Bats are fascinating creatures. They’re nocturnal flying mammals (in fact, they’re the only mammals that can fly at all), and they hang upside down when they sleep. Because bats prefer to live in dark recesses like caves and abandoned buildings, the question is often asked, “Do bats hibernate?”
In this article, we’re going to be addressing this question and answering it definitively. We’ll also be looking at the various behaviors that different bat species exhibit. After reading this, you’ll be well on your way to becoming a bat expert!
Photo Credit: David Morimoto on Flickr
How and When Bats Sleep
First of all, it’s important to draw the distinction between hibernation and sleeping. Hibernation is something many mammals do for extended periods of time, often seasonally. Hibernation periods can last for multiple months.
Most bat species sleep in spurts throughout the daytime, often from what are called roosts. A roost is simply a place where a bat or a colony of bats choose to “hang out” and sleep, nurse their young, or wait for the right time to go hunting or foraging for food.
Most bats will wake up from their slumber about 15-30 minutes before sunset, at which time they’ll head out to to get food. When a bat wakes up, all it has to do is release its hold on the roost, drop directly down into the air, and take flight. Pretty cool!
Some bats will stay out all night, snatching insect prey out from mid-air while in flight. Other bats will spend their time foraging for nectar from agaves or cactus fruits. Other activities bats engage in during the night can include mating, nursing bat babies, and migrating to different areas.
The Bat Hibernation Cycle
Let’s go ahead and answer the question, “Do bats hibernate?” definitively: yes, they do! But not all species of bats engage in the act of seasonal hibernation. .
In fact, many bats are capable of taking long, migratory flights to relocate to warmer areas during the winter, areas where food is more plentiful.
Bats That Hibernate
Generally speaking, bats that live in higher altitudes are more likely to hibernate than bats that live in lower altitudes. Some of the more common bat species that are known to hibernate are the Little Brown Bat and the Lesser Horseshoe Bat.
Whether or not a bat or colony of bats will hibernate depends less on its species, however, and more on its location. Also, drastic seasonal changes in temperature can be a factor that plays into whether or not bats hibernate or migrate.
Bats that Migrate
Bats are ubiquitous throughout the world with the exception of the arctics and a few small islands. Their sheer volume makes studying their migratory patterns quite difficult for biologists. But, from the research that has been conducted so far, we know that there are at least 40 species of bats that exhibit migratory behavior.
For the bats that do hibernate, they do so for one main reason, and that reason is because food becomes scarce in the winter months. A bat’s body temperature is too high for it to continue living it’s normal life without a constant food source; it simply cannot consume enough food to ‘feed the machine’. So, hibernation becomes a necessity.
When a bat hibernates, a few very important physiological processes begin:
The bat’s metabolic rate slows down significantly. A metabolic rate is the frequency at which the biological processes within the bats body take place. This includes things like digestion, organ functioning, and breathing. During hibernation, all of these processes slow way, way down, significantly reducing the amount of energy required to keep the bat alive.
The bat’s heart rate drops. The average heart rate for a bat out of hibernation is around 200-300 beats per minute. During hibernation, this rate is reduced to a mere ten beats per minute. This is just barely enough of a heart rate to keep the minimally required amount of blood flowing through the bat’s circulatory system. Any slower, and the bat might die. Any faster, and the bat would end up consuming too much energy, and it might die from starvation.
[Sidenote: A common infection among bats in hibernation, known as ‘white-nose syndrome’, is responsible for an alarming amount of bat deaths every year in the US and Canada. This fungal infection develops on the bat’s snout, and it often causes the bats to wake from hibernation too early.
When this happens, the bat will head out to look for food, only to find that the winter temperatures are still much too low for hunting. Sadly, the bat will often die from starvation.]
The bat’s body temperature drops. Did you know that during hibernation, the body temperature of a bat can come close to freezing? This biological state is known as ‘torpor’, and it’s an excellent example of just how efficient bats are at conserving energy.
