Hunting scenes, geometric patterns, hand stencils and other works of prehistoric art may have survived on well-preserved cave walls for tens of thousands of years – but only if bats didn’t roam the galleries.
These flying mammals are simply looking for a safe place to settle, but they also become furry benefactors who within a few decades wipe out ancient paintings and other cave wall marks because of their feces, or guano. corrosive property, according to research by a team of geologists and archaeologists Published in May in the Journal of Geomorphology.
In the early 2000s in Jamaica’s Green Grotto Caves, two scientists, Joyce Lundberg and Don McFarlane, showed that a colony of bats settled Create your own microclimate which can gradually erode the limestone of a tropical cave. In the following decades, more research narrowed down the disastrous details. Studies have shown how large numbers of bats generate heat and humidity within the closed confines of a cave, lining the walls with an acidic, carbon dioxide-rich film. In addition, large amounts of bat guano and urine can ferment and saturate the air with aerosolized particles of phosphoric acid. This powerful assemblage eats away limestone walls and ceilings, a process known as biocorrosion.
A group of geomorphologists in France wanted to know whether the same process played out in bat-filled caves across Europe, where prized cave paintings, like France’s Chauvet and Lascaux caves, offer ornate windows into our past.
He focused in particular on one cave system, known as the Eze Caves, in eastern France. Bones found in the cave suggest that it was home to cave bears about 150,000 years ago. Humans lived and worked in the cave throughout the Bronze Age, about 3,000 years ago. And it has been visited for centuries by tourists who are drawn to its limestone labyrinths and underground river. In other tourist caves in the region, visitors have scattered frescoes over the years, but the entrance hall of Eze is much more ancient, said Lionel Barricand, a geomorphologist at the University of Savoy Mont Blanc and lead author of the study.
Eze has also been a major bat roasting site for the past 45,000 years. As human development encroached upon the cave population, many thousands of bats once covered the walls and ceiling of the cave with layers of guano. Yet the bats were blocked from the interior of the Eze about 22,000 years ago by a thick plug of calcite. This inner sanctum was closed in 1963, offering scientists a natural experiment to compare its walls to the cave entrance.
They found that the walls of the long-barred section of the cave were more jagged, with fewer and shallower grooves in the ceiling than the entrance. The inner cave also has several bear claw marks along its walls, while none are present in the parts of the cave where bats live. By comparing measurements from both volumes, the scientists determined that the presence of bats had caused the cave entrance walls to retreat about 3 to 7 millimeters every thousand years. The cave entrance lacks any cave art, frescoes or claw marks, he concluded, because bat-driven erosion reduced all such markings to dust.
“The more bats you have, the more rapid the process will be,” said Philippe Audra, a geomorphologist at the University of Cte d’Azur and co-author of the study. The researchers said a surface painting on the walls of Eze would disappear within about 25 years.
Biocorrosion is an important yet underappreciated aspect of understanding why prehistoric cave paintings are so often found in caves that have been closed off from the outside world or never hosted bats, said Laurent Bruxelles, A geologist at the French National Center for Scientific Research, who collaborates with Dr. Barricand’s team but was not involved in the recent study.
“Paintings are the first things to erode due to bio corrosion,” he said. “In every cave where there are bats and paintings, the paintings disappear.”
Dr. McFarlane, who helped advance the bat’s biodegradation work and is a paleontologist at Claremont McKenna College in California, said the study was a useful application of his earlier research on archaeology. He said that anthropologists should consider these influences by looking at the patterns of where cave art is and where it is not found.
“The lack of cave art may simply reflect bat possession,” he said, “rather than some hypothetical anthropological explanation.”