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As winter approaches, the birds fly south to warmer climates while bears and squirrels hunker down and hibernate through the cold weather. But, if that’s the case, why do you keep seeing squirrels running around outside during the winter? Come to think of it, if squirrels fall into a winter-long sleep, why do they store nuts for the winter? Do they sleep-eat? Do squirrels even actually hibernate? First of All, What Exactly Is Hibernation? Hibernation is something like a really deep sleep and something like a trance or even a state of suspended animation. During this state, the hibernating creature undergoes a series of seasonal changes. The animal’s metabolism slows way down, to the point where it can seem like the creature is near death. Heart rate slows to almost nothing. In this state, the creature’s need for calories and nutrients becomes almost zero. This allows it to live off of its fat reserves for the entire winter season. Brain activity falls to essentially nothing as well, and this is one of the things that distinguishes hibernation from sleep. In a state of sleep, the brain is still pretty active. In the torpor of hibernation, the brain pretty much isn’t doing anything. So the creature doesn’t experience the harsh weather conditions because it spends the winter entirely unconscious. When the warmer months arrive again, these changes slowly reverse themselves, and the hibernating creatures once more become conscious and active. So Do Squirrels Hibernate? The short answer is no. The slightly longer answer is mostly no. There are actually a lot of different types of squirrels across North America, ranging from grey squirrels to fox squirrels to flying squirrels. For the most part, they don’t hibernate. But if they don’t hibernate, how do they survive the cold winter months? How Do Most Squirrels Handle the Cold Winter Months? All common squirrel species, such as gray squirrels and red squirrels, spend the winter mostly awake. So do most of the uncommon species. None of them hibernate. Instead, they prepare a nice cozy nest for themselves, build up a very large supply of food, prepare their bodies, and settle in for a fairly comfortable, lazy winter. They spend more time sleeping than they usually do, but that’s just increased napping, not hibernation. On average, a squirrel spends 16 hours a day sleeping during the winter. During the warmer daylight hours, they will briefly venture outside to raid bird feeders and gather nuts and water, but they spend as much time in their nests as possible. Also, as winter approaches, most squirrels grow thicker coats in order to conserve more warmth. Bulking Up for the Winter In order to prepare their bodies for the coming cold, squirrels do the same thing that hibernating creatures do: eat as much food as they possibly can, starting in late summer and throughout the autumn months. Putting on fat accomplishes two things. One is insulation. The additional layer of fat acts like a winter coat and keeps the squirrel warm. Eastern gray squirrels, commonly known as tree squirrels, can even run around outside comfortably when the weather is below freezing. The other thing the extra fat does is give the squirrel an extra reserve of calories to burn if food supplies run short and it’s hard to find a new food source. Hoarding Food Squirrels famously collect nuts and store them for the winter. That way, they don’t have to go looking for it when the food supply is short, and the temperatures are low. With large food caches, a squirrel can eat as much as it likes and keep fat reserves high. Squirrels don’t keep all of their nuts in one hoard, either. They tend to gather several collections of nuts in different places, possibly so that other squirrels don’t come along and steal everything they have. Many people think that squirrels have exceptionally good memories in order to remember where they stashed all their nuts. However, scientists have tested them and found that this isn’t the case. In fact, squirrels have terrible memories. It’s just that there are so many squirrels storing so many caches of nuts that any time a squirrel goes looking for one, it will probably find enough food to get by. Building Nests Squirrels also prepare for the coming winter by building nests. Different types of squirrels build different types of nests, but the most common squirrels, the tree squirrels, make sturdy nests high up in the branches of tall trees. They do this by weaving twigs, leaves, bark, and moss together to form a nest that looks kind of like a large, fully enclosed bird nest. It’s actually surprisingly well insulated, and since squirrels sometimes winter together with friends and family, the shared body heat can make these nests almost comfortable. Squirrels also build more than one nest. They try to keep a nest near every place where they hid a store of nuts. That way, when one of them is used up, they don’t have to travel long distances to get more. This helps limit the amount of time they spend out in the cold temperatures. Some squirrels nest in the spaces inside hollow tree trunks. Some use the spaces hollowed out by woodpeckers. And some squirrels take over nests made by small birds and other wild animals. The Exception to the Rule As noted above, squirrels mostly don’t hibernate. The exception to the rule is the ground squirrel. Ground squirrel is the generic term for a great variety of squirrel species that don’t make nests, but instead burrow tunnels into the ground. The ground squirrel is the one type of squirrel that does hibernate. Most of them aren’t called ground squirrels, though. That term is generally only used for medium-sized ground squirrels. Smaller ground squirrels are known as chipmunks. The larger ones are often known as marmots. Ground squirrels’ burrows are often large and complex, with many squirrels living together in a single burrow in extended family units. Because they’re much more social than other types of squirrels, they can form larger groups that can snuggle together during the winter for greater warmth, allowing ground squirrels to survive much harsher winter conditions than other types. But shared body heat is not enough in many of the places where ground squirrels live. So they need to enter a state of hibernation. With it, ground squirrels can survive temperatures lower than 30 degrees below zero. Arctic ground squirrels survive some of the most extreme conditions on Earth for long periods on a yearly basis. In fact, the Arctic winters are so harsh that these squirrels spend up to 8 months out of every year in a state of hibernation. Their bodies are so well adapted to these extreme conditions that their body temperatures even drop below the freezing point without their blood turning to ice. However, while ground squirrels can spend up to 8 months in a state of hibernation, they don’t do it all in one stretch. Instead, once every 2 or 3 weeks, they rouse themselves from their hibernation torpor without actually waking up. Instead, they enter a state of deep sleep. Remember, hibernation isn’t sleeping, and long-term sleep deprivation is harmful. They then spend 12 to 15 hours sleeping deeply and shivering violently. This shivering warms them up a bit, and the combination of this warmth and the dreaming time keeps their brains alive and healthy for the long, harsh winter. The violent shivering isn’t typical of all hibernating creatures. However, because squirrels are such small mammals, they have very little body mass, so they lose heat very easily. They must therefore use the violent shivering trick to keep their temperatures up to the minimum level needed for survival. How Do They Handle Thirst? Of course, members of the animal kingdom need more than just food and sleep, they also need water. So how do hibernating squirrels get by without water for so long? A big part of it is the greatly reduced metabolism. When the metabolism isn’t doing much of anything, it doesn’t use much water at all. There’s more to it than that, though. When the body of any animal gets dehydrated, electrolyte concentrations in the blood get dangerously high, causing all sorts of health problems. Ground squirrels have a mechanism for removing some of those electrolytes from their blood and temporarily storing them elsewhere in their bodies. This keeps their electrolyte concentrations from getting too high even when they get dehydrated. Ground squirrels also have special mechanisms in their bodies that suppress the compounds in the blood that trigger feelings of thirst. The Exception to the Exception Although hibernation is literally in the blood of ground squirrels of all species, not all ground squirrels hibernate. California ground squirrels often don’t. This is not surprising, as California is not known for its long, cold winters. In high elevations where winters get cold, they do. However, across much of their range, they are active year-round, as it never gets cold enough to trigger the hibernation reflex. In Conclusion So, in conclusion, tree squirrels and flying squirrels do not hibernate, and ground squirrels do. Mostly. And, with those types of squirrels that do hibernate, the main reason why they are able to do so is an incredible series of specialized adaptations that allow their blood to stay liquid even in sub-freezing temperatures and keep them from getting too dehydrated. Research into these adaptations may even have applications in human medicine.

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