Do Squirrels Hibernate? Facts About Animals in Winter

Do squirrels hibernate? Some do, some don't. Let's find out...

Out of sight, out of mind. It’s curious how our mind works. In the summer, we take a walk along the towpath, let a snapping turtle cross in front of us, and watch it slide back into the canal. 

Next, we’re having some friends over for dinner on the backyard deck. It’s late fall and wasps are very active. And then it gets colder and they’re gone. But where did they go? Where do animals hibernate? 

Now it’s winter. We won’t see any snapping turtles. They’re gone. No wasps either. Out of sight, out of mind. But unlike migratory animals (like some bird species, butterflies, and others), a lot of animals spend their winters right here right now. Some are hibernating out of sight, some are at our bird feeders, others survive as caterpillars - and incredibly, some butterflies overwinter as adults. And then there are the “cheaters:” some mice, ladybugs, and others will find a way into our cozy homes during the winter. 

Why do some hibernate and some not? Is it not having access to food or avoiding the cold temperature? Why do some animals put on body fat to use during the long winter while others won’t, but instead will cache food all summer long, to be used during the colder months? Have you ever heard of frozen-solid spring peepers, only to thaw out and then return in perfect condition in the spring? There are so many incredible survival strategies and it all boils down to one: physiologically preparing for the winter. 

So my curious mind asks “where are they now and how can they survive the cold?” Let’s have a look at some amazing survival strategies of our scaly, feathery, and furry friends out there in the cold, while we’re enjoying a nice cup of coffee next to the fireplace. And hopefully your curiosity will be piqued and you’ll throw on some warm clothing (that’s our way of physiologically preparing 😄), go outside, and look for some clues of who is doing what and where out there.

How Do Birds Survive Winter?

Birds have wings and that’s great. A lot of bird species take advantage of this feature and head south for the winter. Bird migration is absolutely fascinating, and to learn more, check out “On the Wing - The Magic of Bird Migration."

But not all birds use this option. Migration is very dangerous and the fatality rate, especially for birds on their first trip, is nearly 30%! Many bird species have adapted to stay put or to only move far enough to find food, not warmth. Waterfowl and gulls are perfect examples. To find out how these guys don’t lose their feet in the freezing waters, read “Ducks, Gulls, Feathers, Feet, and Freezing Temperatures.”

The Incredible Kinglets

Of all the birds that stay behind, the most fascinating bird to me is, no doubt, the kinglet. There are two species of kinglets living in our area, the Golden-crowned Kinglet (Regulus satrapa) and the Ruby-crowned Kinglet (Regulus calendula). These tiny birds weigh less than a quarter of an ounce and have an energy level that’s through the roof! I’ve never seen a kinglet not moving. Ask any bird photographer what’s one of the hardest birds to take pictures of and the answer could very well be “those damn kinglets, they never want to stay still.”

Besides weighing less than a quarter of an ounce (just slightly more than the weight of two pennies) and measuring no bigger than the end of a thumb (plus feathers), how do they maintain a body temperature of 109.5F? Eat, huddle, shiver, repeat. 

These guys forage all day long, finding tiny insects and caterpillars in trees. Kinglet beaks are tiny and relatively weak, so they don’t have access to grubs under the bark, like nuthatches or woodpeckers do. It’s just mind boggling how they are able to find enough food in the shortened winter days, and put on enough fat in one day only to burn it up overnight to stay alive. Because of their unreliable food source, they’re constantly on the move. This constant movement prevents them from building shelters or caching food. Why waste energy for such a project when you might be a mile away the next evening? But kinglets are known to huddle. To reduce the loss of body temperature, kinglets might huddle overnight in small groups. 

Mammals

From the classic hibernating bear to the active squirrels, mammals have developed techniques to cope with the cold and lack of food in adaptive ways. This changes from species to species, but also the severity of winter can affect hibernation strategies, as well. For example, a brown bear can spend 6 months hibernating further north, but only goes into semi-hibernation for short periods of time in our area. 

A familiar and interesting group to me are the “squirrels.” Where we live, we share our outdoors with chipmunks; flying, red, and gray squirrels; and groundhogs (yes, groundhogs are part of the Sciuridae family, which embraces all tree squirrels, ground squirrels, prairie dogs, and marmots)!

And now it gets interesting. The smallest (chipmunk) and the largest (groundhog) mostly hibernate, while the three medium-size species do not. Classic animal physique states that within the same family, smaller species are more likely to hibernate. This is because the smaller the animal is, its surface area is proportionally larger. A small animal cools quicker since most of its volume is close to the surface, where heat is lost. 

