How bats and their poop erase ancient rock art
Hunting scenes, geometric patterns, hand stencils, and other prehistoric artwork can last for tens of thousands of years on the walls of well-protected caves, but only if bats don’t hang around. the galleries.
These flying mammals are simply looking for a safe place to roost, but they also become furry philistines who erase old paintings and other markings on cave walls within decades due to the corrosive property of their droppings, or guano, according to the research of a team. by geologists and archaeologists published in May in the journal Geomorphology.
In the caves of Green Grotto in Jamaica in the early 2000s, two scientists, Joyce Lundberg and Don McFarlane, showed that perched bat colonies create their own microclimates that can gradually erode the limestone of a tropical cave. Over the following decades, more and more research identified the destructive details. Studies have shown how large masses of bats generate heat and humidity within the closed confines of a cave, smoothing the walls with an acidic film rich in carbon dioxide. Additionally, large amounts of bat guano and urine can ferment and saturate the air with aerosolized phosphoric acid particles. This powerful combination eats away at limestone walls and ceilings, a process called biocorrosion.
A group of geomorphologists in France wanted to know if the same process occurs in bat-filled caves across Europe, where prized cave paintings like those from the French caves of Chauvet and Lascaux offer ornate windows on our past.
They focused on one particular cave system, known as the Caves of Azé, in eastern France. Bones found in the cave suggest that it was a home of cave bears for around 150,000 years. Humans lived and worked in the cave throughout the Bronze Age around 3,000 years ago. And for centuries, it has been visited by tourists attracted by its limestone labyrinths and underground river. In other tourist caves in the area, visitors have scrawled graffiti over the years, but Azé’s entrance chamber is incredibly pristine, said Lionel Barriquand, a geomorphologist at Savoy Mont Blanc University and lead author. of the study.
Azé has also been a major bats resting site for 45,000 years. While the encroachment of human development dramatically reduced the cave’s population, several thousand bats once filled the cave walls and ceiling, covering the surfaces with layers of guano. Yet bats were stranded in the most remote parts of Azé by a thick plug of calcite about 22,000 years ago. This inner shrine was unplugged in 1963, giving scientists a natural experiment to compare its walls with those at the cave entrance.
They found that the walls of the long blocked section of the cave were more jagged, with fewer and shallower recesses in the ceiling than the entrance. The inner cave also had many bear claw marks along its walls, while none exist in the parts of the cave where the bats lived. By comparing the measurements of the two sections, the scientists determined that the presence of bats had caused the walls of the cave entrance to recede by about 3 to 7 millimeters every thousand years. The entrance to the cave lacks rock art, graffiti or claw marks, they concluded, as erosion from bats has reduced all of those marks to dust.
“The more bats you have, the more intense the process will be,” said Philippe Audra, a geomorphologist at the Université Côte d’Azur and co-author of the study. Surface paint on Azé’s walls would disappear in about 25 years, the researchers said.
Biocorrosion is an important but underestimated aspect in understanding why prehistoric cave paintings are so often found in caves isolated from the outside world or having never harbored bats, said Laurent Bruxelles, geoarchaeologist at the French National Center of scientific research that collaborates with Dr. Barriquand’s team, but did not participate in the recent study.
“Paints are the first things to erode due to biocorrosion,” he said. “In every cave where there are bats and paintings, the paintings disappear. “
Dr McFarlane, who helped start the bat biocorrosion work and is a paleobiologist at Claremont McKenna College in California, said the study was a useful application of his previous archeology research. He added that anthropologists should take these effects into account when looking at models where rock art is and is not found.
“A lack of rock art might simply reflect the bat occupation,” he said, “rather than a fanciful anthropological explanation.”