A City Frog’s Love Song Attracts More Mates Than Countryside Croaks

Fewer predators and heightened competition for mates allowed urban tungara frogs to add more notes and frills to their calls, with big results Read more: h

Brian Gratwicke via Wikicommons under CC BY 2.0

City-living frogs in Central and South America sing a different tune than their croaking countryside counterparts. Their new-and-improved sweet serenades even attract more mates, according to a new study published in the journal Nature Ecology and Evolution.

The ways in which species adapt to urbanized landscapes intrigue researchers studying all kinds of organisms. “Just as we change our social relationships in cities, animals are changing their relationships and their behavior in the radically-altered biological communities we are creating across the globe,” says study co-author Rachel Page, an ecologist at STRI.

The new amphibious discovery was led by ecologist Wouter Halfwerk of Vrije University Amsterdam, who studies túngara frogs, or Engystomops pustulosus, in Gamboa, Panama, an urban area full of the inch-long, super-loud amphibians. After the Colombian government and the country's largest rebel group, FARC, negotiated a ceasefire in 2016, Halfwerk and his team jumped at the chance to search for frogs in an area that had been off-limits to outsiders for years because of the long-standing conflict.

When the researchers made it to the dense forest, however, it was much harder to locate the amphibian crooners than it was in Gamboa where they could easily pluck them up off the street. In the forest, the frogs were shy, didn’t call as much, and when they did, their song was not the same as in the city. So, Halfwerk and his team decided to investigate the differences between the country frogs and city frogs.

The team listened to frogs in 11 areas near urbanized sections of the Panama Canal and 11 rural regions in the same zone during the rainy season when the male túngara frogs are noisiest, producing a whiny call ending with several “chuck” sounds that can be as loud a telephone ring.

Urban frogs, they found, actually had a larger catalogue of melodies that are comprised of more complex vocalizations, reports Nicola Davis at The Guardian. To test whether mates preferred one refrain to the other, the team played both the city and country songs for 20 urban female frogs and 20 country female frogs at the Smithsonian Tropical Research Institute (STRI). Thirty out of 40 female frogs hopped toward the speaker playing the smooth city frog soundtrack over the country frog anthem.

Halfwerk, a visiting scientist at STRI, says two factors contribute to the city frog songs’ unique popularity. First, male frogs need to work harder to find mates in an urban environment because there is more competition, so they make their calls more frequently. Second, in the city there are fewer predators, like bats, which listen for singing amphibians in order to locate their next meal.

“There is no constraint, they can go wild,” Halfwerk tells Davis.

Because of that, study co-author Michael J. Ryan of the University of Texas tells Christina Larson at Associated Press says this allows the male frogs to sing more of the notes that attract female frogs, including particularly high notes and low bellows that stimulate their inner and outer ears in an appealing way.

The country frogs, on the other hand, can’t risk the extended cut of their love song. By playing the urban song in rural environments, the team was also able to show that the extra notes just wouldn’t work away from city lights. “In the forest, these more attractive calls have a higher cost,” Ryan says in a statement. “The sound can attract frog-eating bats and bloodsucking midges.”

Luckily for any city frog that might find itself moving to the country, the team also found that the urban troubadours were able to dial things back when necessary. The country frogs, however, were unable to up their game when abruptly relocated to an urban landscape.

Yong reports that the study is actually a positive for the frogs. As urbanization, light pollution and noise pollution increasingly encroach on natural areas, some animals are adapting. But it wasn’t clear if reptiles and amphibians have the same flexibility.

“Birds are singing through the night because of light pollution, and raccoons and squirrels are becoming cleverer in overcoming barriers to access food,” Danielle Lee, an urban ecologist from Southern Illinois University Edwardsville tells Yong. The fact that this species of frog can adapt “is good news.”

Yong also reports that another study conducted in North America showed that wood frogs raised in ponds near traffic noise were able to thrive while their cousins from quiet ponds had problems adjusting to noise. But Andrew Blaustein, an ecologist from Oregon State University, tells AP’s Larson that this doesn’t mean frogs and salamanders in general are adapting to all the changes we’re throwing at them.

“Amphibian populations are declining worldwide, mostly due to habitat destruction,” he says. “This is a rare case — and a very interesting case — of an animal adapting quickly, in evolutionary terms, to new circumstances.”

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