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We tend to think of our brains, at least by adulthood, as being pretty static. There will be some changes, but for the most part, the large structural shifts are over.

But that’s not the case with hibernating ground squirrels. Several species actually deconstruct part of their brains, only to rebuild them again. And they don’t just do it once, when they come out of hibernation for the summer, they do it every 2-3 weeks during winter!

For ground squirrels, hibernation is really more of a cycle than an annual event. To save energy, they curl up in cute balls in underground burrows, slow their metabolisms, and drop their temperatures way down, sometimes below freezing. And every few weeks they naturally warm their little bodies back up.

Less than 24 hours later though, they return to their uber-cold suspended animation. And so on, until spring. In the early 1990s, Russian researchers decided to see what was happening in the brains of.

Siberian ground squirrels during this cycle. So they focused on the hippocampus, since it was easy to study, and is a part of the brain that helps control hibernation. But what they found was pretty shocking.

Compared to fully active squirrels, animals 3-4 days into a bout of hibernation had shorter and less branching dendrites, or the extensions that neurons use to make contact with other cells. The dendrites themselves also had fewer knobby contact points, or spines. Plus, the cell bodies of the neurons had shrunk by more than 30%.

And weirder still, when the scientists looked at squirrels that had woken up from hibernation just two hours earlier, they found a completely different scene: The dendrites were nice and bushy, even more so than in active squirrels, and the cell bodies were also bigger. This meant that in an extremely short period of time, the squirrels had re-sprouted everything they had lost before. And other scientists later found that this process also happens in other parts of the brain, and concluded it was probably happening all over.

Neuroscientists call this ability to change synaptic plasticity. And while we and other non-hibernators do this to a certain extent, this speed is virtually unheard of. Rats, for instance, take 4 months to do what squirrels do in 2 hours.

So obviously, this has gotten a lot of scientists more interested in hibernating squirrels than they might otherwise be. But it’s still a mystery exactly why this massive neural construction project is helpful to them. One idea is that having a stripped-down brain could save a lot of energy, which is basically what hibernation is all about.

But rebuilding your brain doesn’t seem especially efficient, so we’re not really sure. Though scientists have noticed that paring things back in the brain when it’s cold is fairly common, and that it’s even more destructive in non-hibernating animals. Mice, for instance, lose all of their spines when the temperature drops, whereas squirrels lose just a quarter.

This has led some researchers to think that maybe part of what makes hibernating squirrels so special is that they lose fewer connections than you would otherwise expect. So, while the rebuilding is certainly impressive, so might be the relatively limited demolition. Scientists will keep investigating these animals, and some believe there’s even a chance we could borrow their tricks to help with human diseases or strokes.

So, at least one thing’s for sure: These adorable little squirrels have awesome brains! Thanks for watching this episode of SciShow! If you’d like to learn even more mind-blowing fast facts, you can watch our episode about the world’s most terrifying caterpillar. [ ♪ Outro ].