You can hear them from a long way off. They sound like a high-speed train trying to screech to a halt with that nerve-shattering metal-on-metal grind. It gets more frantic as you inch closer into the darkness. Then, you spot them – hanging from the dank roof inside a stone temple in Tirunelveli district, Tamil Nadu.

Rising from the banks of the river Tamiraparani, the temple in Kallidaikurichi, part of the Thiruvaduthurai Mutt, is more than 600 years old. It holds some deliciously cool and dark corners within its stone bosom, areas loved by both sun-baked visitors as well as insectivorous bats. Right outside, giant fruit bats roost in massive arjuna trees above the heads of farmers busy sowing paddy.

More than 150 stone buildings, some as old as 1,800 years, dot the banks of the 125-kilometre long perennial river. They are excellent bat roosting sites, researchers from the Ashoka Trust for Research in Ecology and the Environment have found.

In a study likely to be published soon, T Ganesh, A Saravanan and M Mathivanan of Atree point out that the temples provide exactly what these insect and fruit-eating bats need – dark, quiet places in large halls with high ceilings, abandoned rooms, damp corners and gaps between pillars made of granite stones stacked over each other.

The team studied 58 of the temples in the Tamiraparani river basin and found 32 of them housed as many as 4,136 bats across five species. What makes the temples important is that the natural habitats for the bats – caves and gaps in boulders – have been razed either for agriculture or for construction. “Some of these natural boulders and rocks that were removed actually made these temples,” said Ganesh, a conservation ecologist and a fellow at Atree. Pushed away, the cave-dwellers flew to these temples.

Unfortunately, for these flying mammals, most temple authorities find them smelly, dirty, noisy pests. Theresearch showed the primary threat to these remaining bat refugees is temple renovation. Mathivanan said that the power-washing, whitewashing and painting of these walls could put the bat population in the region at risk. More recent surveys showed that the bats have deserted some of the temples completely because of the renovations. “In fact, when we visit these temples for research," said Ganesh, laughingly, "they always first ask if we have come to clear the bats.”