If you’ve ever spent a warm summer night on a pond, you’ve undoubtedly heard a chorus of frogs calling into the distance. These calls come from a variety of male frogs, each searching for a potential mate, and most of them reliant on the strength of their call to find one. But understanding how these calls work and what makes them attractive can be difficult in that environment.
"I don't know if you've ever been at a pond at night when the chorus is really, really rolling. Trying to do any type of experiment there would be really hard because your speaker would get drowned out by the natural sounds of the frogs," says Dr. Gerlinde Höbel, an associate professor at UW-Milwaukee and the head of the Höbel Lab of anuran behavioral ecology.
She wanted to create a space where she could study the Eastern Gray tree frog without the distraction of the pond. So she and her students created the ideal environment by converting a commercial freezer into a sound proof room with a small frog arena in the center. There, they can play male mating calls through speakers set up around the arena and observe which ones female frogs seem most attracted to (no male frogs are present for the experiment).
The setup is simple: a small metal pen at the center of the space, surrounded by speakers. The fencing is covered by black fabric, so the frogs are unable to see where the sound is coming from. The frog is lowered into the pen and allowed to listen to different types of male calls with different frequencies.
Dr. Höbel explains, "In here we can control everything and still get the female to do exactly the natural behavior. And if she doesn't want to she just sits there and doesn't move. So we can't force her to do anything. If she plays with us, we get data. If she doesn't play along, we get the next one."
She and her students are studying the different attributes that make these calls attractive to potential mates. So far, they've concluded that there isn't just one type of call that attracts an Eastern Gray tree frog, but instead two main preferences: very long calls or calls that are repeated with more frequency.
"If you kind of add those together it basically means a male that produces a lot of sound energy is very attractive to the female," says Dr. Höbel.
