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Illusionist Frog Attracts Mates Without Unwanted Attention From Predators

The simultaneous mating calls of the male pug-nosed tree frog confuses bats but not female frogs

Male pug-nosed tree frogs confuse predators by overlapping their mating calls with those their neighbors. (Purdue University / Henry Legett)
smithsonianmag.com

Male tungara frogs of Central and South America call out to potential mates with reckless abandon. During the rainy season, they wait for pockets of relative silence amid the cacophony of the rainforest and belt out a song that could attract females’ attention or get them eaten by an eavesdropping bat. Even worse, their most seductive calls are also more likely to turn them into someone’s dinner.

It might seem like a rough trade off, but trying to stand out from the acoustic lineup is typical among frogs, explains Ximena Bernal, an ecologist at Purdue University and researcher at the Smithsonian Tropical Research Institute in Panama.

In the rainforest’s dry season, another frog species has a more confusing way of flirting. When it’s time for male pug-nosed tree frogs to turn on the charm, they all call out at the same time.

“Synchronizing calls is like talking over other people which, as we all know, reduces our ability to understand what the person is saying,” says Bernal via email. Calling out at the same time seemed like a confusing strategy for pug-nosed frogs to get dates, but the tungara’s sometimes fatal bids for attention gave Bernal and her colleagues a clue.

After studying the pug-nosed frogs in the rainforests of Panama and in the lab, the researchers have found that the near-perfect synchrony of the frogs’ mating calls confuses their would-be predators—all while remaining plenty alluring to females, reports Pratik Pawa for Science News.

When one pug-nosed tree frog (Smilisca sila) trumpets his love song, other nearby males start their calls almost instantly. With all the frogs calling out at once, bats and most other vertebrates think the sound is all coming from the frog that started the chorus.

“Humans experience this illusion too, it’s called the ‘Precedence Effect’. When we hear two short sounds in quick succession, we think the sound is only coming from the location of the first sound,” says Bernal, who is also affiliated with Purdue University in Indiana, in a statement.

This auditory illusion obscures the locations of all the frogs who joined in late and protects them from predators, the researchers report in the journal American Naturalist.

This places the poor saps leading the call at a big disadvantage, which drives each frog to hold its note as long as possible—resulting in gulfs of silence between the bouts of song, Bernal tells Science News.

But what do the female frogs think? Surprisingly, the team’s experiments suggest females don’t show any preference for the bold males who initiated the calls. What remains a mystery is how the females avoid falling prey to their species’ own illusory tactics and remain capable of choosing their mate.

This phenomenon is something Bernal hopes to explore in future research. “Is there something specific about their hearing mechanisms that allows them to detect and accurately locate two signals even though they are produced milliseconds apart?” she wonders.

Synchronous calls aren’t this illusionist amphibian’s only tactics for evading predators. Males are known to prefer to sing near waterfalls. This placement isn’t just for ambiance; the sound of the rushing water overlaps with the frequency of the males’ calls and helps obscure them to hungry bats.

Prior research has also shown they vary their calls in accordance with the moon. Males are more vocal on nights when moonlight is brighter and they can more easily spot marauding bats, and quieter when it’s darker.

Bernal speculates that the pug-nosed frog’s choice of mating season may account for its multiple strategies for avoiding predators: “This is the main species calling in the dry season so it may be that it is under strong selection from many frog-eating beasts.”

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