Archive image shows Coimbra University students lining-up with their yellow hats and sticks during the end of term festivities May 6, 2003 . EPA PHOTO LUSA/PAULO NOVAIS

A student stands in front of the University of Coimbra the day it was classified by the United Nations Educational, Scientific and Cultural Organization (UNESCO) as a World Heritage site, in Coimbra, Portugal, June 22, 2013. EPA/PAULO NOVAIS

Archive file of the Joanina Royal library at Coimbra University shows the ground floor's portal and national coat of arms leading to three great rooms in the same style as the portal and entirely executed by Portuguese artists. The walls are covered by two storied shelves, in gilded or painted exotic woods; Its construction was ordered by Joao V. EFE/BARRIOPEDRO/jgv

For half a millennium, a colony of bats has lived and thrived in Portugal's venerable Royal Library located in Coimbra.

These flying furry mammals were appointed to the unlikely role of custodians of the Biblioteca Joanina of Coimbra University due to their voracious appetite for booklouse, bookworms, and other paper-munching insects that, as ravenous bibliophages do, would have otherwise shredded the 60,000 priceless Royal collection's leather-bound volumes dating back to the 16th century.

Coimbra's University's other General Library, available to its students, holds over a million volumes.

Each of the library bats that inhabit this UNESCO World Heritage site (since 2013), is capable of eating, in a single night, 500 insects buzzing about the centuries-old volumes stretching across the three floors of this impressive, baroque library.

The library's construction was ordered by King Joao V (John), who gave his name and funded this repository of knowledge, in this, one of Europe's oldest universities (1290) and Portugal's only university until 1911.

The building was designed to become a royal library as its building technique demonstrates, from the width of the walls, the choice of woods and its many ventilation inlets, which are also used by the little critters seeking a diet based on written slices of history.

Each evening, before the library locks up its doors, the librarians cover the volumes with large leather covers to avoid the priceless books getting covered in bat droppings.

The bat colony is comprised of two different species and their presence was recently studied and confirmed by Lisbon researcher, Jorge Palmeirim.

Although he was not able to catch a glimpse of the bats, he was able to measure their squeaks when flying at night.

One of the bats' most valuable attributes is their voracious appetite for the "nicobium castaneum" or Library Beetle larvae, borer insects capable of destroying complete book collections, parchments, leather bindings or covers.

Another Librarian's scourge is the Silverfish, a little wingless silvery insect, that enjoys dining on book glue, centuries-old paper and living in dark, moist places.

However, the increasing number of volumes and the passage of time have taken a toll on the centuries-old symbiosis between bats and books.

According to a University of Coimbra source contacted by EFE, in order to protect their bibliographical treasure, the institution invested in a six cubic meter (212 cubic feet) anoxia chamber, a 70,000 euro ($82,000) ecological disinfestation method which uses an inert gas (nitrogen) to kill book-eating insects by asphyxia and dehydration.

The Joanina also houses newspapers, magazines, special manuscripts, a noteworthy cartographical collection and ancient musical scores dating between the 16th-18th centuries.

The Joanina is one of the world's most important historical libraries with such unique titles as a first edition of "Os Lusiadas," an epic tale written by Luís de Camões or a Hebrew Bible edited in the late 15th century of which only some 20 copies exist worldwide.

Also worth mentioning is the "48 line Latin Bible," whose name relates to its 48 text lines per page layout, printed in 1462 by two Gutemberg associates and considered one of the four most beautifully printed early Bible editions.

The Joanina library was built on top of a medieval prison and its cells were later used as a University prison for bad students.

By Carlos García

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