For the female túngara frog, a low-pitched, fast-singing male is music to her ears.
But a new study shows that males with less desirable songs aren’t completely out of luck: They can trick a female into choosing them as mates by serenading her next to an even less attractive male.
Tiny, mud-colored natives of Central America, túngara frogs are well known for their outsized vocal abilities and ballooning vocal sacs: Their songs, meant to attract females, can sometimes unwittingly catch the attention of hungry bats.
The amphibians are onomatopoetically named for the sound they make, which starts out with a whining tung and ends with a croaking gara. (See "What Can Sexy Robot Frogs Teach Us About Evolution?")
“The calls, they sound like a video game ping. It’s the funniest sound,” says study co-author Amanda Lea, a Ph.D. student in integrative biology at the University of Texas, Austin.
“They’re tiny little frogs, but they’re really loud.”
The Dating Game
To see what a female looks for in a male's singing voice, Lea and team played previously recorded male túngara frog calls to 78 female frogs they captured in Gamboa, Panama.
The scientists placed the females in the center of a room, bracketed by two speakers, and watched which speaker each female hopped toward—an indication of which male’s song she preferred.
Lea found that females liked a fast pace and low pitch in a male’s mating call—since low-pitched calls come from more desirable, larger males—but they weighted the pace of the call more heavily. (Also see "Weird Purple Frog Seduces Females From Underground.")
“If a male calls really, really fast, females just go crazy. They love it,” says Lea, whose study was published August 27 in the journal Science.
In her second set of experiments, Lea made 120 female frogs decide between the songs of potential mates. When choosing between Contestant Number One—a tenor with a fast call—or Contestant Number Two—a baritone with a slow call—the female frog usually goes with Contestant Number One, the guy with the fast delivery but less attractive, higher voice.
But that all changes when Contestant Number Three is thrown into the mix. “This third male… also has an attractive voice—or, attractive call—but has a reallyyyy slow call rate,” Lea says. “The slowest of all three.”
When the female frog hears Contestant Number Three’s slow bass, she flips her weighting system, the experiments show: Now she values the lower pitch over the faster pace. (See “World’s Loudest Animals—Bug With ‘Singing’ Penis, More.”)
So, when she chooses between Contestants One, Two, and Three, she picks former loser Contestant Number Two—the male with the attractive baritone, but slower delivery.
"Not Just Little Robotic Creatures"
This phenomenon could be similar to the human strategy of hitting the bars with one’s less attractive friends to seem more appealing in comparison, speculates Rick Shine, a professor at the University of Sydney who studies reptiles and amphibians and was not involved in this research.
Lea said the female’s choice switch is “irrational." That's because a rational female frog would stay strong in her selection of Contestant Number One in the presence of a male that was less attractive than both Contestants Number One and Two. (Also see "New Frog Mates Doing Handstands, Does “Pottery.””
But, Lea says, just because the choice is irrational doesn’t necessarily mean that the female is making a mistake when she leaves with Contestant Number Two—it just means that we don’t yet understand what the females’ priorities are when she chooses her mate.
Joan Roughgarden, an emeritus evolutionary biologist at Stanford University, thinks that when the female is presented with Contestant Number Three, it's possible "she learned something indirectly from that information about the other two that led her to change her preference quite rationally," Roughgarden suggests.
In that way, “it might be rational in light of the new information that [Contestant Number Three] supplied,” she says.
Rational or irrational, experts say the results shed light on frogs' complicated sex lives.
“These are not just little robotic creatures that are wandering around having very simple interactions,” says Shine.
“They’re aware of the neighbors and they’re listening to what the neighbors have got to say—and that’s affecting the kinds of decisions that they’re making."