Bats in the Library: How Portugal's Ancient University Protects Its Books With Winged Guardians

A Library Built in the 18th Century for Books That Were Already Ancient

The Joanina Library at the University of Coimbra was completed in 1728, commissioned by King João V as part of a broader program of royal patronage for arts and learning in Portugal. The building itself is a masterpiece of Portuguese Baroque architecture: three interconnected halls with gilded woodwork, painted ceilings, and shelving that rises to the vaulted ceilings in tiers accessed by carved wooden balconies. The space is designed as much as a statement of royal magnificence as a repository for books, and the effect it achieves — overwhelming, warm, deeply impressive — has made it one of the most photographed library interiors in the world.

When the library opened, it housed a collection of books that included volumes hundreds of years old. The University of Coimbra itself is one of the oldest universities in the world, continuously operating since 1290, and its library had been accumulating books for centuries before the Joanina building was commissioned. The new building provided a magnificent home for the collection, but it immediately faced the problem that all libraries face: insects.

The Insect Threat to Ancient Books

Books are organic materials — paper or vellum, leather bindings, glue derived from animal products, wooden boards — and many insects find them attractive as food or habitat. The primary culprits in European libraries have historically been silverfish, booklice, and certain species of beetle whose larvae bore through paper and binding materials. A single infestation in a library can cause irreparable damage to irreplaceable volumes. Modern libraries address this with climate control, pest monitoring, and careful quarantine of incoming materials. The Joanina Library addressed it with bats.

The colony of free-tailed bats that lives in the Joanina Library occupies the space inside the walls and in the cavities behind the ornate wooden shelving. At dusk, when the library closes to visitors, the bats emerge and spend the night hunting the insects that would otherwise find the book collection attractive. Their echolocation and aerial agility allow them to hunt effectively in the enclosed space without damaging the shelving or the books. By dawn, they retreat to their roosts, and library staff cover the reading tables with leather cloth before closing to protect the surfaces from bat droppings during the night.

A Symbiosis That Has Persisted for Centuries

How long the bats have inhabited the Joanina Library is uncertain, but the arrangement is old enough that it has been integrated into the library's maintenance routines for as long as anyone can document. The relationship is genuinely mutualistic: the bats have a warm, insect-rich environment with shelter from predators, and the library has a free, continuous, highly effective insect control system that requires no chemicals and generates no risk of inadvertently damaging the books.

Modern pest management in libraries involves careful tradeoffs. Chemical pesticides are effective against insects but can damage paper and bindings, and their long-term impact on organic library materials is a genuine concern for conservators. Physical exclusion, climate control, and monitoring are preferred but require significant infrastructure investment. The Joanina Library's bats provide biological control — a method that conservation biologists have increasingly recognized as both effective and environmentally appropriate — without any of the chemical risks.

The Tension Between Conservation and Tourism

The Joanina Library is a UNESCO World Heritage Site and one of Coimbra's most-visited attractions. Hundreds of thousands of visitors move through the three halls annually, creating a tension between the building's function as an active library, its status as a heritage site, and the needs of the bat colony that has been part of its ecosystem for centuries.

Visitor access is carefully managed, with the library open for limited hours and visitor numbers controlled to reduce the impact on the collection and the microclimate. The bats' presence is now acknowledged openly and has become part of the library's identity — the winged guardians of an 18th-century treasure house, patrolling the stacks in darkness while the golden woodwork gleams by day. The arrangement is unusual enough to be remarkable and old enough to have become a tradition. In a place devoted to the preservation of accumulated human knowledge, the partnership with non-human protectors has proved remarkably durable.