Male frogs have tremendously diverse vocalizations, and their sounds can mean different things depending on the listener. Female frogs might assess the quality of a male through the nuances of his call. Male frogs might decide whether or not to fight based on a the sound of another male's call. Predators might think the song signals a tasty treat. People might interpret the songs as a sign of the change of seasons.
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The playlist to the left shows four short videos of American Toad breeding behavior. The first video shows two toads calling to each other. The second video shows a toad calling next to a spring peeper. The third video shows some male toads trying to break apart two amplexed pairs of toads. The fourth video shows a group of four males battling each other to mate with a single female toad.
This video shows a complication of observations from Michigan in 2010. It starts with a view of a pond at sunset; peepers can be heard around the pond. Then a view of a spring peeper calling is shown. At 24 seconds into the video, a head-on shot of a spring peeper is shown. At 35 seconds, there is a view of a a male and female spring peeper in amplexus. The video ends with another view of a calling male spring peeper.
Many hypotheses have been proposed for why frogs have evolved large vocal sacs. One hypothesis is that the pulsating vocal sac serves as a visual signal to attract females. The head-on video of the calling male peeper at 24 seconds seems suggestive of a visual signal.
The video to the left shows a top-down view of a calling male pacific chorus frog. This video provides a nice visual illustration of another function of the vocal sac: to refill the lungs after calling. As you watch the video, you can see the relationship between the vocal sac and the lungs. Sound is produced as the frog forces air out of its lungs into the vocal sac. The air rushing out of the lungs passes through the vocal cords, creating the frog call. As the lungs deflate, the body appears to get smaller as the vocal sac expands. But after the frog is done calling, the air is forced from the vocal sac back into the lungs. You can see this happen as the vocal sac gets smaller and the frog's body appears to get larger as it fills with air.