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Researchers Probe the Language of Bats

  • Charles Ray

New technology is boosting the global effort to build a library of bat calls and decipher the language of these night flying mammals. It will help researchers protect an important -- and misunderstood -- creature.

On this warm spring evening in the Black Hills of South Dakota, the coyotes howl at the full moon as it rises through the pines. Other nocturnal creatures start to stir.

A group of research scientists waits outside the mouth of a limestone sinkhole called Earskin Cave. It's a popular daytime roosting site for a few species of bats. As the sky grows darker, they fly out of the cave's large opening, straight into huge nets the researchers have strung up around the entrance.

Nestled between his thumb and forefinger, biologist Joel Tigner holds a small tan-colored bat that he's just untangled from the net, and she isn't very happy. This little bat, known as the Northern Myotis, has a rough resemblance to a tiny bulldog with wings. Under her snub nose, she reveals a row of sharp teeth made for snatching up bugs.

Dan Taylor, with Bat Conservation International, is helping retrieve bats from the net. He says they're really intelligent animals. "When people have them for rehabilitation or caring for them, they learn their names like a dog, they'll respond, they can be trained -- not to go get the paper or anything -- but they're actually very smart animals."

Besides being smart, they also have a huge appetite. Studies show that a small bat can eat up to 1000 mosquito-sized insects every hour, but relatively little else is known about these nocturnal creatures. According to Joe Szewczak, a leading bat researcher, that's because they are so hard to study. "They make these sounds we can't hear. They operate in the night where we can't see and they're up in the air where we don't go," he points out, "so any means that we have to improve our ways of keeping track of them has been a bonus to us."

Professor Szewczak has developed a new, improved way to keep track of bats: a computer program and sonar sensor, aptly named Sonobat. With a small hand-held detector and a laptop computer, high frequency bat echo-location sounds can be recorded and analyzed to help identify the individual species.

In one recording, a bat identified as an Eastern Pipistrelle flies by the sensors. The frequency of the call has been dropped to a level humans can hear, and Professor Szewzak says if you listen carefully you can hear the actual sonar echo, the part of the call the bat uses to effectively 'see' in the dark. But this recording has been slowed down 10 times. He re-plays it in real time for a reporter, who is amazed at the rapid-fire pace of the clicks. Professor Szewzak explains people can't actually hear the sounds as bats fly by, "but these clicks are a representation of [the call]. You can at least hear the cadence or how many times per second these sounds are coming out."

Researchers are working to record entire repertoires of various bat species. Joe Szewczak plays a recording of an Eastern Pipistrelle using its sonar to zero in on a flying insect. The bat calls more frequently as it nears its prey. "As it's approaching something, it needs to [speed] up its repetition rate to get more information back." The biologist compares it to something easier to visualize: "Picture trying to catch a ball by making a flashlight go on and off, and if the ball's coming toward you, you would have to flash it many more times to keep track of the trajectory of the ball."

Researchers are not only using this technology to listen to bats snag dinner -- they are also collecting calls from around the world, as bats search for mates and shelter, as well as food. This global database will allow scientists to quickly identify various species as they fly overhead. That will help researchers track bat populations without having to net them, and the creatures won't suffer the stress of being caught.

The Northern Myotis in Joel Tigner's hand is about to be released. Her sonar signal will be analyzed in flight, and added to that library of bat calls. With new tools like these sonar recording devices, researchers say they are deciphering the dialects of various species, giving them a better understanding of bat behavior, which in the long run will help protect these animals, so they can help keep insect populations in check.