Bats are adapting to human noise by changing their hunting strategies, according to a new study. That’s good news for bats, since the number of city-dwellers in the world is expected to increase to 6.5 billion in 2050, from nearly 4 billion today.

In noisy environments, instead of relying only on hearing to find their food, bats also activate echolocation, according to the study published today in Science. It’s the first time that scientists show bats can switch sensory systems, and it raises questions about how other animals are adapting to noise.

Bats, the only flying mammals, are particularly vulnerable to noise because they use echolocation to get around — they basically use sound waves and echoes to determine where objects are. One study in 2010 showed that bats took much longer to find their prey when hunting around highways; traffic noise made it harder for the bats to hear the rustling sounds made by the bugs and spiders they feed on.

"It’s really important to be able to show that animals can cope with [noise]," says Brock Fenton, who studies the behavior and ecology of bats at Western University, and was not involved in the study. "Some animals probably can’t."

Human noise — coming from cities, highways, factories, and agricultural fields — puts big strains on the environment. It makes it harder for animals to detect prey or hear an approaching predator. Previous research has shown that human-made noise is linked to higher mortality rates for certain animals and that it disrupts communication among others. "It’s having a big effect on animals and plants," Fenton says.

Human noise puts big strains on the environment

The authors of today’s study wanted to see whether bats are using alternative strategies to adapt to noise when hunting. In particular, they wanted to see whether fringe-lipped bats, which are specialized to find frogs by hearing their mating calls, also used echolocation to hunt when the environment became noisy. (Echolocation also relies on sound, but most noises are well below the range of echolocation sound waves — so background noise doesn’t affect it.)

In the study, the researchers exposed 12 bats to two robotic frogs. One frog only released the mating sounds; the other also inflated a balloon mimicking the movement of a vocal sac. (The frogs inflate and deflate the vocal sac when calling.) The researchers found that, when they added background noise, the bats chose the frog with the moving vocal sac 75 percent of the time. That’s because the bats stopped relying completely on hearing and instead used more echolocation to detect the frogs. The movement of the vocal sac made them easier to find.

"We show that they switch," says W.H. Halfwerk, an assistant professor of ecological sciences at VU University Amsterdam and one of the study’s authors. "They are flexible and ... better resistant than we previously thought."

Some say the findings aren’t that surprising. We already know that animals use a variety of senses and, when one sense is impaired, they compensate with another one. The same happens with humans, too: if you’re in a loud bar and are trying to talk to friends, you start paying more attention to their lip movement and facial expressions. In short, your vision compensates for your impaired hearing. That’s what the bats are doing when hunting. "I’m not blown away," says Gary McCracken, a professor of ecology and evolutionary biology at the University of Tennessee, who did not take part in the study. "It’s not an unexpected result, I don’t think."

"It could affect the composition of urban wildlife."

The study, however, has important implications for the survival of bats — and other animals — in the future. Nearly 4 billion people in the world already live in cities, and that number is expected to grow to 6.5 billion in 2050. That means that human-made noise — and its impact on ecosystems — is only bound to increase. Knowing how animals cope with noise and which ones can adapt to it will tell us how they will fare in the future. "It could affect the composition of urban wildlife," Halfwerk says.

"This is way beyond bats now. This is about thinking about any animals," says Paul Faure, the director of the Bat Lab at McMaster University, who was not involved in the study. "We are domesticating our planet, we’re creating noise pollution, we’re creating light pollution. We’re fundamentally altering the world that we live in."