Do Squirrels Hibernate? And What About Chipmunks and Groundhogs?

So what’s going on here? I’m sure that wildlife biologists will figure it out one day. But my curiosity came to the following conclusion: if size is not the defining factor for their winter behavior, how else could I group the hibernating ones against the non-hibernating ones? What about foraging techniques and habitats? 

Chipmunks and groundhogs live on the ground and forage on the ground, whereas tree squirrels forage on the ground and in trees. Animals foraging on the ground can lose their access to food when there’s heavy snow, while animals also using trees will still have access to food - this may be another reason why chipmunks and groundhogs hibernate.

But what about caching food? All squirrels cache food. Kind of. Tree squirrels collect food and bury it about 1” under the soil. They do this all over their territory, a technique called “scatter hoarding.”  Squirrels are even known to crack open a nut before burying it, to keep it from germinating.

Chipmunks hibernate, but they also cache food for the winter (they are outfitted with large cheek pouches to carry food). Around November, they go underground, plug the burrow entrance, and become torpid. Their breathing rate drops from 60 to 20 breaths a minute and their body temperature plunges from around 100℉ to 45℉. But sometimes it gets warm outside and they wake up in the middle of winter. As they don’t really store body fat, stored food is imperative for survival during this time. 

How Do Frogs Survive Winter (Reptiles and Amphibians)

Where are all the snakes, frogs, and toads once Mother Nature shuts down for the winter? In general, amphibians in our area bury themselves around October and emerge from their dormancy in March and April. 

The first spring after I built my backyard pond, I saw a large tadpole wriggling around after the ice melted. Where did that dude come from? I hadn’t heard or seen any frogs yet, so it must have survived the winter in the pond. 

It turns out that tadpoles are pretty good at overwintering. If their mom lays her eggs too late in summer, food availability and water temperature can delay their metamorphosis from tadpole to adult until next spring. 

Frozen Spring Peepers

I was on a hike in the middle of January. The temperature was in the mid-fifties with a light drizzle. And sure enough, I heard a Spring Peeper (Pseudacris crucifer) singing. But aren’t they supposed to hibernate?

Spring peepers and a few other frogs have somehow developed a different strategy to survive the winter. They hide under leaves, barks, or logs, and literally freeze. As temperatures dip below 32℉, they produce their own “antifreeze” to protect their most essential organs.The rest of their body freezes to the point that the heart stops pumping and the frog appears to be dead. Once spring arrives, they will thaw out and go about their business as usual.

Insects

Insects rule the world in many ways. They are the most versatile, adapted, living things we know. They are -  without a doubt - the most important animal group and the most successful. They have mind boggling winter survival strategies that cryobiologists can only dream of and learn from. 

I just now came back inside from looking for some insects in the backyard. It’s 27℉ today with no snow on the ground. I was turning over some leaves and found a couple of woolly bear caterpillars. They were curled up and appeared to be frozen solid. Are they going to be okay? What about other insects? Where are they? 

Egg, Caterpillar, Pupa, or Adult - Which Life Cycle Survives the Winter?

Insects have developed winter survival strategies that can be drastically different from species to species. Take butterflies and moths for example. 

We’re familiar with the beautiful woolly bear caterpillar. We see them in late fall and wonder how they’re going to survive the winter. Simple: they crawl under some leaves, curl up, and freeze. They have shown to survive temperatures as low as -90℉, as well as being frozen in ice! And then they cooly transform via pupa to an adult Isabella Tiger Moth (Pyrrharctia isabella) in the spring.

The family of moth species that is best known as tent caterpillars lay their eggs in late fall. In these species, it’s the eggs that survive the winter.

But one of the coolest sights in winter is seeing a fully grown, large butterfly flitting and gliding through the air on a sunny day - the beautiful Mourning Cloak (Nymphalis antiopa). It uses its own “antifreeze” to survive as an adult in a sheltered place. They emerge on warm, sunny days and can be seen throughout the winter months in eastern Pennsylvania. But don’t they need nectar to survive? Not this one. It can feed on tree sap and - Wait What?!?! - animal droppings. 

It’s a winter wonderland out there. Most of nature’s life may escape our eyes. But once we look a little closer, it’s there - surviving and getting ready to burst out of winter as soon as spring has sprung. 

Please share your own awesome winter nature stories. There’s still so much we can learn out there. 

Have you seen some of these animals in winter? Let us know in the comments below!

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