For people who aren't fans of winter, animals that hibernate seem to have the right idea. After all, hibernation is the equivalent of burying your head under the covers until spring comes, isn't it? Not quite—the science of hibernation is very different from what happens when you sleep.
What Is Hibernation?
Despite what you may have heard, species that hibernate don't "sleep" during the winter.
Hibernation is an extended form of torpor, a state where metabolism is reduced to less than 5 percent of normal. Metabolism is the chemical process that takes place in plants and animals to keep them alive. It is how our cells convert the food we eat into energy our bodies can use. During hibernation, metabolism is "extremely slowed down or completely halted," scientist Marina Blanco says. She studies the dwarf lemurs (Cheirogaleus spp.) of Madagascar, the only primates that hibernate on a regular schedule.
For example, when dwarf lemurs hibernate, they reduce their heart rates from over 300 beats per minute to fewer than six, Blanco says. Instead of breathing about every second, they can go up to 10 minutes without taking a breath. Their brain activity "becomes undetectable."
This is very different from sleep, which is a gentle resting state where unconscious functions are still performed. In fact, Blanco found that hibernators have to undergo occasional arousals to catch some sleep.
Why Do Animals Hibernate?
"Hibernation is a means of energy conservation," scientist Kelly Drew says. She studies the brain chemistry of hibernating Arctic ground squirrels (Spermophilus parryii).
While hibernation is most often seen as a seasonal behavior, it's not exclusive to cold-weather critters. There are tropical hibernators that may do so to stay cool in the heat.
Temperature isn't always a factor. "Some species hibernate in response to food shortages," Drew notes. For example, a group of spiny mammals in Australia called echidnas will hibernate after fires, waiting until food resources grow back to return to their normal activities.
Recent studies have even suggested a third reason: protection. When hibernating, "you don't smell, you don't make any noise, you don't make any movements," scientist Thomas Ruf said. That makes it hard for predators to find you. Ruf's work has shown that small mammals are five times more likely to die each month when active than when hibernating.
What Actually Happens when Animals Hibernate?
To slow their metabolism, animals cool their bodies by 9 to 18 degrees Fahrenheit on average. The Arctic ground squirrels Drew works on can take this much further, supercooling to subfreezing temperatures.
Drew's research has shown that cooling is likely regulated by levels of the chemical adenosine in the brain. During winter, the production of adenosine ramps up in ground squirrels.
However, species don't stay in their cold, comatose state for all of their hibernation period. About 80 percent of their energy is spent occasionally waking and warming up.
Why they do this is "one of the greatest mysteries" of the field, Ruf said. Some think they need to turn back on their immune systems to fight disease, while others think they may simply awaken so they can sleep.
Unfortunately, these arousals may drive hibernating species to extinction as our climate changes. Scientists have found that animals stay active longer during arousal periods as temperatures rise. As a result, they lose more of the energy they are trying to conserve.
What Kinds of Animals Hibernate?
One bird and a variety of amphibians, reptiles, and insects also exhibit hibernation-like states. There is even at least one fish—the Antarctic cod (Dissostichus mawsoni)—that slows down its metabolism in winter.
Of course, there are many mammals that hibernate. While bears might be the first that come to mind, for years questions have surrounded whether bears really are true hibernators. Unlike animals that stir regularly during hibernation, bears can go for 100 days or so without needing to wake to consume or pass anything. They can also be aroused much more easily than typical hibernators.
Most mammalian hibernators are on the smaller side. The average hibernator weighs less than 91 grams (one-fifth of a pound), Ruf said. That's because little bodies have more surface area relative to their volume. This means they lose relatively more heat than larger animals. It is more challenging for them to stay warm in cold weather, so they need the additional energy savings.
What Animal Hibernates the Longest?
It's harder than you'd think to award a prize for the longest period of hibernation. The obvious choice would be the edible dormice Ruf works with. The tiny rodents can hibernate for more than 11 months at a time in the wild. To pull that off, they have to double or even triple their body weight while they are active.
In one experiment, a big brown bat (Eptesicus fuscus) hibernated in a refrigerator for 344 days. That suggests that bats may deserve the title (however, the animal didn't choose to hibernate, and it didn't survive the feat).