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When playing in the snow, few things are more fun than building a fort. But building a shelter out of snow also turns out to be a pretty ingenious way to make it through the winter, because even though snow is cold, it’s a great insulator.

And we’re not the only species that’s figured this out: a tiny bat that lives in Asia has also mastered the art of making snow caves. The Ussurian tube-nosed bat is native to Korea, Japan, and parts of Russia. Adults max out at about eight grams, less than the weight of two American nickels.

And like most bats, they’re nocturnal insect-eaters. During the warmer months, these bats spend their days holed up in dead leaves or tree cavities. But during winter, the region where they live turns cold and snowy.

And in response, they dig out little bat-sized snow dens to hibernate in. The dens are pretty similar to the snow caves you might have excavated as a kid. And while keeping warm in one seems counterintuitive because, like, snow is cold, it’s actually a pretty good idea.

See, a lot of air gets trapped between snowflakes as they pile up. Freshly fallen snow is as much as 95% air. That air can’t move around, so it doesn’t transfer heat well, making piles of snow excellent insulators!

So, researchers think that Ussurian tube-nosed bats dig out little cavities, curl up, and go into torpor, a state in which they drop their body temperature and slow down their metabolism to reduce energy use. At around zero degrees Celsius, the snow isn’t exactly warm inside, but it doesn’t easily get colder than that, either, even when the environment outside does. Also, most predators aren’t rooting around inside snowbanks for food, which helps keep the bats safe during their period of hibernation.

And since snow is made out of frozen water, the environment inside a snow den is pretty moist, too. This means the bats don’t lose as much water while they’re resting and can easily grab a sip if they do wake up thirsty! Depending on the weather in a given year, it could be anywhere from a few days to six months before the snow melts and the sleepy bat is exposed.

And then, it’s back to hiding out in a tree cavity. We only know about one other non-human mammal that builds dens in the snow to keep snug over the winter, and it’s way at the other end of the size spectrum: the polar bear. But maybe other species do something similar, and it’s just that no one has noticed yet!

In fact, it was luck that we learned about these snow dens. The researchers who described the behavior just happened to hear about some bats found in the snow and decided to investigate. Just goes to show, you can learn a lot by following your curiosity.

And on that note… this episode is brought to you by everyone who supports SciShow, including you! From viewers to channel members to patrons, we wouldn’t be able to do all of this without you and your curiosity. So, thank you.

If you want to watch one more episode after this, we recommend our episode about what the first animal ever might’ve looked like. [♪ OUTRO].