Why do animals hibernate?
Every living thing needs to burn energy to survive. Energy is needed to breathe, digest food, pump blood and regulate body temperature. During times of physical activity, such as walking, running, swimming or flying animals need even more energy. When there is lots of food available to burn for the energy, there is no issue for animals to maintain the energy supply levels that it needs to survive.
So why do animals hibernate? Sometimes food is scarce especially during winter. In order to survive, some animals hibernate, using minimal amounts of energy, which allows them to conserve the energy they have for long periods of time, long enough for the seasons to change and food to become plentiful once again.
What is hibernation?
Animals hibernate for many different reasons, but it all comes down to conserving energy. During hibernation the hibernating animal will lower its metabolic rate which lowers its body temperature. Hibernation can also slow breathing and heart rate in an attempt to conserve energy. Hibernation drastically reduces the amount of food that the animal needs to survive, which is the key to survival for many different species. The amount of time that animals may hibernate for varies - it can last for days, weeks or months.
Some creatures that have to endure freezing conditions, especially the cold blooded animals that produce natural antifreeze to stop their blood and body from freezing. Heart rates of hibernating animals can fall as much as 400 to 11 beats per minute and metabolic rates can be as low as 2% of the normal rate.
When preparing to hibernate larger animals in particular become hyperphagic, which means that they eat vast amounts of food to store as fat. Smaller animals tend to store food away, squirrels for example will bury nuts underground.
Types of Hibernation
The term hibernation is used to describe periods of inactivity, in biology the term hibernation only applies to obligate hibernation, where animals hibernate at the same time of year every year. This is sometimes called true hibernation.
Obligate hibernators also known as true hibernators, is the term given to animals that hibernate no matter what the temperature or amount of food is available to them. The body temperature of animals practicing this form of hibernation will drop to around the temperature of its environment, and their breathing and heart rate will slow significantly. During the period of hibernation, obligate hibernators will sporadically wake up from their state of torpor and their body temperature and heart rate returns to normal, then they enter a state torpor again. It is not known why this occurs but some say it could be to fight infection and disease or aid sleep. Some examples of obligate hibernators include: some squirrels, hedgehogs, rodents, marsupials and some types of butterfly.
Facultative hibernators is the term given to animals that hibernate due to extreme temperature or food deprivation. This means that the length of period that the animal is hibernating for depends on weather conditions and food supplies, so hibernation patterns are not as consistent as obligate hibernators'. Some examples of facultative hibernators include: bears, pocket mice, Syrian hamsters and black tailed prairie dogs.
Alternative Survival Tactics to Hibernation
There are many different strategies that animals use to survive difficult times other than hibernation. Some animals enter a state of suspended animation, some migrate, and animals that have to endure scorching conditions enter a state of hibernation called aestivation. Larger animals tend to just migrate as it is not energy efficient to warm up a large body mass, and having a large body means that the animal can keep warmer in the first place having more capacity for insulating fat and fur.
Aestivation is the summer equivalent of hibernation. In places of the world where it gets too hot in summer, drought and scorching heat can destroy food and water supplies. During aestivation, animals tend to burrow down into the ground where it is cooler, where they enter a state of torpor while it waits for rain and cooler weather. Some creatures, such as land snails, will climb trees and hide within their shells while they wait out the extreme conditions. Aestivation is carried out by insects, fish, amphibians and some reptiles. Some animals that use aestivation include the Long-Eared Hedgehog, Nile Crocodile and Gopher Tortoise.
Torpor is the term used for when an animal is inactive for a short period of time, usually this also means a lower body temperature and metabolic rate. It is sort of a short term hibernation and just like hibernation allows an animal to survive in poor conditions. Some animals enter a state of torpor every day, like bats for example. In the UK, animals such as dormice, bats, hedgehogs and wood mice enter states of torpor during cold nights in order to survive. The problem with a state of torpor is that it slows down the reactions of the animals and leaves them vulnerable to predators.
Denning is a form of hibernation that is mainly associated with bears. Whilst denning, the animal is more aware of its surroundings than in other forms of hibernation, allowing it to be easily awoken so it can defend itself from any potential threats. Whilst in this state, an animal can also give birth and take care of its offspring during the early stages. Animals that do this must build up stores of fat that will last the entire denning period. The downside of denning is if the animal is awoken too early the animal may not have enough energy to survive the winter.
Examples of animals that hibernate
Hedgehogs hibernate throughout the coldest months of winter to conserve energy. However, hedgehogs don't always hibernate. If the weather is warm enough and there is enough food available, some hedgehogs will not hibernate as they don't feel the need to.
Snakes also hibernate, but it really depends on the climate. Being reptiles, snakes cannot produce their own body heat and are heavily reliant on the sun for warmth.
Dormice hibernate for roughly 5 months during winter. They prepare for this by foraging and building up fat reserves during summer. However, despite this preparation, a large number of them don't actually survive.
Frogs are ectothermic, this means that they rely on their environment to regulate their temperature. Being ectothermic, frogs must hibernate in certain weather conditions in order to survive. Frogs also require oxygen, so when they hibernate in cold weather, they're typically found only partially buried just below the frost line.
- Common box turtle
As reptiles, box turtles cannot produce their own body heat, which means they do not function well during the colder months. This as well as a scarcity of food requires them to hibernate. A large number of wild hatchlings do not survive their first winter.
Bears hibernate continuously for between 3 and 7 months. When hibernating bears remain at their normal body temperature but they do not eat, drink, urinate or defecate. Bears also do not lose any muscle during hibernation, they retrieve energy from solely their fat supplies that they build up just before hibernation.
To find out more about different animal species that enter the state of hibernation, check our article on animals that hibernate.