Squirrels are busy animals. When it is nice outside, it is hard to go anywhere or spend more than a minute looking out your window without seeing a squirrel. While these animals are busy throughout the warmer months, I started to wonder about how they got through the winter.
More specifically I wondered, “Do squirrels hibernate?” I did some research and found that the answer is:
Trees squirrels like the Gray, Fox and Red squirrels do not hibernate but many ground squirrels do. Ground squirrels are “light sleepers” and will often come out of hibernation for short periods of time. The Arctic Ground Squirrel is a serious sleeper that truly hibernates for up to eight months!
Tree Squirrels Do Not Hibernate
Tree squirrels like the Flying, Grey, Red and Fox squirrels are warm-blooded homeotherms. Homeotherms maintain a constant body temperature throughout the seasons and do not need to hibernate.
The primary mechanisms tree squirrels employ for winter survival include:
- Building protected nests
- Sharing body heat
- Reducing physical activity
- Creation of food caches
- Development of fat stores
- Development of a thicker coat
- Utilization of shivering
Tree squirrels make it through the winter by building warm nests or dreys. Tree squirrels will share their nests with other squirrels for the benefit of the shared body heat. Grey squirrels will share a nest with just a few other squirrels while Flying squirrels have been known to share a nest with more than 20 other squirrels.
Once the squirrels are in their nests they tend to stay there as much as possible. Tree squirrels will often stay in their nests for 2-3 days at a time and only come out to search for food around midday. By staying in their nests they use less energy, stay warmer and help maintain the shared body heat.
Here is a peek inside a squirrel nest…it looks kind of cozy in there! By the way, if you want to make winter easier for the squirrels in your yard then you can buy a nest box for them!
Tree squirrels spend much of the fall preparing food stores for winter. Squirrels stash away acorns and pine cones for winter munching and eat voracious amounts of extra food to pack on extra weight. Some estimates say that the grey squirrels consumes 32% more food then energetically required during the fall and increase their weight by 25%.
In addition to gaining weight, squirrels adapt their bodies for winter by developing a thicker coat. Some species, such as the Eurasion red squirrel, also develop tufts of fur about an inch long above their ears.
Tree squirrels utilize shivering to generate extra body heat. An interesting study has found that black squirrels, which are genetic variations of the Grey or Fox Squirrels, are better at shivering and may eventually become dominant species in colder climates.
Flying squirrels, like the American Northern and Southern, have the ability to go into mild states of torpor, which is the lowering of metabolic rate and body temperature, but this does not appear to be a primary mechanism for winter survival.
Several Species of Ground Squirrels Do Hibernate
There is not a blanket statement that can be made about the winter activity of ground squirrels because of the extreme differences between them. For example, the Arctic Ground Squirrel is one of the most fascinating hibernating creatures in the world while the White Tailed Antelope Ground Squirrel just enjoys the break from the scorching desert heat.
Let’s take a look at two species of ground squirrels that are confirmed to hibernate.
Arctic Ground Squirrel
The Arctic ground squirrel is one of the few species of squirrels that truly hibernates and this species is fascinating. I like to call these critters “Zombie Squirrels”!
The Arctic ground squirrel spends up to eight months hibernating and will let its core body temperature drop to as low as -2.9 degrees Celsius. This is the lowest core body temperature ever recorded in a mammal. There is some fascinating biology at work that lets this squirrel get this cold without its blood freezing solid.
I call these squirrels “zombies” because of what happens to their brains while their body temperature is so low. The brain of an Arctic ground squirrel gets completely fried during hibernation and loses almost all of the synapses needed for neural connectivity. Neurons in the brains of these hibernating squirrels were greatly shrunken and had fewer dendrites than the same species during the summer.
The remarkable aspect is that as soon as it is warm enough to come back outside, these squirrels regrow their brain functions! This crazy ability to regrow their brain has exciting possibilities for researchers studying degenerative brain diseases in humans.
Thirteen Lined Ground Squirrel
The Thirteen Lined ground squirrel is also a true hibernator. These squirrels hibernate from August through March and lose up to 33% of their body weight in the process. The core body temperature of these squirrels will drop to just above freezing and their heart rate will slow to around 20 beats per minute.
The biological features that allow this ground squirrel to hibernate is also of interest to medical researchers. Specifically, researchers are interested in what prevents these squirrels from developing blood clots, if they can teach us about cold storage of platelets and if they can shed any insights into reversal of bone loss.
Here is what these cute guys look like when they are not hibernating!
Some Squirrels Hibernate and Some Do Not
There are many different species of squirrels and they have different winter survival mechanisms.
Tree squirrels tend to bulk up, put on a thicker coat and hunker down in warm nests with a few buddies.
Ground squirrels that live in extreme climates will hibernate while those that live in temperate climates will not. The ground squirrels that do hibernate offer potential insights into groundbreaking medical research for humans.