Bat sounds reveal bat secrets

Bats are difficult to track, but by recording their calls, researchers hope to study them.

GREENSBORO—File this under: things you might not realize were happening while you were sleeping. “When you’re asleep the sky is filled with multiple species of bats,” said Matina Kalcounis-Ruppell, Ph.D., professor and Biology department head at the University of North Carolina at Greensboro. “We don’t’ see them, we don’t hear them, so we don’t know much about the 17 species of bats in North Carolina. So my lab wants to find out what those species are doing."

So to find out what the bats are doing, the researchers in Kalcounis-Ruppell’s lab listen. That’s because bats are a very acoustic species. However they don’t listen to bats in a traditional way. Bats produce ultrasonic sounds. That means the sounds are at a higher frequency that humans can hear.

“In all of our studies, we put out microphones across the landscape so we can eavesdrop on them,” explains Kalcounis-Ruppell. “That way we can study them by eavesdropping on those sounds.”

If you keep watch and walk slowly through a woods near the UNCG campus, you’ll spot trees with green boxes attached by bungee-cord at the base. The wire snakes up the tree to the middle branches. A long microphone is attached to the end of the wire. The microphone is anchored into the tree branches.

Rada Petric, a Ph.D. student in Biology, is at the base of a tree checking one of the boxes. The recording device, memory card and battery are inside.

“We typically set these up to record from sunset to sunrise, which is when bats are most active,” explains Petric. “While the recording box is here, the microphone is several meters above because some bats are high flyers, some are low, but this gives us a chance to record more of the vocalizations of the bats that are passing through.”

Bats produce sounds in the ultrasonic range, which means humans are unable to hear the sounds. But using special computer software, researchers are able to slow down bat recordings and change the frequency so they are in the audible range.

“And what’s amazing about these bats is you can assign different species to the calls,” adds Petric. “Each species produces a different type of call, so that is one way we can understand what types of bats we have in the area. We can also learn from that how often they come through here and why they are here.”

Bats use the sounds to communicate with each other as well as find their way. That way-finding system is called echo-location: bouncing sound waves off obstacles and prey. To actually hear the sound, Brian Springall, a second-year masters student in Biology, plays back the sound of a Big Brown Bat at 1/8 the regular speed. It sounds like a high-pitched, short squeak.

On the computer screen, the sound is represented by a short wave, then a space, and then another wave, and then another space. The calls get smaller and closer together as the recording continues.

“The regular echo-location calls are when the bats are navigating and searching for food items,” explains Springall, pointing at the calls marked on the screen. “The bat produces a call and then it waits until it hears the previous call before sending out another. That’s why there is a call and then a space. As it find it’s prey, and then as it gets closer to an insect, the calls are going to be made and returned at a much more rapid pace until the prey is caught.”

The research team is collecting bat calls from almost 200 sites across North Carolina with the help of citizen scientists. It’s part of the Carolinas regional acoustic bat survey. Microphones are placed across the landscape of the state; in urban, suburban, ex-urban, agricultural and even pristine areas. The recordings are then checked to see which species are active and when and how they are being active.

“This allows us to understand if bats are doing well and their range is expanding or if they are not doing well and their range is shrinking,” said Han Li, a post-doctoral Biology student. “What we have found so far is that in some areas bats are doing well and going into new areas over years. In some places unfortunately, the range is shrinking and the bats in the area are becoming very specialized.”

The team has also found that the bats that are on the coast, in bottomland hardwood swamps, are different than the species that are in the mountains. However in the Piedmont, there is a mix of those species. However in all of those areas in North Carolina, all of the bat species are eating insects

“That’s what bats do for us, in terms of service,” explains Kalcounis-Ruppell. “They control insects. Just image what it would be like for people and crops if the bats weren’t around.”