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Bats Stand Accused

The onset of the coronavirus is not the first time that bats have received a fair amount of (unwarranted) negative attention. Why is it that these mammals get such a bad rap?

No, the original cause of COVID-19 was not someone eating bats in China. That is a myth. But bats are not off the hook because there is a chance that the SARS CoV-2 virus did originate in bat poop from where it migrated to an animal, likely the pangolin, from where it jumped to humans. Interestingly, this is not the first time that bat droppings have been accused of crimes against humanity.

The newspaper headline in 1993 sure got everyone’s attention: “Holy Blam, Batman! Bat Dung Brews Methane; Park Building Explodes!” The story seemed to justify the sensational title. It intrigued readers with its description of a remarkable event that occurred in a State Park in Michigan. The area was shaken by a reverberating explosion that left an old abandoned ranger station in smithereens.

At first, police investigators could find no cause for the explosion; there was a propane furnace in the house but it had long been disconnected and in any case, the gas had been shut off at the outside propane tank, which was still completely intact. No one held any insurance on the dilapidated building so foul play was quickly ruled out. But then one of the police officers noticed that the floor inside the building was sprinkled with a black gunk that had apparently been dripping down from the attic. He decided to investigate. As he opened the attic door, he was startled by the frantic beating of hundreds of wings! Bats had invaded the abandoned house and had roosted in the attic. The extent of the slimy stuff on the floor was evidence that they had been there a long time; it was bat guano! Bat poop for those unfamiliar with the scientific term.

The officer, it seems, recalled some of his basic scientific education and remembered that organic waste could produce a combustible gas. A possible explanation for the explosion now began to emerge. He hit the books and quickly learned that manure contains bacteria that can convert the various organic molecules that have survived digestion into methane gas. We normally refer to this as “natural gas.” Methane burns, and under the right conditions, it can explode. Consultation with a zoologist seemed to seal the case. The expert confirmed the potential of guano to produce methane and added that methane was heavier than air and probably settled in the basement. The Fire Marshal’s report concluded that the explosion likely occurred when the methane was ignited by a spark from a sump pump that was still functioning in the basement. A neat, interesting, and obviously sensation provoking report. But there was a problem. It had to be wrong!

The investigators should have consulted a chemist instead of a zoologist. They would have learned that methane is lighter, not heavier than air. It would have escaped through the openings in the roof and could not have accumulated in the basement. Had they consulted a microbiologist, they would also have discovered that the bacteria that produce methane from waste are anaerobic, meaning they can only survive in the absence of air. Bat droppings are quite porous and the bacteria they contain would be exposed to the air. It is most unlikely that any appreciable amount of methane would be produced. If this were the case, we would have had all kinds of reports of visitors to bat-infested caves being blown away when they struck a match.

So if bat droppings weren’t responsible for blowing the house away, what was? In light of the new scientific revelations, the investigation was reopened. The culprit turned out to be manure alright, but of a decidedly human variety. It seems the toilets in the house were connected to a septic tank, with the usual U-shaped trap built into the pipes. Under normal conditions, these traps are filled with water which prevents any gas from backing up into the toilet. Since septic tanks provide the anaerobic conditions under which flammable methane can be generated, this is an essential safety measure.

The toilets had not been used since the house had been abandoned and the water had evaporated from the plumbing. Methane was now free to back up into the basement and had apparently done so. As the original investigator had proposed, a spark from the sump pump probably set off the blast. The bats undoubtedly had been falsely accused.


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