Do Dormant Bats Still Sleep in Old Mines?

In his 1943 book Coal Dust on the Fiddle, Pennsylvania folklorist George Korson passes on "a curious tale about a Rip Van Winkle-like bat" reported "some years ago" by a mine engineer exploring "an abandoned old working in the Georges Creek region of Maryland." Here, verbatim, is the engineer's story, as quoted by Korson.

Photo by Ann Froschauer, U.S. Fish and Wildlife Service

"The mines in which I was making surveys were abandoned for over fifty years. The roof had fallen; rooms had been clogged shut; and only by the most circuitous routes, often entailing a creeping on one’s hands and knees for over a half mile, could we get to the unsurveyed part. Arrow marks of chalk were necessary to find one’s way out, and a full supply of oil, sufficient to last a day or two, was taken along. Back in the recesses of this cavern we wandered and we found colonies of bats. At first we did not know what they were. Welsh miners with the party enlightened us. In certain sections of the mine, where the air was so foul that the miners’ lamps shed but a faint light, sticking to the roof were little frost-coated objects. In many places there were hundreds of them. By putting a light under the little lumps on the roof a burning of fur and hair would be followed by a falling object, which would flop and emit shrill cries. On investigation they proved to be bats.

"The Welsh miners thought nothing of the incident, and said that in abandoned mines in Wales they were found in places that had been shut up for over half a century. How they got there and how they lived is the riddle. 

"With a view to experimenting, I placed a chalk mark around one of the bats, disturbing his slumber not at all, and then awaited developments. I did this a few weeks after I went there, and every time I was in that particular section of the workings I would see whether my bat had awakened or not. I was there over a year, and to my personal knowledge, this bat, in a semicomatose condition, was glued fast to the roof of the mine, covered with mold and sleeping away the years." (208) 

Biologists generally agree that in cold climates, bats can hibernate for six months or more, however long their insect prey is scarce. (Bats in warmer climes may not hibernate at all.) Any claim of years-long hibernation, however, would give most bat experts pause.  

Note, moreover, that Korson's unnamed engineer seems to imply the bats have been in place for more than fifty years. This is so startling an idea that we almost miss the head-scratching question of why bats should be less active when there are fewer people around.

What's remarkable about bat hibernation, scientists say, is not how long it lasts, but how efficient it is. In the words of the National Park Service: 

A bat's heart rate drops from 200-300 beats per minute to 10 beats per minute, and it may go minutes without taking a breath. The bat's body temperature can also drop to near freezing, depending on the temperature of the bat's surroundings. Other bodily functions also slow down, which reduces energy costs by about 98%. In this state of "torpor," bats are experts in high energy efficiency!

In light of these facts, one easily can see why claims of extremely long sleeps for entire bat colonies might get started. If bats basically can put their lives on hold for six months, why not six years, or sixty? -- especially if the bats were visiting the surface world while no humans were around to see them, aiding the illusion of dormancy. As any exterminator can attest, bats are extremely good at turning improbably tiny fissures and cracks into bat superhighways that enable them to come and go as they please. And if emissaries aren't making occasional visits to the surface, how does a colony, living deep underground at a controlled temperature, know when spring has arrived?

I'd love to know whether any current research has extended the possible length of bat hibernation. In the meantime, I suspect this engineer's Welsh guides were leading him on in more than one way!

(If you're interested in bats, please consider helping the vital work of Bat Conservation International.)

Sources:  

Korson, George. Coal Dust on the Fiddle: Songs and Stories of the Bituminous Industry. Philadelphia: U of Pennsylvania P, 1943. 

National Park Service. "All About Bats: Hibernate or Migrate." Updated 12 June 2020. https://www.nps.gov/subjects/bats/hibernate-or-migrate.htm. Accessed 20 Sept. 2021.