By Victoria Gill
Science and nature reporter, BBC News
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.
Greater mouse-eared bats eat large, ground-running creatures, such as carabid beetles, hunting spiders and centipedes.
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.
The flight experiment simulated the foraging behaviour of bats
"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.
Traffic noise does not appear to affect the high frequency pulses that many bats emit in order to navigate.
But this echolocation, whereby bats detect echoes as the pulses bounce off other objects, can be interrupted by another man-made structure - wind turbines.
In a recent study, Chloe Long from Loughborough University, UK, and her colleagues, found slow-moving turbine blades might be invisible to echolocating bats.
In some cases, the bats might even be attracted to the turbines, because the echoes they return are so similar to the "acoustic glints" produced by the motion of a flying insects' wings.
"This could be attractive to bats in some way, or at least attract the curiosity of the bat," Ms Long told BBC News.
And in almost half of those trials, the animals failed to locate the food.
"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."