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In the winter of 1975, a biologist named Merlin Tuttle bought himself a state-of-the-art digital thermometer and set out on a road trip from Wisconsin down to Florida. Tuttle, who was in his mid-30s and sporting a brown, push-broom mustache, was trying to measure something that no one had really measured before. He wanted to know the precise attributes that turned a hole in the ground into a winter home for bats.

The six months of winter in the Northeastern United States are life-or-death for cave-dwelling bats: Their usual food source, bugs, are too scarce to sustain them, and they must find a suitable roost to conk out in and ration their energy. But if it’s too warm in their subterranean abode, they can’t slow their metabolism down enough to hibernate. And if it drops below freezing, they have to wake and burn precious calories to avoid being turned into a fur-covered ice cube. “It has to be cold without being too cold,” Tuttle says. The ideal home is harder to find than you might imagine: Out of Alabama’s 1635 known caves, for instance, only two of them have bats in winter.

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The perfect bat cave lives and breathes.

During Tuttle’s journey, he visited 25 different caves and mines, drawing elaborate maps of them and logging thousands of temperature measurements at different locations, times of the day, and times of the year. He visited caves with a single entrance, and caves with two or more entrances. He visited caves with wide openings and caves with narrow openings. He visited caves that were shaped like a U, and others looked like a U turned sidewise. He visited caves that had domes in their ceilings and pits in their floors, and some that had both.

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So what does the perfect bat cave look like? Tuttle points to the Pearson Cave in eastern Tennessee. That cave has multiple entrances to create a chimney effect that sucks in just the right amount of warm air in the summer, trapping it in a dome in the ceiling, and then reverses in the winter, trapping a pocket of cool air in a depression inside. The cave itself lives and breathes. And, for centuries, 100,000 gray bats have been hibernating there peacefully every winter, their tiny hearts pumping every few seconds at temperatures decimals above the freezing point. “That’s quite a trick,” Tuttle says.

Like bats, some early humans used caves to endure harsh seasons. As our tropically bred ancestors migrated from Africa into chilly parts of Europe, Asia, and the Americas, some of our first winter shelters were caves populated by bats who roosted in the ceiling as we lit fires for warmth down below. But while bats could only survive in caves with just the right environmental conditions, we knew how to modify our habitats, even going so far as plugging drafty entrances. 

“Bats use what is available to them,” says Timothy Carter, a wildlife biologist at Ball State University in Muncie, Indiana. “Using our minds, we were able to devise and create inventions that allowed us to occupy a diversity of habitats. That’s why we can occupy the South Pole, Siberia, and elsewhere.” We moved out of caves and built homes of ice and snow, wood and straw, animal skin and manure, mud and clay, metal and brick.

Now some people want to do for bats what we have traditionally done for ourselves: build them a home. In 2010, more than 30 years after Merlin Tuttle’s field work on bat caves, he received an urgent call from The Nature Conservancy in Tennessee. Cory Holliday, a bat biologist there, wanted help in building a better bat cave.

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An itchy fungus that grows on the tips of bats’ noses in winter and rouses them during hibernation had arrived at some of Tuttle’s old field sites. Soon, it would be found in Pearson Cave. Every time a bat wakes, it burns as much energy as it would in a full 30 to 60 days of hibernation—energy it desperately needs to store to survive the winter. White nose syndrome, which was first identified in a New York cave in 2006, has now killed over 5 million bats, who  in 19 U.S. states.

Holliday wanted the bats to spend their winters in an artificial cave that could be sterilized at the end of the year, keeping the fungus in check. Following Tuttle’s advice, his cave would be about the size of a trailer home, constructed from a series of rectangular culvert pieces which could be sunk into the hillside. At one end, there would be an entrance for bats, and at the other end, a chimney that poked up through 20 feet of earth. The ceiling would be textured like the inside of a cave and have pieces of black mesh that the bats could hang from.

In October 2012, Holliday’s cave was complete, and he set out a speaker system blasting high-pitched bat calls to make it more attractive to the gregarious creatures. A few gray bats visited, but none roosted that winter. Nevertheless, inside a control shack up on a hill, Holliday monitored the cave temperature with data loggers and found that it met the characteristics that Tuttle had identified in his research: cold, but not too cold. 

It’s the first time that a winter roost like this had ever been built, but it could take years before the bats are comfortable enough to hibernate in it. Even if the temperature is just right, maybe something else—the humidity or the texture of the ceiling—doesn’t quite fit the bill. And so, each year, the Nature Conservancy may make a little tweak, trying to turn the interior of this cave into something that the bats can call home.

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“Even when we build our own homes, we don’t always get to make them perfect,” Tuttle says. “We are playing a little game with the bats and trying to provide them with something that is better than what they already have, and are hoping they will choose to live there.”

Brendan Borrell is a journalist and a fellow with the Alicia Patterson Foundation.

Lead image: Paul Kingsbury/The Nature Conservancy

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