“Typically, what happens with these treefrogs is if you go to a pond a lot of males are calling all at once and on any given evening, a relatively small percentage of the female population have eggs and are ready to breed,” explains Dyson Professor of Biology Joshua Schwartz, PhD. “So the females listen to the males calling and then they make a mate-choice decision. They pick a partner for the night based on what they hear. If one male’s call sounds better than another’s, then she moves in his direction.”
The female gray treefrogs are collected from a nearby field-site by Schwartz and his student researchers. Once collected, the female treefrogs are brought to the basement of Wilcox Hall, where Schwartz has set up a lab that is ideal for his female-choice experiments. An individual female treefrog is put under a screen cup inside a large enclosed chamber lined with acoustic foam which reduces echoes of the sounds of the synthetic male treefrog calls which are played on speakers that have been strategically set up in the room.
“We can manipulate individual features of the synthetic call and how the call is delivered,” says Schwartz. “After the female has had a listen, we raise the screen cup using a pulley and she will hop in the direction of the call she likes best and we record that as a mate-choice decision.”
The experiments are performed in pitch blackness and Schwartz and his students monitor the females’ movements using closed circuit television and infrared illumination, otherwise known as night-vision.
“By manipulating the synthetic call, we are trying to tease apart what females like in a mate. We can vary the acoustic background, and we can set up more complicated situations by using four, six, or eight speakers all playing a variation on a call,” he says.
But Schwartz’s research isn’t a females-only endeavor. On the Pleasantville Campus, Schwartz has set up a greenhouse dome that contains a habitat that resembles the environment gray treefrogs would inhabit in the wild. The enclosure contains a population of male gray treefrogs that Schwartz monitors in the evening. Using directional microphones, he records each frog’s individual vocalizations. Those are then fed into a computer interface that he built that allows him to analyze vocal interactions among males. Sometimes, a female is brought into the greenhouse so that her mate-choice can be observed in a more realistic environment than the testing chamber in Willcox.
“The dome is a venue for setting up an artificial chorus with real frogs; whereas with the chamber, we are using artificial sounds created with a computer,” Schwartz says. “We are trying to get an understanding of the communication system of these animals and try to understand how they can successfully communicate in extremely noisy conditions.”
For those who have never experienced a frog chorus firsthand, imagine the deafening din created by upwards of a hundred male treefrogs all vying for a bit of female attention. Despite the incredible levels of noise, females have to identify males of their own species, decide which male of their own species they want to mate with, and then finally locate him.
“In many ways, the challenges for communication in a treefrog chorus are similar to those faced by human beings trying to carry on a conversation at a crowded cocktail party,” concludes Schwartz.