipmunks
If you were to see a hibernating chipmunk you could be forgiven for thinking that it was dead. These animals drop their body temperature to freezing, and their hearts beat just one a minute.
Chipmunks are a type of squirrel although their cousins don’t actually hibernate. Even the hibernation habits of the chipmunk are vastly different from other animals. For example, they don’t build up their body fat before hibernating but instead will wake up from time to time to feed on food stores they stashed before winter.
Even when they’re not hibernating, these cute critters need as much as 15 hours of sleep every day owing to their high energy levels. These high energy levels make it even more fascinating that chipmunks are able to hibernate. Sadly, due to climate change, the winter survival rates of chipmunks are declining as they’re not as easily able to hibernate.
8. Bumblebees
We don’t see bees in winter and that’s largely because the conditions are too unfavorable for them to fly and forage. However, some bees, like the honey bee, don’t hibernate but instead remain in the hive in a huddle, living on honey stores they create during spring and summer.
However, bumblebees are much different, and the queen will hibernate under the ground in a burrow before emerging in spring to lay her eggs. When she emerges, she will still spend a significant amount of time hiding in leaf litter and foliage on the ground. The males will all die off by the time winter comes around.
Queen bumblebees have been seen to be emerging much earlier as a result of climate change. But this is detrimental to the species as the nectar sources are not quite as abundant as required.
9. Marmots
During the spring and summer months, marmots, a type of ground squirrel, will gorge themselves on food to bulk up ready for hibernation. They’ll also prepare a cozy den where they will hunker down over winter in pairs or as a family.
In order to keep out predators, the marmots will seal their dens with their own poop as well as materials like soil and grass.
The marmot will lower its body temperature and slow their rate of breathing down during hibernation in order to conserve energy. Burning just 0.03 oz (1g) of body fat each day, scientists believe that these animals can essentially halt the aging process while they hibernate.
While many animals are negatively affected by climate change, it seems as though the marmot could be an exception to this. They wake up earlier in spring and are therefore able to feed more. The result is that over the course of 33 years, each generation within a study group seems to have grown bigger and healthier.
10. Bats
There are more than 1400 species of bats but not all of them hibernate. There are some that migrate and others that do both. Of course, for the purposes of this guide, I’ll be looking at those that hibernate.
Some bat species may have an active heart rate of up to 1000 bpm, but when they’re hibernating, this can drop to as low as 20 bpm. What’s more, their oxygen requirements may drop by up to 100 times, and they build up their fat reserves to ensure their survival over winter. It’s thought that hibernation is the key to some bats’ long life spans. For example, the big brown bat can live for up to 19 years which is a long time for such a small animal.
During hibernation, it’s important that bats rest somewhere with high humidity otherwise they risk dehydration. That said, many bat species will periodically rouse to take water before going back into their dormant state.
11. Groundhogs
Dropping their body temperature to as low as 37°F (3°C), the groundhog is considered to be a true hibernator, and it can remain in hibernation for many months at a time. On top of this, groundhogs will drop their rate of breathing to just two breaths per minute compared to their usual 16 and drop their heart rate by more than ten times per minute.
Groundhogs will burrow in an underground nest and will typically hibernate around the first frost. During the summer, they’ll gorge on food to give them energy during hibernation as they won’t re-emerge until around April.
Scientists have noticed that the groundhog’s brain doesn’t need as much oxygen when it is hibernating.
12. Crocodiles
Crocodiles are ancient reptiles that are thought to have walked the planet for hundreds of millions of years. They’re not the first animal you’d think of when discussing hibernation, but this is something that they do. At least, they enter into aestivation when the weather gets too hot.
During this time, the crocodile will dig a burrow under the ground where it’s nice and cool. It will also slow down it’s body processes to conserve energy. What’s more, since crocs are unable to generate their own body heat, they may also go into aestivation during winter as a way of staying warm.
Their cousins, the alligators also behave in a certain way, entering a period of brumation between November and April. However, they may emerge on warmer days to bask in the sun.
13. Snakes
I’ve heard many a new snake owner panicking because their beloved pet appears to be dead. But this isn’t a fatal situation, and the snake is likely in a period of brumation. While this isn’t as common in captive snakes owing to the consistent vivarium conditions, it can still happen, and it’s a trait that is ingrained in them from their wild cousins.
Brumation is essentially the reptile form of hibernation, and snakes do this in the winter to conserve energy.