The bat cycles in and out of torpor throughout it’s hibernation period. Once ambient temperatures return to the range that is suitable for hunting and foraging, a natural signal produced by the bat’s body arouses it from hibernation. After a few hours or shaking off the side effects of such a long rest, the bat takes to the air to find food.
Some bat species can enter torpor for just a few hours during an especially cold day. Other bats can spend entire weeks or months in this state of suspended animation. One of the techniques employed by entire bat colonies in hibernation involves roosting very close to one another. This technique provides a kind of heat shield for the bats during hibernation, thus protecting the bats from the elements and even further conserving their much-needed energy.
Where Do Bats Go to Hibernate?
When the time comes to hibernate, bats instinctively isolate themselves from predators and the elements by choosing a roost in a dark, secluded area. By far, the most common location of choice for hibernating bats are caves.
However, abandoned buildings, rock outcroppings, mines, crevices, and other such areas are all candidates for bat hibernacula (hibernacula is the plural form of hibernaculum, a term used to describe places where bats hibernate—fun fact!)
Now, you might be thinking, “If a bat is hibernating, does it still need to pee or poop?” The answer to this question is, yes and yes. Even though a hibernating bat is in a reduced state of biological functioning, it will ‘wake up’ from time to time in order to pass excrement. After it has done its business, however, it will sink back into a hibernatory sleep for as long as the winter period lasts.
How Long Do Bats Hibernate?
As we mentioned earlier, it’s common for some bats to enter a hibernation state for brief periods of time, especially during colder days. For those bats who do not migrate to warmer areas during the winter, the hibernation period can stretch on for months on end.
The average amount of time a non-migratory bat spends in hibernation is 183 days. During this time, there may be brief spells of wakefulness that are useful for the bat to ‘recondition’ to being in it’s own skin.
When a bat is out of hibernation and it’s body returns to a natural state of functioning, the energy required for it to maintain its body temperature is quite high. This is why bats cannot go longer than 24 hours without food or water. In fact, during flight in the night when the bat is actively hunting or foraging, it’s common for the bat to consume more than 100% of its body weight in pure energy.
Even though bats are remarkably light creatures (most bat species weigh only one or two ounces), the caloric expenditure required for flight is very high.
How to Tell If a Bat Is in Hibernation
Bats that are hibernating are very unlikely to be found in broad daylight. Because they choose to hibernate in places like caves and abandoned mines, it’s very rare for the average person to come across them. However, hikers, spelunkers, or others out and about in nature can sometimes find bats in hibernation.
This is especially true if the bats in question are found during the winter months.
One of the first things you’ll notice about a bat colony in hibernation is how still and quiet they are. Because the hibernating bats number one priority is to conserve energy, it’s not going to be moving around at all, and it certainly isn’t going to be flying anywhere. Instead, it’s most likely that the hibernating bat is going to be hanging upside down, completely motionless, and completely silent.
If you ever find yourself confronted with a colony of bats in hibernation, do not disturb them under any circumstances. Why? Because, if a bat in hibernation is ‘woken up’ too soon, it may not have the energy reserves required to defend itself or to escape, meaning it’s possible that the bat or bats will die of starvation.
Bats are a crucial part of our ecosystem here on planet Earth. They are largely responsible for maintaining the balance required for all animals to flourish, including humans! We need bats to keep insect populations in check and to ensure that pollination of certain plant species happens effectively.
Bats do hibernate when food becomes scarce. They choose to do this in areas where they are least likely to be disturbed—places like caves, abandoned buildings, and barren rock outcroppings.
Even though some bats migrate to warmer climates in order to continue feeding, many bats spend multiple months on end in a state of ‘torpor’, or hibernation. Hibernating bats are very fragile and easily disturbed, and they should be left alone if they’re ever discovered.
We hope you’ve learned a lot about the hibernating behavior of bats. For more information on bats, feel free to explore the other resources listed on this page.