Skip-the-line available The Bats That Guard the Biblioteca Joanina
Two colonies, a nightly ritual of leather covers, and roughly two and a half centuries of pest control — the library's strangest story, told properly.
Every great building accumulates one story that travels further than all the others, and for the Biblioteca Joanina it is the bats. Somewhere behind the gilded shelving of King João V's baroque library at the University of Coimbra live colonies of small bats, and each night, when the last visitor has gone and the teak door is shut, they emerge to hunt the insects that would otherwise eat the books. The tale sounds like something a tour guide invented — too neat, too gothic, too good. It is none of those things. The university itself describes the arrangement plainly on its visitor pages: two colonies, resident for roughly 250 years, tolerated and quietly accommodated because they work. The full story is stranger and more interesting than the one-line version that circulates online, involving leather sheets laid out like table settings each evening, a morning cleaning ritual, and exactly one other library on Earth that does the same thing. Here it is, told properly.
The nocturnal staff — what actually lives in the library
Strip away the mythology and the facts are these. The Biblioteca Joanina houses not one bat but two colonies of them, according to the University of Coimbra's own account — small insect-eating bats that roost by day in the dim spaces behind the towering bookcases and in the fabric of the building itself, invisible to the visitors filing through below. They are not pets, not introduced, not managed in any modern sense; they are wild animals that took up residence in a warm, dark, insect-supplied building and were never evicted, because somewhere along the line the librarians understood what they were getting in exchange.
What they were getting was pest control. Old books are, from an insect's point of view, food: the paper, the paste and glue in the bindings, the leather covers themselves all sustain the small armies of book-eating insects that have destroyed collections all over the world. A bat is a superbly efficient predator of exactly those insects. Each night the colonies leave their roosts and work the three halls, taking the moths and crawling things drawn into the building, then slip out through the building's crevices to continue hunting in the Coimbra night. By morning they are back behind the shelves, and the library opens to visitors with no sign of its night shift beyond a freshly cleaned floor.
It is worth being precise about what the bats are not. They are not fluttering overhead during your visit — daytime visitors will almost never see one, and the twenty-minute timed entries pass without so much as a squeak. They are not a health hazard the university is ignoring; the morning routine exists precisely to keep the reading spaces pristine. And they are not a marketing invention. The bats appear on the university's own visitor pages, in serious press coverage and in the testimony of the people who tend the library — which is more documentation than most 250-year-old animal stories can produce.
The nightly ritual — leather covers and the morning clean
The most charming evidence that the bats are real is the furniture protocol. The noble floor of the Joanina holds a set of magnificent tables made from precious tropical woods — working furniture, three centuries old, on which readers once spread their books beneath the painted ceilings. Bats, however elegant their service to literature, produce droppings, and droppings fall on whatever stands beneath the flight paths. So every evening, after closing, the staff cover the tables and credenzas with sheets of leather, laid out with the regularity of a butler setting a table. The library is, quite literally, tucked in for the night.
In the morning the process reverses. The leather sheets are lifted, carried out and cleaned; the floors are swept of guano; and the halls are returned to their daytime perfection before the first timed slot of visitors arrives. It is a routine that has been performed, in some form, for generations — a small, unglamorous domestic ceremony hidden inside one of the most extravagant rooms in Europe. Few visitors ever learn it happens, which is a pity, because it says something profound about the institution: the same establishment that gilded its shelves and painted its ceilings also decided, centuries ago, that the practical thing to do about bats in the library was to buy good leather and get up early.
The ritual also explains why the arrangement has endured. The costs of hosting the colonies — some evening labour, some leather, some sweeping — are trivial and predictable. The benefit, an every-night patrol against the insects that eat rare books, would be difficult and expensive to replicate any other way. Institutions rarely keep a practice going for centuries out of sentiment; they keep it going because the ledger favours it. The Joanina's ledger has favoured the bats since before anyone alive can remember.
How long has this really gone on?
Ask three sources how long the bats have guarded the Joanina and you will get three romantic answers, most of them '300 years' — implying the colonies moved in with the first books. The university's own visitor pages are slightly more sober, crediting the colonies with approximately 250 years of residence. The distinction hardly matters in practice: either figure predates every modern technique of collection care by a very long margin, and either makes the bats one of the longest-running pest-management systems in continuous operation anywhere. The library received its first books in 1750; on the university's own timeline, the bats arrived within a generation or so of the collection itself.
What can be said with confidence is that the arrangement was never a formal decision recorded in a ledger — no eighteenth-century librarian wrote a memorandum engaging the services of bats. The colonies established themselves as bats do, in a dark, stable building full of food, and the practice of accommodating them hardened gradually into custom and then into identity. That is how the best institutional traditions form: not by design but by the slow recognition that something unplanned is working, and the wisdom not to interfere with it.
The bats have outlasted extraordinary things. Since they took up residence, Portugal has seen the Napoleonic invasions, the civil war of the 1830s, the fall of the monarchy, a republic, a dictatorship and a revolution — and through all of it the colonies kept their unwritten contract with the librarians of Coimbra. Collections elsewhere in Europe burned, were looted or simply rotted. The Joanina's survived, still on the original gilded shelves, protected by walls over two metres thick, a teak door, and a night shift that never once failed to report for duty.
