The male Tungara frog wants to be sexy for his potential lady frog partners. Who doesn’t? But lady frogs have very specific tastes, they like their males to make two sounds, a whine and a chuck.
This is what they sound like. The first part is the whine, followed by three short chucks.
Lady frogs like males who do both. But chucking can be dangerous. Just like lady frogs, bats are highly tuned to the sound. The more the male frog chucks, the more attractive he is to the ladies, and the easier he is to find for the bats.
The preference for chucking isn’t trivial. Males that chuck are five times more attractive to females than males who just whine. The whine-chuck combination is called the complex call – where the whine alone is the simple call. “In a nutshell the complex call is much ‘sexier’ than the simple call,” Ryan Taylor, a biologist from Salisbury University wrote in an email. So male frogs have a choice: risk chucking and increase their attractiveness, or play it safe and risk going solo.
Males seem to know the risks of chucking. If there are only a few males in the pond they don’t chuck. There are enough females to go around, so they don’t have to out-sexy each other. It’s only when males have to compete for females that they start to be daring. When they do, males can add up to seven chucks, but they rarely go for more than three, just enough to lure the lady, but hopefully not enough to tip off the bat.
This is a classic example of how natural selection can limit sexual selection. Without the bats, Tungara frogs would probably chuck all the time. But the bats impose an extra constraint, making chucking more dangerous. Lady frogs – like many lady people – are attracted to that risk taking; they like a frog who chucks in the face of danger.
So if the chuck is so important, why bother with the whine at all? The answer is really cool. Females don’t recognize males unless they whine. If you play a female frog a million chucks she simply won’t respond. The whine tells her, “Hey lady frog, I’m a man frog of your species, let’s mate.” Then the chuck tells her just how attractive he really is.
Bonus fact: the Tungara frog’s Spanish name is “sapito de pustulas” which literally means “pustulated toadlet.”
Sound credit: Michael Ryan – These are computer-simulated recordings, used by researchers in the lab to test females responses. The female frogs cannot tell the difference between these computer calls and the calls of real male frogs.