There's More to Hibernation Than You Think

Hibernating is just one way animals survive the coldest part of the year, but it takes on many different forms

_{| Story by Meghan McMahon |}

_11/15/2019

Who hasn’t taken a look outside on a frigid winter day and thought maybe groundhogs have the right idea, hibernating all winter long in their underground burrows?

In the animal kingdom, many animals are inactive or far less active in the winter as a means of surviving the harshest season of the year. While we often say these animals are hibernating, there’s more to the story.

Some animals, like the groundhogs, are indeed hibernating. But there’s different ways to hibernate.

Take bears – the most famous of all hibernating animals – as an example. Bears do hibernate in winter, but not quite like groundhogs. For bears, it’s a lighter kind of sleep, more akin to taking long naps throughout the season rather than sleeping the whole winter away, said Sara Russell, an interpretive naturalist for the Forest Preserve District of Will County.

Here’s a closer look at hibernation and other ways animals survive the winter.

Hibernation vs. torpor

We like to think of animals that hibernate as sleeping the winter away, but sleeping and hibernating are not one and the same. Hibernation is a form of torpor, which is a way for animals to conserve energy by reducing their metabolic functions during extreme temperatures.

There are two distinct kinds of torpor: hibernation and daily torpor, Russell said. When a period of torpor lasts longer than 24 hours, it is hibernation. When animals are in torpor for less than 24 hours, it is considered daily torpor.

Hibernation

_{Groundhog. (Photo via Shutterstock)}

Hibernation can last anywhere from a period of days to weeks to even months, depending on the species. Some animals, like groundhogs, hibernate for as long as 150 days, according to the National Wildlife Federation.

Animals such as these are considered true hibernators. They enter into a period of inactivity in which their metabolism is just 5 percent its normal rate, according to National Geographic. Their body temperature drops and heart and breathing rates are just a fraction of what they are during their active periods.

While hibernating, a groundhog’s body temperature drops from 99 degrees Fahrenheit to as low as 33 degrees, reports the National Wildlife Federation. Their heart rate plummets from about 80 beats per minute to 5, and their breathing rate goes from 16 breaths a minute to as few as 2.

In addition to groundhogs, ground squirrels and many species of bats are true hibernators. Some animals that hibernate are considered light sleep hibernators rather than true hibernators. Their state of hibernation is not as deep as a true hibernator’s and does not last as long.

“They are easily awakened,” Russell said of light sleep hibernators. “They are basically taking long winter naps that can last several days.”

Bears are the most famous example of this kind of hibernation. Closer to home, opossums, raccoons and skunks are also light sleep hibernators, she said.

One misconception about hibernation is that animals do not wake while hibernating. They do wake up, but how and how often they do depends on whether they are true hibernators or light sleep hibernators, Russell said.

True hibernating animals sleep so deeply that waking is difficult and takes a lot of time and energy, she said. These animals may wake every few weeks to eat and, like in the case of groundhogs, use the bathroom in their burrow. As spring inches closer, they wake more frequently.

Light sleep hibernators wake more often throughout winter and carry on as usual while they are awake. Their metabolic functions — body temperature, breathing rate and heart rate — return to normal when they wake, then drop again when they once again begin to hibernate.

Torpor

_{Black-capped chickadee. (Photo by Glenn P. Knoblock)}

Meanwhile, animals that go through daily torpor experience shorter periods of inactivity to help them prevent losing too much energy in cold weather. Daily torpor is more common in small animals that are susceptible to rapid heat loss, Russell said.

One key difference between hibernation and daily torpor is that hibernation is voluntary while daily torpor is involuntary, Russell said. Daily torpor is triggered by temperature, so it’s more common and more dramatic in areas where the weather can change drastically in a single day.

“Think of (daily torpor) as a power nap that can happen for just a few minutes or up to a couple hours,” she said.

Locally, animals that experience daily torpor as a means of survival include mice and some other rodents, some bat species and several types of birds, including doves, chickadees, nighthawks and swifts.

Brumation

_{Fox snake. (Photo by Chad Merda)}

Brumation is a metabolic state similar to hibernation, although it isn’t as well known.

“Brumation is basically just hibernation for cold-blooded animals like reptiles and amphibians,” Russell said.

During brumation, these animals’ body temperature, heart rate and respiratory rate all drop, just like animals that are hibernating. Cold-blooded animals, or ectotherms, that experience brumation include many species of snakes, turtles and frogs.

As temperatures start to drop, these animals will start to burrow underground, in crevices or under rocks and logs, according to Discovery Place. Some animals also stay underwater while in brumation.

One difference between brumation and true hibernation is longevity. During winter or cold spells, ectotherms will wake on warmer days to find water or relocate before re-entering a state of brumation when temperatures drop again.

Diapause

_{Wooly bear caterpillar. (Photo courtesy of Barb Ferry)}

Many insects survive winter by entering a period of suspended development called diapause.

“Diapause is the way insects survive our freezing temperatures,” Russell said. “It is when the insect goes through a period of inactivity and they stop growing.”

How insects enter diapause varies. Some slow their metabolism and survive off their fat stores, Russell said. Others make an alcohol that acts as an anti-freeze for their bodies. The alcohol stops ice crystals from forming, which would puncture and destroy cell walls. (The forming of ice crystals that puncture cell walls is very similar to frostbite, Russell said.)

Not all insects enter diapause at the same stage of their life cycle, Russell said. For example, woolly bear caterpillars enter diapause as larvae, while mourning cloak butterflies enter diapause as adults. Where they experience diapause varies as well. For instance, ladybugs form masses under plant debris, while the larvae of June bugs burrow underground and let the soil and mulch above serve as a protective insulated layer.

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_{(Lead image via Shutterstock)}