Ecology

Hibernation – A long winter’s nap

Daniel Brannen
Executive Director, Strawberry Hill

(11/2022) As winter approaches, we notice many changes in our environment. It begins to get colder, the sun sets earlier and earlier, and we see changes to much of our flora and fauna. Many plants either die or go dormant. Leaves change colors then fall from their trees – at least from deciduous trees and shrubs. And many animals are less visible – but we know they are all around us, hibernating. We think of hibernating animals as sleeping from late fall and early winter to the beginning of spring and warmer weather. But more is really going on during hibernation than just sleeping including distinct stages of what we think of as hibernation.

Not all animals hibernate, so what gives? Well, for those animals who cannot forage or that do not migrate during the winter months, saving energy, or hibernating, is key to their survival. Many animals will go into a form of sleep – from bats to butterflies, from bears to beavers. However, each of their hibernations may look a bit different. We will take a closer look at some of the differences in how animals hibernate in the cold, dark months of winter or even in extremely hot and dry climates.

So, what is hibernation? And, what causes hibernation in the first place? Is it always a good thing, or can it be dangerous for animals as well?

Hibernation is a state of minimal activity where metabolic depression occurs. Think of it as "when a body’s systems slow down." Hibernation is seasonal; most commonly occurring during the winter season. It is characterized by low body-temperature, slow breathing and heart rate, and a lower metabolic rate. Metabolism is the set of life-supporting chemical processes that occur in organisms. Key, among these, is the conversion of energy found in food to types of energies that living things utilize. When less food is available, like during the winter months or times of extreme temperature gradation, hibernation will occur for those animals that need to conserve energy. This affords an animal a much better chance of survival. As one might expect, hibernating animals may also be more prone to the dangers of extreme temperatures (cold or hot) as they are not in a condition to escape these. Plus, a "sleeping" animal is much more prone to become prey for its predators.

While we mostly attribute hibernation to the cold season, there are other types of "hibernation." Aestivation is like hibernation but generally occurs in response to high temperatures and arid conditions. Animals that need to regulate their body temperatures or conserve energy (due to lack of a food source or water) will go through the process of aestivation. Certain insects, reptiles, amphibians, fish, and mammals go through aestivation: these may include mosquitos, land snails, lady beetles, turtles, and salamanders.

Torpor is a state of decreased physiological activity in animals typically marked by a reduced body temperature and metabolic rate as well. Torpor enables animals to survive periods of reduced food availability. Torpor is when a hibernating animal spends time with a low body temperature, which may last for days or weeks or can be a brief period lasting less than 24 hours. The latter is known as daily torpor. Animals that undergo daily torpor include some birds, hummingbirds, and mammals – such as mice and bats. Torpor occurs when an animal lowers its heart and respiratory rate down to a point that conserves energy, but not as extreme as full hibernation. While we lower our heart and respiratory rate every night when we go to sleep, animals in torpor slow the rate down even further – for a prolonged sleep state during the coldest weeks of winter when food is especially scarce.

Sometimes when we think of hibernation, we are really thinking of torpor – or "hibernation light." Animals that we know go into torpor during the winter months are skunks, chipmunks, squirrels, raccoons and even bears. Yes, bears in some locations, like Pennsylvania and Maryland (think the American black bear), do not go into full-blown hibernation for the entire winter, but into torpor; waking occasionally when the weather warms, though very groggy, then back to sleep. So, if you happen to come upon an animal in the wild, such as a groggy black bear or skunk, do not move closer just because they are moving slowly, they are still dangerous and can be even more aggressive when confused or startled.

As previously mentioned, hibernation, aestivation, and torpor are all survival modes that once activated due to a lack of a regular food source, keep animals alive. Also, these metabolic slowdowns carry the risk that hibernating animals can then become more vulnerable to predators or unpredictable climate extremes. Current studies regarding climate change have shown that some animals are coming out of hibernation or torpor earlier than expected while their food availability is still too low. This may ultimately impact their survivability as food sources are scarcer. Further, this may also drive certain animals into more populated human areas as they search for food. And this, as we have seen with grizzly bears in Yellowstone, is usually not good for either wild animals or humans.

You may be wondering why certain "wild" pets don’t hibernate, aestivate, or go into torpor. You may own a turtle, or a frog, or a snake, a ferret or even a skunk (scent glands removed of course) and have noticed that they are awake all winter long. A couple of things are at play here: 1) they have a regular food source – us, their pet owners and 2) their climate is fairly well regulated to avoid extremes – not too cold, hot, or dry. So, they will typically not hibernate. With that said, some reptiles and amphibians may still go into what is called – brumation. Since they cannot lower their body temperatures like warm-blooded mammals can, they must seek out warmer spots in their own climate and surroundings where they can possibly maintain their body temperatures to between 40-50 degrees Fahrenheit. So, if you see your pet turtle burrowing to stay warmer, or eating less or becoming more lethargic, then you know that they are trying to brumate, whether their surroundings say they need to or not.