How to build a better bat cave (Nautilus)
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.
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.
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.