Instead of going into a deep state of sleep, snakes will simply stop moving around as much and become much less active. During this period, the snake will stop eating, but it’s not uncommon for them to eat more than usual in preparation for brumation.
However, the garter snake is known to hibernate in a truer sense of the word and it does so in groups. Individuals will coil up around one another to keep warm and may remain this way for up to six months at a time.
14. Common Poorwill
Most bird species will migrate during winter, and it’s uncommon to hear of them hibernating. Where they do, they usually enter into torpor which is the case with the common poorwill. This usually only happens in specific areas, most notably the southern part of its North American range.
The common poorwill will spend most of the winter in a state of torpor which is not something we see in other avian species, making this a rather unique bird.
A type of nightjar, the common poorwill lowers its body temperature and heart rate in order to maintain its state of torpor for so long. Interesting, while this behavior was first observed in the early 1800s, it was not properly acknowledged or documented until the 1940s.
What Happens if you Disturb a Hibernating Animal?
Nobody likes to be woken up, much less a hibernating animal. In fact, disturbing a hibernating animal can be detrimental to its survival, so if you ever find one, leave it alone. The very act of waking up requires a good amount of energy which these animals just don’t have during this time. Wake up an animal too many times, and its fat reserves will be prematurely depleted, thus preventing its survival for the rest of winter.
What’s more, when animals like bears hibernate, they don’t lower their body temperature as much, meaning it’s easier for them to wake up. A human could easily disturb a bear which is not only bad for the bear but could be dangerous for the human. While bears are generally docile animals, they will attack if they feel threatened. Being woken from hibernation will certainly alarm the bear and make it feel as though it’s under threat.
Impact of Climate Change on Hibernation
Animals don’t have calendars and clocks that tell them when it’s time to hibernate. They are able to sense subtle changes within the seasons that are more than enough for them to follow their natural rhythm. However, with climate change, we are seeing seasons merging into one another and fewer changes can be seen between them. Seasons are lasting much longer and this is having a direct effect on animals that hibernate.
Many animals from insects to frogs and hedgehogs are all affected by climate change, and in a lot of cases, they are entering hibernation too late or emerging too early. The problem with this is that they will not have time to properly prepare for hibernation or may come out of it during a time when food is still scarce, threatening their survival.
In captive environments, it has been observed that bears are entering and coming out of hibernation at the wrong time, and the whole process is being cut short. While it’s harder to determine whether this is happening in the wild, scientists think this could very well be the case.
The fat-tailed lemur was discovered as being the only primate that hibernates. In many cases, it can spend up to seven months at a time hibernating inside trees. However, with inconsistent temperatures, these animals are being woken too early, and the fruit that they feed on is not yet abundant enough for them to survive.
And it isn’t just hibernators that are at risk, animals that enter into aestivation may also be affected by climate change. Take the diamondback rattlesnake, for example, which aestivates during the summer months. However, since lower temperatures are conducive with reproduction, the rising temperatures associated with climate change could affect the populations of these snakes.
Bats are among some of the most affected animals when it comes to climate change and are the most likely to prematurely emerge from hibernation. This is especially true of cave-dwelling bats that may use up to two-thirds of their energy reserves just from waking up.
Could Humans Hibernate?
During winter, I often feel more tired than usual, and the cold weather makes me want to curl up in bed and sleep for as long as possible. I’m sure many humans feel the same, but did you know that human hibernation could be a real possibility?
In fact, it’s thought that early humans may have used hibernation as a way of getting through harsh winters. While there are many defects that are believed to have occurred as a result of this hibernation, such as abnormal growth spurts in teenagers and vitamin D deficiency, scientists think that hominids may have attempted to hibernate.
However, naturally, humans don’t really have the capacity to hibernate but that hasn’t stopped scientists from looking into a type of forced torpor. In fact, it’s something that could come in handy when sending astronauts on long-distance trips. In the future, there’s hope that we might be able to send humans to the outer reaches of the solar system but this could take years. Entering a form of induced hibernation would preserve resources and astronauts’ energy.
While there is still a lot of research to be done on the potential benefits of hibernation for humans, scientists believe that it could be a way to tackle obesity. What’s more, scientists are looking at how the physical ability of bears to shut down genes that cause bone breakdown during hibernation could help in the fight against osteoporosis in humans.
It has also been theorized that induced hibernation or at least suspended animation could be useful in treating life-threatening injuries where doctors need to buy time to perform surgery and administer treatments.
