Bats Pick Up Rustling Sounds Against Highway Background Noise

Listening for faint rustling noises made by tasty beetles on a quiet day is simple for bats hunting with their exquisitely sensitive hearing. So try imagining what it must be like trying to locate rustling treats just metres from a roaring highway. It would seem to be almost impossible to pick out a centipede's footsteps as a juggernaut hurtles past; or is it?

How animals that locate their prey by sound alone cope in our increasingly noisy world puzzles Björn Siemers from the Max Planck Institute for Ornithology in Germany. Siemers explains that no one had ever measured whether bats that hunt by listening for rustling insects are affected by man-made noise.

However, this is a question that Siemers is frequently asked by urban planners keen to minimise our impact on local wildlife populations. Curious to know how sharp-eared bats react to loud background noise, Siemers and his colleagues Andrea Schaub and Joachim Ostwald monitored foraging bats' responses to rustling mealworms in noisy environments.

Working with a group of young male greater mouse-eared bats, Siemers and Schaub allowed individual bats to forage freely in a large soundproof room. Dividing the back of the room in two, the duo provided the bats with a choice of rustling mealworm snacks in each half of the room to dine on. Over the course of several days, the bats divided their attention equally between the two halves of the room, easily locating the rustling nibbles. But how would the bats react when the team switched on a noise in one of the two dining areas?

First, the team synthesized true white noise before playing the sound in one half of the flight arena. The bats instantly avoided the unpleasant buzzing sound, spending more than 80% of their time hunting in the quiet dining area.

Next, Siemers and Schaub headed out to a local highway to record traffic sounds within 15·m of the busy road. Back in the lab the animals were less bothered by the loud traffic noise than they had been by the white noise buzz. But they still preferred hunting in the quiet dining area, only spending 38.7% of their time gathering mealworms from the traffic noise booth. However, when the bats ventured into the noisy dining area they had no obvious problems locating their rustling prey against the traffic background.

More surprisingly, when the team played a simulation of a high wind rattling reed beds, the bats seemed to find it difficult to locate their prey and preferred foraging away from the sound, even though it was a noise that they encounter naturally in their day-to-day activities. So, man-made noise does interfere with bat foraging, but less than a very high wind rattling through vegetation.

Siemers admits that it isn't yet clear how man-made noise interferes with foraging bats as they listen out for a rustling lunch, but it probably does discourage these animals from foraging close to busy road networks. Keen to find out whether noise pollution affects foraging bat's hearing, or interferes with some other aspect of the bat's behaviour, Siemers is optimistic that his work will eventually result in bat-friendly road construction guidelines that will help to protect endangered bat species from our increasingly mechanised world.