Why bats beat chemistry — the conservation logic
To understand why the bats matter, consider the enemy. The insects that attack old libraries — moths, beetles, silverfish and their larvae — feed on paper, on the starch pastes and animal glues that hold bindings together, and on leather covers. Left unchecked in a warm building, they tunnel through texts that survived centuries and reduce them to lace. Modern archives fight this war with quarantine chambers, freezing treatments, controlled atmospheres and constant monitoring — an expensive apparatus that must be paid for and staffed forever. The Joanina fights it with predators who work every night, for free, and reproduce their own workforce.
The bats are one half of a two-part system, and the halves reinforce each other. The building itself is a passive conservation machine: exterior walls about 2.11 metres thick and a heavy teak door seal the interior into a remarkably stable climate — around 18–20°C with humidity near 60% year-round — which is precisely the environment in which paper and leather age slowest. The stable, sealed vault suppresses the conditions in which insect populations explode; the bats then suppress the insects that appear anyway. Eighteenth-century builders and an unplanned colony of wild animals arrived, between them, at something modern conservation science would recognise as a defensible integrated pest-management strategy.
There is a lesson in the comparison that curators elsewhere have noticed with something like envy. The Joanina's system requires no electricity, no chemicals near the books, and no maintenance contract that can lapse. Its weakness is that it cannot be replicated — you cannot install a 250-year-old bat colony — which is why the library's approach is admired rather than copied. Conservation literature tends to describe the arrangement with a mixture of scientific respect and open delight, because it is that rare thing: a heritage solution that is simultaneously a working one.
Mafra — the only other library on Earth that does this
The Joanina is not quite unique in its nocturnal staffing, and the exception proves the pattern gloriously: the only other library known to rely on resident bats for pest control is the library of the Mafra National Palace, the vast baroque palace-convent outside Lisbon. Mafra's library is another creation of the same era and, remarkably, the same king — João V, whose Brazilian gold built both — which makes bat-guarded libraries not a global curiosity randomly distributed, but a specifically Portuguese, specifically Joanine phenomenon: two eighteenth-century royal libraries, two colonies, one crown.
Why only these two? Partly architecture: both are massive stone buildings with the deep wall cavities, stable interiors and generous roof spaces that let a colony roost undisturbed within metres of the books. Partly continuity: both collections stayed in place, under uninterrupted institutional care, for the entire period — no evacuations, no modernising clear-outs that would have evicted the animals. And partly culture: at some point both institutions made the quiet choice to see the bats as staff rather than infestation. Plenty of historic buildings host bats; only in Mafra and Coimbra did the librarians decide the bats were the point.
For the visitor, the pairing is a gift. The two libraries make natural bookends to a Portuguese itinerary — Mafra an easy trip from Lisbon, the Joanina the crown of Coimbra between Lisbon and Porto — and seeing both is the closest thing book lovers have to a pilgrimage route. But of the two stories, Coimbra's is the more intimate: Mafra's library is enormous and echoing, while the Joanina is a jewel-box where the whole system — the sealed vault, the gilded shelves, the hidden colony — operates within three rooms you can cross in a minute.
Will you see them? Honest expectations for visitors
Here is the honest answer to the question everyone asks: no, you will almost certainly not see a bat. Visits to the Joanina happen in short timed slots during opening hours, when the colonies are roosting deep behind the shelving and the woodwork, and the animals have every incentive to stay hidden from the procession of daylight visitors. The library is not a bat-watching experience and does not pretend to be one; the university's evening events happen indoors under lights, and even the staff see the residents rarely. The bats' whole value lies in being active precisely when no one is there.
What you can do is read the room with the story in mind, which transforms the visit. Those magnificent dark-wood tables are the ones dressed in leather every night. The dim upper reaches behind the gilded balustrades are the roosting territory. The teak door that closes behind your group with such finality is the seal on the vault the bats patrol. Twenty minutes is a short audience with a room this rich, and knowing the nocturnal half of its life lets you spend those minutes seeing a working conservation system rather than simply a beautiful interior — which is, in the end, what the Joanina actually is.
And if the bats are the story that brought you here, let them do one more service: let them get you to book properly. Library entry is by timed slot as part of the University of Coimbra's visitor circuits, capacity in each slot is small, and the good times go first. The colony has kept its side of the arrangement for a quarter of a millennium; securing your twenty minutes inside the room they guard is the visitor's side, and it is considerably easier — it just needs doing in advance.
Frequently asked
Are the bats in the Biblioteca Joanina real?
Yes. The University of Coimbra's own visitor pages describe two colonies of bats resident in the library for roughly 250 years. They roost behind the shelving by day and emerge at night to eat the insects that would otherwise damage the books.
Will I see bats during my visit?
Almost certainly not. Visits run in short timed slots during the day, when the colonies are roosting out of sight. The bats are active at night, after the library closes — that is exactly why they are useful.
Why doesn't the library remove the bats?
Because they work. The bats eat the moths, beetles and other insects that feed on paper, glue and leather bindings — a natural pest-control system that has helped keep a centuries-old collection intact on its original shelves, at almost no cost.
What are the leather covers for?
Each evening after closing, staff cover the library's tables of precious tropical wood with sheets of leather to protect them from bat droppings. In the morning the covers are removed and the floors cleaned before the first visitors arrive.
Is the Joanina the only library with bats?
One of exactly two known. The other is the library of the Mafra National Palace near Lisbon — also built in the era of King João V. Bat-guarded libraries are a distinctly Portuguese phenomenon.
Do the bats damage the books?
No — their contribution is protective. The nightly leather covers and morning cleaning deal with the droppings, while the bats remove the insects that genuinely destroy books. The collection's survival across three centuries speaks for the arrangement.