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Hundreds of bats watch over two of the oldest Portuguese libraries daily, in the University of Coimbra and the palace of Mafra. It's their ability to catch bugs that makes sure the books are kept safe.

Bats are the only mammal capable of flying and only do it during the night, making high-pitched sounds inaudible to the human ear, making it hard to study the 26 species known to exist in Portugal.

One night in 2008, Professor Jorge Palmeirim from the Science Faculty in the University of Lisbon, gathered his sound-equipment and went to King John's Library in Coimbra to try and understand which bats have been using the shelters in that place for over 200 years.

"I couldn't see them, just hear them, but according to the droppings I found, I can say that there live at least 2 different species of bats", says the professor.

In the entire country only one other library is known to shelter bats, in the Palace of Mafra and is thought that their preference has to do with the antique wood paneling.

"Bats are also very conservative when it comes to shelters. They tend to use the same shelters for several generations and prefer old buildings", the professor explains.

In Coimbra there are 200 years-old documents testifying the purchase of the same type of animal hide still in use today to protect the antique tables in King John's Library from bat droppings daily.

"Bats have been living in the library since ever. The library tables are protected with hide every night, because they are also antiques and the bats fly freely, eating the bugs," says the director of King John's Library, Carlos Fiolhais.

In Mafra it's also the bats and the good environment provided by the high ceilings and the 18th century wood paneling, that explain the "excellent state" the books are in, explains librarian Teresa Amaral.

A bat can eat up to 500 bugs daily, so no wonders that they leave the libraries during the night to hunt. The coming and going of the bats is made possible by the holes common to old libraries.