Iron ore caves where bats roost in Brazil have become much larger than caves without bats – due to the weathering effect of thousands of years of guano.
The iron-rich earth beneath the Carajás National Forest in the Brazilian Amazon hosts more than 1500 caves, only 10 of which currently house large bat populations or used to.
Most other cave-dwellers – such as bacteria, fungi and invertebrates like beetles and scorpions – don’t leave their underground haunts. But bats forage for fruit and insects outside and release energy and organic matter in the cave through their guano, says at the Federal University of Pernambuco in Recife, Brazil.
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Bernard’s team noticed that caves with bats were larger and had more formations: stalactites, stalagmites and coral-like nubs of rock. They wondered if the bats played a role in carving the caves.
A comparison of bat-inhabited and bat-free caves revealed that, where bats roosted, caves ballooned. On average, bat caves were nearly 200 metres long – while regular caves were only 30 metres long – and their volumes were five times larger.
With colonies sometimes numbering more than 150,000 bats, the guano piles up. “Some of these deposits are 1 metre deep,” says Bernard.
Sampling the water that seeps through these caves, the team found that faeces make a slew of acids that can dissolve the iron. Radiocarbon dating at different depths of a guano pile revealed that bats have been using the same caves for millennia. Two samples dated to the end of the Pleistocene, with the oldest from around 22,000 years ago.
Normally, caves develop in limestone, says at the Côte d’Azur University in Nice, France, who wasn’t part of this work. Researchers know less about how guano sculpts caves in iron ore. It now seems the way acid etches this rock is “surprisingly rather similar” to the process in limestone, he says. Microbes may also play a role, but researchers are still pinning down the ways that they help.
The caves bats have lived in for thousands of years are in one of Brazil’s most important mining regions. Companies want to quadruple the area available for mineral extraction in the Carajás, says Bernard. An outcry from scientists halted a move last year to open protected caves to mining.
But with a new administration in Brazil, researchers are worried again that regulators may give in to pressure from the mining sector, says Bernard. “It’s very clear that those caves are unique – we cannot lose that.”
PLoS One
Article amended on 25 May 2023
We corrected a statement about how many caves house bat populations.
