Volume 40
Issue 1
Photo by Yushi & Keiko Osawa

These insectivores snuggle into snow for a long winter’s slumber

By Christie Wilcox

Scientific name: Murina ussuriensis
Family: Vespertilionidae
Colony size: Estimated 2–22
Weight: 4–8 grams
Diet: Insectivorous
Status: Least Concern
Region: Kurile Islands, Sakhalin, southeastern Siberia, Korea, and Japan

The small bodies of bats are ill-suited to cold winters, yet bats persist in myriad frigid environments.

The animals tend to mysteriously disappear as the temperature drops, then reappear come spring. That’s led to the general assumption that they find an insulated spot where they can hunker down and hibernate—a nice tree cavity, perhaps, or even a cozy burrow. But in the snowiest regions of Japan, the Ussurian Tube-nosed Bat (Murina ussuriensis) does something truly unexpected: It hibernates in snowbanks.

Dr. Hirofumi Hirakawa, a wildlife biologist for the Forestry Research Institute in Japan, set out to explore this bewildering behavior. Back in 2005, he heard of cases where people discovered what appeared to be half-dead bats on top of melting snowbanks. These anecdotes roused his interest, and he began digging into the phenomenon. He searched through newspaper clippings, museum notes, and scientific journals to find more reports. He found some dating as far back as 1964, though the accounts were vague and lacked the detail he needed to figure out what was going on. He contacted the people who found the bats and questioned them to learn the circumstances surrounding each.

Soon, he was convinced it was no accident that Ussurian tube-nosed bats are occasionally found in the snow. They hibernate there—a behavior almost unknown in mammals.

The Astonishing Snow Bat
After poring over nearly two dozen reports of bats found in snow, Dr. Hirakawa hypothesized that the bats were using the snow as insulation to survive subzero temperatures.

It’s essentially the same idea as digging a snow cave or even building an igloo; because snow traps a lot of air, it doesn’t conduct heat very well. And that means anything under or encased in snow stays at freezing (32°F, 0°C), even when the outside world drops well below that.

There are also other benefits. “It’s a safer space,” Dr. Hirakawa says. Few winter predators search the snow for their meals, as they would in tree cavities or other potential hibernation spots. In regions with heavy snowfall, burrowing into the fluff could mean staying warmer and safer. Then, when the snow melts enough, they can rouse from their winter-long rest and fly away.

If that’s true, the bats found in shallow divots atop the snow were serendipitous discoveries during the brief window when they were no longer covered by snow but were waiting for nightfall to return to their usual roosts in trees or under leaves.

However, demonstrating all of this would prove difficult. It was also possible the bats ended up in the snow under grimmer circumstances, and the reports rarely observed the bats long enough to determine if the animals survived.

Dr. Hirakawa needed to locate and properly study the animals. So, every spring, when the snow started to melt, he searched through snow piles hoping to find them. But year after year, his searches came up empty.

Luckily, word of his quest spread, and sightings continued to stream in, giving him even more data to narrow down his hunt. Finally, eight years after he began his research, he found his first snow bat—a mere 175 yards (160 meters) away from his office.

Revealing Snowbound Secrets
While Dr. Hirakawa has found a little over a dozen of these snow-hibernating bats to date, he’s obtained detailed data on dozens more thanks to the keen eyes of his colleague, Yu Nagasaka.

Nagasaka is not a bat scientist by trade. “His specialty is forest, river, and stream ecology,” Dr. Hirakawa explains. But he, too, was intrigued by the snow bats. And it just so happens that “he has a very good eye for things in nature in general,” Dr. Hirakawa says. Nagasaka has found more than 50 bats so far and has compiled detailed notes on the circumstances of each one he’s discovered.

Those data, as well as Dr. Hirakawa’s own observations, strongly suggest the bats really do hibernate in a place constructed entirely out of the snow—something only one other mammal (the polar bear) is known to do.

They also found that bats usually survive their snowbound stints. In their 2018 paper on the phenomenon, Dr. Hirakawa and Nagasaka noted that only one of 37 bats they personally observed atop a snowbank was dead, and of 23 bats watched into the night, 22 flew away shortly after sunset—observed in the narrow window between when the snow melted enough to expose them and when they flew away.

The duo has also continued to hone their locating skills and collect data since their paper was published. They are now trying to find bats in the snowbanks early in the snow season, often in December, to weigh them before weighing them again in the spring.

The ultimate goal is to weigh the bats at different points in time to get an idea about how much energy they’re using. Dr. Hirakawa presumes that, like other hibernating mammals, they spend the vast majority of time in a state of torpor, where their energy use is greatly reduced, but he’d like the hard evidence to back up that hunch.

He also hopes someone with expertise in bat physiology will study the animals in greater depth. “Usually, mammals cannot survive below 0°C … We don’t know how they do it at all,” he notes.

Uncovering their cold-enduring tricks would shine more light on this surprising behavior, as well as provide clues as to how other bat species might survive harsh winters. Although these are the only snow-hibernating bats known so far, it’s possible, if not likely, they’re not the only ones to have evolved this remarkable ability.