These Bats Make Tiny Igloos to Survive the Winter

The Ussurian tube-nosed bat of Japan joins the polar bear as the only animals known to hibernate in snow.

Ussurian tube-nosed bats spend the coldest months in snow-covered dens—the only mammal species known to do so besides polar bears.

These Bats Make Tiny Igloos to Survive the Winter

The Ussurian tube-nosed bat of Japan joins the polar bear as the only animals known to hibernate in snow.

Ussurian tube-nosed bats spend the coldest months in snow-covered dens—the only mammal species known to do so besides polar bears.

Until recently, polar bears were the only mammals known to hibernate in dens made of snow. But now, after more than a decade of research, scientists in Japan have added another mammal to the club—the Ussurian tube-nosed bat.

How does a tiny bat survive being entombed in snow?

It’s thought that when temperatures plummet in fall and winter, the bats trade their roosts in leaves and tree cavities for snow accumulating on the ground. Once there, they create small depressions and then become covered by more snow as it falls. Their movements and body heat create a small cavity in which they curl up and wait for spring.

Believe it or not, a hollow within a snowbank is actually a warmer and more stable environment than the tree cavities these bats typically roost in, says Hirofumi Hirakawa, a wildlife biologist at the Forestry and Forest Products Research Institute in Japan and lead author of a new study published this month in the Nature journal Scientific Reports.

This is because trees are exposed to wind, and snow is actually a better insulator than wood, says Hirakawa.

Even still, the bats must survive perilously low temperatures inside their makeshift igloos. They do this by slowing their breathing and heartrate and keeping their body temperatures just above that of the frozen world around them.

This state requires a minimal amount of energy to stay alive, says Hirakawa. “I think this made their survival in the snow possible.”

Snow Bats

While the tube-nosed bats may be the first chiropterans found to cozy up in a snowbank, plenty of small mammals have been known to hibernate just under the snow, says Justin Boyles, a physiological ecologist at Southern Illinois University.

In fact, the red bats Boyles studied as part of his Master’s Degree were known to sometimes get covered in snow while hibernating beneath the leaf litter.

The difference here is two-fold. First, the Ussurian tube-nosed bats appear to seek out snow specifically. And second, red bats might be under snow for a week or two, while the tube-nosed bats observed in the new paper seem capable of much greater feats.

There’s evidence that one bat in the study hibernated in a snowbank from at least early December 2017 to mid-April 2018—a span of over four months.

Life at such extremes comes at a cost, though. The researchers discovered one bat that died while hibernating in its igloo.

However, the fact that 36 other individuals survived suggests that “the failure rate must be very, very low,” says Hirakawa.

Indeed, if too many bats died while hibernating in snow, natural selection would have likely have rooted out the behavior long ago.

Breakfast In Bed

A stable temperature may just be one advantage of hibernating in the snow. The researchers suggest that the bats may also have evolved this trick to avoid predators, a hypothesis that makes perfect sense to Boyles.

Regarding the leaf-litter-hibernating-red bats he studied, Boyles says, “We thought anything and everything would eat them. But they don’t. Things just don’t seem to find them.”

Most interesting of all, wintering in a snowbank may supply bats with an inexhaustible, indispensable resource—water.

“We always thought hibernation was purely to save fat and energy over the winter,” says Boyles. “But there’s more and more evidence that water may be the real thing, the real limiting factor in hibernation.”

You see, unlike bears which only lower their body temperatures a little bit during hibernation, small mammals such as bats have to rouse themselves from time to time to get warm. During these bouts, bats also go out in search of water.

But in a snowbank hibernaculum, the walls are literally made of water. All you have to do is wake up and start licking.

Slow Science

The discovery of snow-hibernating bats did not come easy. Hirakawa and his coauthor, Yu Nagasaka, first heard anecdotal reports of bats in snow in 2005, and it would be another eight years before the two would see the phenomenon for themselves.

“It took 13 years from the start to publication,” says Hirakawa, noting that the study was completed without any formal funding.

“We are proud of having achieved this work without huge resources, except for time,” he says.

For his part, Boyles says this is exactly the kind of work he likes to share with his students. It begins with a “cool little natural history observation” but progresses to evidence collection, analyses, and—finally—publication.

“This is how science should be done,” says Boyles.