Noise "degrades" bat habitat for 50-60m either side of a busy highway
Traffic noise reduces bats' ability to locate their prey, say scientists.Researchers in Germany found that road noise affected the bats' ability to listen for the "rustling sound" of the beetles and spiders they feed on.
This is the first study to examine the impact of traffic on predators that listen for their prey.
The researchers report in the Royal Society journal Proceedings B that the same effect could be true for other "acoustic predators", including owls.
With their remarkably sensitive hearing, the bats detect and track down their prey by listening for the faint rustling sounds they produce when walking.
The bats are protected under the European Habitats directive, so the scientists' aim was to measure how any planned highways might affect their habitat.
To do this, they set up a flight test.
"We attempted to simulate the bats' foraging behaviour in our flight room," explained lead researcher Bjorn Siemers from the Max Planck Institute for Ornithology in Seewiesen.
In the wild, the bats fly about one metre above ground listening for rustling sounds.
"We had an array of 64 plates [on the floor of the flight room] each containing a speaker through which we could play this rustling sound," explained Dr Siemers.
When a bat landed on the right plate - the one from which the sound was being played - there would be a food reward waiting for it.
"On average it took five seconds for the bats to find the right plate," said Dr Siemers.
But when the team introduced traffic noise - via more loud speakers - into the flight room, the bats' performance declined.
Under the "strongest noise profiles" - which mimicked the sound of a busy highway just a few metres away - the bats took an average of 25 seconds to locate their treat.
"But even with the sound of a busy highway seven and a half metres away, they could still forage," said Dr Siemers.
"We were astonished by how well they coped with the noise, but their efficiency was greatly reduced."
Noise levels mimicking traffic up to 50m away affected the bats' ability to locate a meal.
This means, the researchers say, that each highway "degrades" an area of 50-60m of foraging habitat either side.
"It might not sound like much," Dr Siemers said, "but when you look at the thousands of kilometres of highway in a country like Germany, it adds up to quite a lot.