The 60,000 books in the Joanine Library are all hundreds of years old. Keeping texts readable for that long, safe from mold and moisture and nibbling bugs, requires dedication. The library’s original architects designed 6-foot (1.8 meters) stone walls to keep out the elements. Employees dust all day, every day.

And then there are the bats. For centuries, small colonies of these helpful creatures have lent their considerable pest control expertise to the library. In the daytime—as scholars lean over historic works and visitors admire the architecture—the bats roost quietly behind the two-story bookshelves. At night, they swoop around the darkened building, eating the beetles and moths that would otherwise do a number on all that old paper and binding glue.

The library dates the bats’ entry to the late 18th century. That’s when records indicate the purchase of large leather sheets from Russia, presumably to protect the hall’s desks and tables from the nightly rain of guano. Employees use the same system today, while the books themselves are behind wire mesh, says the library’s deputy director, António Eugénio Maia do Amaral. (The bats’ tendency to pee next to a portrait of the library’s namesake, King John V, is harder to address.)