Most male frogs want their mating call to stand out from the crowd, and they do that by calling when nobody else is. This makes sure that the females hear them loud and clear, and know where they are. That’s why it was baffling that pug-nosed tree frogs all call together.
“Why would all frogs call at the same time? That made no sense,” says Ximena Bernal, a behavioral ecologist at Purdue University in West Lafayette, Ind. She suspected it might have something to do with evading predators who eavesdrop on these mating calls.
Now, she and her colleagues have found that pug-nosed tree frogs calling out in near-synchrony creates an auditory illusion that tricks predators while still successfully wooing females. The research is reported in the May American Naturalist.
When male frogs call in the wild, their audience doesn’t consist of just females. Bats and bloodsucking midges, frogs’ natural predators, eavesdrop on the calls too (SN: 9/15/16). So sending out a single isolated call might be attractive to a female, but it also exposes a frog’s location to those predators.
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Pug-nosed tree frogs (Smilisca sila), however, have evolved a work-around. Calling out nearly simultaneously — the first, leading call is quickly followed by successive calls by other males — creates an auditory illusion that tricks predators into perceiving the sounds as coming from the same source. This illusion, called the precedence effect, is widespread, and most vertebrates, including humans, are susceptible to it. So synchronizing calls in groups helps all other frogs, except the first one, to go unnoticed from predators.
But there’s one glaring issue with this way of sending signals. The first male, which gives out the leading call, is at a disadvantage because he’s the one being heard by predators. So why would any frog call first? Bernal says pug-nosed tree frogs are in a situation that ecologists call “war of attrition,” where every frog holds out calling until some neighboring frog gives in. This is in line with what scientists see in the wild, Bernal says, “where these synchronous calls are followed by long episodes of silence.”
What’s even more surprising is how the females respond to these mixed calls. The study finds that the female pug-nosed tree frogs are somehow immune to this illusion. They aren’t attracted only to the first leading call, like most frog species would be. Instead, females can distinguish between the overlapping calls and show no preference toward either the leader or followers.
To understand how both predators and females respond to this unique way of calling, the researchers conducted field and lab experiments. In both settings, the team used two speakers, one playing the leading call and another following with a call 79 milliseconds later, emulating the overlapping chorus of pug-nosed tree frogs in the wild. (Such small choruses of two males do occur in the wild, though often there are more callers.)
Using these simple tools, the researchers observed the attraction of predators toward either speaker. Both bats and midges frequented the speaker sending out the leading call much more than the speaker broadcasting the following call. The female frogs, however, choose either speaker roughly equally.
“Their experiment clearly shows that [the female frog] is somehow solving that problem,” says Viraj Torsekar, an ecologist at the Hebrew University of Jerusalem, who was not involved in the study. But how they overcome this illusion is still a mystery.
“You would think that vertebrate brains were all set up to be susceptible to this illusion, so why not these frogs? Who knows?” Bernal says.
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One way to go about understanding how this way of calling evolved would be to look at how closely related species are calling. If related species all inherited the same trick, that “might throw some light on how this might have come about,” Torsekar says.
But, Bernal notes, that will require more research on frog choruses. One closely related tree frog sometimes synchronizes, and sometimes doesn’t, and it’s not yet clear whether females of that species are immune to the illusion.
Chorusing is a common phenomenon in the animal world, crossing sensory boundaries. “Cicadas with their calls, fireflies with their lights, even wolves often howl in sync,” Bernal says. There have been several hypotheses explaining such synchronization in different species, but the precedence effect has not yet been widely tested.