A robot, which resembles a real tungara frog, made this finding possible. The machine has a speaker that broadcasts a realistic mating call, and a shiny painted balloon that inflates and vibrates beneath its throat.The researchers used the robo-frog to study different components of communication between the frogs, found the vocal sac to be quite important for wooing females, even though they mate in the dark.According to them, males sing to attract females at night, and their throats inflate.
"The sacs evolved for males to shuttle air back and forth, so they don't have to suck in air each time they sing," New Scientist magazine quoted Michael Ryan of the University of Texas in Austin as saying.
During the study, the robo-frog was attached to an air pump that was driven by the wave on an oscilloscope, to make its fake vocal sac bulge in and out rhythmically.
The researchers had synthesised a male mating call, and fed it into the oscilloscope to make the fake frog's vocal sac to vibrate realistically.
Overall, 20 female frogs were given a choice between an inflating robo-frog producing mating calls and a speaker on its own.
The researchers observed that 16 of the 20 female frogs seemed more attracted to the combination of speaker and frog.
When the researchers switched off the robot's oscilloscope, the females did not find it any more attractive than the speaker.
Based on their observations, the researchers came to the conclusion that a vibrant vocal sac is crucial to finding a mate.
The researchers also tried vibrating the vocal sac out of sync with the songs, and observed that it actually deterred the females.
"We are particularly excited about the implications of this result," said Ryan Taylor, another expert to lead the study.
The researchers believe that their findings show something similar to the "McGurk effect", which shows that the shapes made by a person's lips are relevant to the sound they are perceived to produce.
A research article describing the study has been published in the journal Animal Behaviour.