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Froggy Went A-Courtin', But Lady Frogs Chose Second-Best Guy Instead

Picking a mate can be one of life's most important decisions. But sometimes people make a choice that seems to make no sense at all. And humans aren't the only ones — scientists have now seen apparently irrational romantic decisions in frogs.

Little tungara frogs live in Central America, and they're found everywhere from forests to ditches to parking lot puddles. These frogs are only about 2 centimeters long, but they are loud. The males make calls to woo the females.

Amanda Lea, a biologist in the laboratory of Mike Ryan at the University of Texas, Austin, says past studies have given scientists a pretty good idea of what the females find appealing.

"They tend to like longer calls. They also like lower-frequency calls," says Lea. "Then, the other thing that's a really big one for these gals is the 'call rate.' They love faster call rates. The faster a male can call, the better."

But in real life, love is complicated. Female frogs face countless suitors. So Lea and Ryan wondered: Would a female really always pick the male that scored highest on the froggy love-call meter?

To find out, they put female frogs in a room with some loudspeakers. From one speaker the scientists played a recording of frog call that had a really fast rate. But other features in this voice were less attractive.

Then the researchers played a second, different call for the female frogs. This voice was more attractive, but it was slower. The ladies had to make a choice.

"They have two traits to evaluate," Lea explains. "They have the call rate and they have the attractiveness of the call."

The female frogs consistently picked speed. They seemed to like the fast guy best — they crawled toward that speaker.

They researchers then repeated the experiment, but this time added a third option: a frog with a voice that was as attractive as the previously rejected option. But it was slower -- very, very, very slow.

Though the females didn't pick this loser, his mere presence had a profound effect. When he was around, the superfast guy did not win. Instead, females picked the frog they had rejected the first time around.

"They actually switch their preferences," says Lea. "So now call rate is no longer the most important thing."

They reported the finding Thursday in the journal Science. And it seems to make no sense. It's the latest example of something called the "decoy effect" — that's when adding a third, inferior option inexplicably results in the rejection of the best choice.

Scientists see the decoy effect in all sorts of decision-making — among humans and other animals, too, says David Stephens, a behavioral ecologist at the University of Minnesota.

He says it happens "in hummingbirds and bees. And the most stunning example is actually not an animal at all, but in a slime mold." (In the studies of slime mold, the organism was trying to pick the best place to grow and eat.)

What's interesting about this frog study, says Stephens, is that it found the decoy effect in something as crucial as picking a mate. "Mate choice is, for many people, the gold standard decision," says Stephens.

Lea says she really wants to understand what is going on with these frogs — are they really making an epic mistake? Maybe they're just being smart about love in ways we don't understand.

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Nell Greenfieldboyce
Nell Greenfieldboyce is a NPR science correspondent.