Striking Picture: Bats Hunt Frogs by Detecting Water Ripples

Male túngara frogs (Physalemus pustulosus) are the romantic type.

Instead of roaming around looking for mates, these tiny Central and South American rain forest amphibians sit in puddles singing love songs, attracting females that flock to the aquatic crooner’s home.

A picture of the frog-eating bat with its prey
A frog-eating bat with its amphibian prey. Photograph courtesy Christian Ziegler

The frog’s calls have an unintended effect: They create ripples that draw the attention of predators lurking in the sky.

Scientists already know that the frog-eating bat Trachops cirrhosus eavesdrops on the túngara’s love song. The frog knows this too, falling silent whenever it senses a predator nearby. (Also see “Surprising Photo: Toad Eats Bat.”)

However, according to new research, frog-hunting bats can also detect ripples created by the frogs with their echolocation, or built-in sonar. The bats can do this even after a frog has stopped calling, since ripples last for several seconds afterward.

For the study, published January 24 in the journal Science, researchers caught wild frogs and bats from the Panamanian forest and tested them in the lab. (See pictures of other rain forest animals.)

The experiments showed that bats were much more likely to attack the frogs when they perceived ripples on the water. Because the phenomenon was observed in darkness, the team concluded that the bats use echolocation and not visual cues to zero in on their prey.

Ripple Effects

These ripples may not always spell disaster for the frogs: Sometimes they help. For instance, the size of a song-induced ripple aids rival male frogs in assessing their competition and deciding their next move, which could mean making their call louder or choosing to fight.

“It’s comparable to the use of lip reading,” Wouter Halfwerk, a postdoc from the University of Leiden in the Netherlands, said in a statement.

“While sound is the most obvious component of the frogs’ communication, the call-induced ripples alter the behavior of competing males that sense them.”

And frogs may be already figuring out a way to one-up their bat predators: keeping a messy house.

Researchers found that when a frog’s puddle was littered with debris or vegetation, like leaves, it made them much more difficult for frog-eating bats to detect. (Also see “‘Whispering’ Bat Evolved to Trick Prey.”)

That’s because the debris acts as “echo-acoustic clutter” and breaks up the ripples that the túngara frog’s song would otherwise generate.

This interferes with their bat predator’s echolocation abilities, and offers the frogs some level of protection from being spotted.

Even so, to quote Kermit the Frog: “It’s not easy being green.”

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Meet the Author
Stefan Sirucek is a writer and journalist who reports from both sides of the Atlantic. He's written for the Huffington Post and Wall Street Journal. Follow him on Twitter at @sirstefan.