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By Billy Clapham, Assistant Producer

Snow is magical stuff, transforming the plainest of landscapes into a beautiful wonderland. But for animals, snow is generally a problem. It blocks access to food on the ground. It makes travelling on foot far more demanding. It can soak animals, or even freeze them to death. All of which makes one of the behaviours we filmed for the Frozen North episode of Asia all the more surprising: a golf ball-sized creature spending winter buried alive in snow.

There are only two species of mammal known to make long-term dens under snow: the polar bear and the Ussurian tube-nosed bat. Polar bear mothers excavate winter dens where they give birth and keep relatively warm. But the bat undergoes a true hibernation: its body temperature plummets, and its pulse slows dramatically, which brings the bat strikingly close to death. In April of 2024 I travelled to Japan on a mission to film the mass ‘bat resurrection’ that occurs when temperatures rise.

One spring evening in 2013, two scientists, Hirofumi Hirakawa and Yu Nagasaka, found a bat, lying in a little hollow in melting snow. Researching this bizarre sight, they could only find a few historical mentions of similar discoveries. Ever since that fateful day, the two scientists have gone out year after year on warm spring days, searching for bats in the snow. It took five years of study for them to confirm that the bats were spending the entire winter beneath the snow, and were only being revealed during the spring thaw.

Filming the ‘resurrection moment’, when the bats awaken from their hibernation, was what we set out to do. Every day, myself, the two scientists, three cinematographers, an edit assistant and a local fixer all spread out into the forest of northern Hokkaido to search for bats. The scientists took us to their favourite spots, then we simply split up and searched. Sadly there was no particular technique nor technology that made our search any easier. We just had to keep an eye out for tiny inanimate balls of fluff on the forest floor, sometimes still half-buried, often mixed in with brown leaf litter. We just had to stare at every patch of snow, investigate every small brown blob and forever watch where we were putting our feet.

Seven crew members is quite a lot for a shoot of this kind, but that number was necessary: capturing this behaviour was going to be a challenge. Timing was paramount – the bats would only be revealed when the last thirty centimetres of snow melted away and therefore depended entirely on the weather – never a good place to start in natural history filmmaking. Then for every bat we found, there would only be one chance to film the moment of take-off. But with a big crew, some settled warm weather and Mr Nagasaka proving to be a champion bat-finder, we had multiple chances to get the shot.

Remarkably, the bats would transform from being near-dead, to ready to take off in just about an hour. If they stay out overnight in newly-exposed melting snow, they could freeze to death. The process starts with a twitch of the ears, but soon accelerates to a furious vibration, as the bats warm their flight muscles to operating temperature. When everything is ready, the bats push down their arms with full force, and spring into the air. Not all the bats were successful, and some came crashing back to the ground with a very quiet thud. But most, somehow, rose vertically into the air and vanished.

Only by filming the behaviour with a slow-motion camera, could we witness this event in detail. But the bats’ ability to simply snap their eyes open and fly was a reminder that they are well adapted to do this, and there is a good reason why these bats chose to spend their hibernation entombed in snow: it’s actually a great insulator.

Hokkaido’s winter temperatures regularly plummet far, far, below zero. I experienced myself what it was like standing around waiting to film at minus eighteen degrees Celsius. It wasn’t fun. In Hokkaido, several metres of snow carpet the forest each winter. This thick layer of fluffy snow is not just comprised of ice, but also a lot of air, making it pretty good at retaining warmth. So while air temperatures plummet every night, the temperature below the snow will remain steady and not far off zero degrees. This saves the bat lots of energy on top of the more obvious benefit of hiding it from predators. Not only that, if a bat wakes up early and needs a little drink before sleeping again, it need only take a lick.

In conclusion, being buried alive doesn’t seem so bad, at least not if you’re a bat, surrounded by Japan’s legendary snowdrifts.