In the cartoon Batman, police commissioner Gordon sends for the caped crusader by shining a bat symbol into the dark night sky. In real life, a South American frog can unknowingly call real bats at night by simply making ripples in the water.
While sitting in a shallow pond, the male túngara frog makes the small waves as he tries to lure female frogs to mate with him.
As he calls, the frog inflates and deflates a balloon-like vocal sac that creates the ripples in the water.
Bats are essentially blind, but they have the ability to find their prey by sending out sounds and listening for how long it takes the sound to bounce back – similar to sonar that was developed for ships to detect submarines underwater. That sonar, called echolocation, also helps the fringe-lipped bat detect the ripples that the túngara frogs make, so they can swoop down and capture the frogs for a meal.
A túngara frog will stop calling if it sees a bat, but ripples will continue moving for several seconds after the frog’s call stops. Think of the ripples as a bull’s-eye that the bat can zero in on. Frogs that call from water littered with leaves are more likely to avoid bats because the leaves stop the ripples.
The ripples also seem to encourage rival male frogs to challenge for mating rights. Without ripples, the rivals responded less enthusiastically. And when the ripples were close to a rival frog, the rival tended to call less or stop, maybe to prepare for a fight or to get away.
– Brett French, Gazette Outdoors editor