The frog’s love language not only attracts it’s mate, but also predators.
The tiny brown tungara frog, found in Central and South America, seeks out females by perching in a shallow pond and making a mating call. A large vocal sac under his mouth inflates and deflates as he calls, causing ripples in the water.
“They produce two kinds of calls—a simple one, and a complex one,” says Rachel Page, a researcher at the Smithsonian Tropical Research Institute in Panama who’s been studying the species for several years. “The basic one is this whining sound, and then they make it complex by adding these ‘chuk’ noises. A whine might be necessary and sufficient to bring in a female, but chuks make the call more attractive.”
First researchers from the United States, the Netherlands and Panama discovered these ripples from the mating calls tended to arouse more competitive calls from nearby male frogs than calls that were sent out without making waves.
“With sound, they responded a little bit by calling back, but as soon as you add ripples they go crazy,” study leader Wouter Halfwerk, a postdoctoral researcher at the University of Texas, Austin told LiveScience. “They double their call rates, they start moving around, looking for the potential intruder.”
The frogs compete with other males that share their puddles, Halfwerk said, so the ripples may be a warning that rivals are close. When the researchers put multiple frogs in one puddle at different distances, they found that frogs close to one another respond to ripples and calls by either becoming aggressive, or shrinking into themselves, trying to avoid a fight.
Next, the researchers tested this theory with bats to see if they would also use the ripples. They set up two trays of water, each with a fake frog, sitting on a speaker. From one tray, they played the sound of a male mating call. From the other, they played the same call but added ripples to the water.
They found that the bats dove at the frogs next to rippling pools 36.5 percent more often than than the still ones. They also discovered that bats were using their own biological sonar abilities to find frogs in the dark via the pond ripples. Although the frogs have learned to stop calling if they glimpse a bat overhead, the tactic usually comes too late.
“The interesting thing is that these frogs have evolved a strategy to escape predation,” lead author Wouter Halfwerk, a postdoctoral researcher at UT Austin and also affiliated with STRI and Leiden University. “When a frog detects the shadow of a bat overhead, his first defense is to stop calling immediately. Unfortunately for the frog, the water ripples created by his call do not also stop immediately. The ripples continue to emanate out for several seconds, creating a watery bull’s-eye on the frog. Bats use the ripples, thereby beating the anti-predator strategy.”
The study appears in the US journal Science, and included researchers from UT Austin, the Smithsonian Tropical Research Institute in Panama, Leiden University in The Netherlands and Salisbury University in Maryland.
A Smithsonian scientist in Panama studies frog-eating bats.
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