Hunting scenes, geometric patterns, hand stencils and other works of prehistoric art can exist on the walls of well-protected caves for tens of thousands of years – but only if there are no bats in the galleries.
These flying mammals are simply looking for a safe place to sleep, but they are also turning into furry philistines who, according to a team’s research, erase old paintings and other cave wall markings in May within a few decades due to the corrosive nature of their feces or guano Geomorphology journal published.
In the caves of the Green Grotto in Jamaica in the early 2000s, two scientists, Joyce Lundberg and Don McFarlane, showed that resting bat colonies create their own microclimate that can gradually erode the limestone of a tropical cave. In the decades that followed, the destructive details were determined by further research. Studies have shown how large masses of bats within the closed confines of a cave generate heat and moisture and smear 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 phosphoric acid particles. This powerful combination is eating away at the limestone walls and ceilings, a process known as bio-corrosion.
A group of geomorphologists in France wanted to know if the same process was happening in bat-filled caves across Europe, where valuable cave paintings such as those in the French caves of Chauvet and Lascaux offer ornate windows into our past.
In particular, they focused on one cave system, the Azé Caves in eastern France. Bones found in the cave suggest that it was a home for cave bears around 150,000 years ago. During the entire Bronze Age, approximately 3,000 years ago, people lived and worked in the cave. And it has been visited by tourists for centuries, drawn by its limestone mazes and underground river. Other tourist caves in the area have scribbled graffiti over the years, but Azé’s entrance chamber is amazingly pristine, said Lionel Barriquand, a geomorphologist at Savoy Mont Blanc University and lead author of the study.
Azé has also been an important resting place for bats for 45,000 years. While advancing human development has significantly reduced the cave’s population, many thousands of bats once filled the walls and ceiling of the cave, covering the surfaces with layers of guano. But bats were blocked by a thick plug of calcite from the heart of Azé about 22,000 years ago. This inner sanctuary was uncovered in 1963 and offered scientists a natural experiment to compare its walls to those of the cave entrance.
They found that the walls of the long blocked section of the cave were more rugged and had fewer and shallower depressions in the ceiling than the entrance. The inner cave also had numerous bear claw marks on the walls, while none are present in the parts of the cave where bats have lived. By comparing measurements from the two sections, the scientists found that the walls of the cave entrance had retreated about 3 to 7 millimeters every thousand years due to the presence of bats. The cave entrance lacks cave art, graffiti or claw marks, they concluded, because the bat-driven erosion caused all of these markings to crumble into dust.
“The more bats you have, the more intense the process will be,” said Philippe Audra, geomorphologist at the University of Côte d’Azur and co-author of the study. Surface paint on the walls of Azé would go away within about 25 years, the researchers said.
Biocorrosion is an important but underrated aspect of understanding why prehistoric cave paintings are so often found in caves that have been sealed off from the outside world or have never housed bats, said Laurent Bruxelles, a geoarchaeologist at the French National Center for Scientific Research, who worked with . cooperates with Dr. Barriquand’s team, but was not involved in the most recent study.
“Paints are the first thing that bio-corrosion erodes,” he said. “In every cave where there are bats and paintings, the paintings disappear.”
Dr. McFarlane, who pioneered bat biocorrosion and is a paleobiologist at Claremont McKenna College in California, said the study was a useful application of his previous research on archeology. He added that anthropologists should consider these effects when looking at patterns of where cave art is and is not found.
“A lack of cave art could simply reflect the occupation of bats,” he said, “rather than a fanciful anthropological explanation.”