How and Why Animals Hibernate: Facts and Myths
You might have been taught that hibernation is a way for animals to snooze and avoid facing the cold during the winter months, but actually, hibernation is different from sleep.
What is hibernation? Hibernation (1) describes an extended period of time in which an animal’s metabolism (2), heart rate, and breathing (3) all slow down, while their body temperature drops precipitously, sometimes to temperatures below freezing (4). Animals enter hibernation to conserve their energy during times of short food supply and inhospitable weather (5).
Many different animals hibernate, including mammals (6), birds (7), and even fish (8). The hibernation process might surprise you, because it’s different from what most people have been taught about it.
1. Hibernation Isn’t the Same as Sleeping
Many people commonly think of hibernation as a long winter sleep, but that’s just a myth. How does hibernation differ from sleep? The physiological changes of hibernation (9) are quite severe and designed to enable the animal to save energy and survive without eating for long periods of time.
During hibernation, an animal’s metabolic rate slows to as low as 2% (10) of normal levels. Their body temperatures can drop by as much as -2.9 degrees Celsius. By contrast, human body temperatures drop only slightly (11) during sleep.
An animal's breathing rate slows significantly during hibernation. Some reptiles, like turtles (12), stop breathing altogether. Heart rate is also reduced. Squirrels, for example, may slow their heart rate down to just a few beats per minute (13). Again, both of these processes are slower during sleep, but nowhere near as slow as they are during hibernation.
The largest distinction between hibernation vs. sleep is brain activity. During sleep, our brain waves change as we cycle through the stages of sleep. In hibernating animals, brain wave activity looks a lot like it does when they're awake, just more slow (14).
Animals are difficult to rouse from hibernation. They appear sleep-deprived (15) upon waking up, and they need to catch up on deep sleep afterward. Waking up from sleep, on the other hand, is a fairly quick process, and after a good night’s rest, sleep deprivation is minimal.
Finally, sleep differs from hibernation in that sleep is a regular, daily process for many animals (although not all). Hibernation, however, can last days, weeks, and even months.
2. Animals Aren't Always Still During Hibernation
What do animals do when they hibernate? Animals certainly spend a lot of their time lying still during hibernation, but that’s not all they do. Many animals will periodically get up in order to eat, go to the bathroom, or even give birth.
Some animals prepare for hibernation by building up their fat reserves ahead of time, while others store food (16). Animals that store food need to wake up every so often to eat and drink; otherwise, their metabolism will burn through their fat reserves (17) to maintain a minimal body temperature
When a hibernating animal eats, they’ll also need to go to the bathroom — unless they’re bears. Bears recycle their urea (a waste product of urine), and use water from their body fat, which allows them to stay hydrated. Female bears can even give birth (18) during hibernation.
3. It’s Not Only Bears that Hibernate
Bears are certainly the mascot of hibernation, but it’s a title they haven’t truly earned. Unlike the majority of hibernating animals, bears don’t experience a severe drop in body temperature. So, do bears hibernate? Technically, no. However, they do enter torpor.
During torpor, animals exhibit many of the same physiological changes as they do during hibernation, including decreased metabolic activity, breathing, and heart rate. However, torpor can be short-term, lasting a few hours. By this definition, hibernation is extended torpor.
Some animals hibernate for months at a time, including bears, squirrels, lemurs (19), chipmunks, mice, groundhogs, lizards (20), snakes (21), and bats. Some bats, however, enter torpor on a daily basis, as do some species of birds, like chickadees and hummingbirds.
Animals that enter torpor regularly consume massive amounts of energy during the day, by flying around, eating, and staying warm. Conserving their energy for just a few hours each day helps them stay alive during the colder months. Migrating birds may also enter torpor to save energy before taking flight.
Hibernation is actually more common (22) than torpor for smaller animals, and can even increase their survival rate. Small-sized animals who hibernate have a 15% higher survival rate (23) and 50% longer lifespan than similarly sized animals who don’t hibernate. Scientists theorize this may be because animals who are hibernating are less likely to be spotted, heard, or smelled by predators.
4. Not All Animals Hibernate the Same Way
Animals prepare for hibernation in different ways. Squirrels and chipmunks gather nuts, while bears eat a lot to build up their fat reserves.
Hibernating animals love a cozy place to rest just like we do, but their definition of “cozy” can vary. Squirrels and lemurs find a spot in a tree and insulate it with leaves and mud, while snakes snuggle up together. Bats are perhaps most varied in their choice of hibernation spots (known as hibernaculums), and you can find them enjoying spells of torpor in caves, attics, mine shafts, and under bridges.
Different animals hibernate for varying lengths of time. Bears and tropical lemurs may hibernate in trees for up to seven months, while some bats may hibernate for 40 days.
As a coldblooded species, reptiles can’t regulate their body temperature in the same way mammals can. Instead of hibernating, they brumate. During brumation, turtles, snakes, and frogs burrow underground or underwater to stay warm. When it warms up, they may rouse to get some sun, as the Black and White Tegu lizards do in Florida.
5. Animals Don’t Only Hibernate in the Winter
Many animals hibernate in response to temperature changes. They may hibernate when the weather gets cold, because the winter months require more energy, and food supply tends to dwindle. However, food shortages can occur during other times of the years. For example, echidnas (24) in Australia will enter torpor after a fire to survive until their food supply replenishes.
While the cold months threaten many hibernating animals, weather that’s too hot or too dry can threaten others. When it becomes too hot in the summer, some species enter estivation (25) to survive, which is basically hibernation during the summer. Estivating lungfish burrow into mud of river banks, allowing them to stay cool and moist for up to 2 years.
Though hibernation is different from sleep, hibernation is just one of the many ways animal sleep and rest differ from ours. Each animal has its own unique sleep needs and styles. For example, some animals can sleep standing up, while others can sleep with one eye open. Some animals sleep for just a few minutes at a time, while others, like lions, can sleep for hours and hours.
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