UT researchers study how bats deal with human-created noise

Jack Stenglein

Human-created, or anthropogenic, noise is a serious problem for both humans and animals, according to Mike Ryan, UT professor of integrative biology. Ryan recently co-authored a study showing how bats might deal with this noise and still find prey.

The study focused on the fringe-lipped bat, which, unlike most insect-eating bats, eats Tungara frogs. While most bats use echolocation to find prey, the fringe-lipped bat can also hunt using the lower frequencies of the frog’s mating call.

To study this ability, the researchers played the frog calls on a speaker, which the bats located using low-frequency hearing and minimal echolocation. However, when background noise was introduced, the bats could not find the speakers.

This was when the bats increasingly used their echolocation, according to Ryan. The researchers added robotic frogs with the same calls and moving vocal sacs. Ryan said the bats’ echolocation allowed them to sense the movement of the robot’s vocal sac despite background noise.

“This study shows for this one species of bat, it’s able to adapt and flip between two very different sensory channels: passive hearing and active echolocation,” Ryan said. “We’re very interested in how they handle the different streams of info and make decisions — when one channel is blocked, can you just flip a switch and start paying more attention to another?”

Ryan said the ability to switch between streams of information is not unique to bats. Humans exhibit this trait when talking in a loud room. People watch the lips of the person they are talking to in order to better understand them.

“We’ve discovered that some animals have found ways to combat [background] noise,” Ryan said. “We’re kind of stuck; we don’t have too much ability. We can read lips, but these bats have a much more sophisticated way of dealing with noise.”

George Pollak, UT neuroscience professor, led a study in which he recorded the different calls of Mexican free-tailed bats and played them back to see how the bats’ neurons responded. 

While Pollak said he cannot conclusively link his study with Ryan’s, he said the results of Ryan’s study suggest some nerve cells in the bats’ brains react to the robotic frogs but not the speakers, allowing the bats to distinguish between them.

“When you deal with the nervous system, success is partial,” Pollak said. “We understand aspects, but there are ten million questions unanswered. It’s one of the most complex systems in the universe.”

Answering these questions could increase understanding of our own auditory systems, Pollak said. Humans detect accents, recognize male or female voices and hear imperfections in music, all of which the brain must encode and represent. 

Pollak said understanding what happens when humans — and bats — combine multiple inputs is even more difficult. It is unknown how the auditory system works when the brain is focused on the input, and attention itself is not fully understood either, Pollak said.

Complicating matters further, Ryan said the information humans receive can sometimes clash, leading to bizarre perceptions. Harry McGurk documented this in 1976, finding that when people watch videos of someone speaking but hear different audio, “their vision influenced their speech perception to hear a third sound distinct from the audio and the video.”

Ryan said the next step toward answering questions about fringe-lipped bats processing information is to introduce interference, like vegetation movement, that affects the bat’s ability to detect the frog’s vocal sac.

However, on the neuroscience side, the way forward — determining what causes the brain to handle information the way it does — is less clear, Pollak said.

“We know a lot, but there’s a lot more we don’t know,” Pollak said. “I don’t know how [the auditory system] works, I don’t know how long it would take to figure out and I don’t know exactly how to go about it. It’s such a complicated question.”