WASHINGTON – City frogs and raven frogs are not singing the same melody, researchers found.
A study, published Monday, examines why small crowds in Panama's dark frog are adapting their mating in urban areas – an unexpected example of how animals change their communication strategies when cities fall into the forests.
These frogs take advantage of the relative absence of eavesdropping predators in cities to pull out longer love songs that are more appealing to female frogs.
Tigers do not scream like American bullfighters. For human ears, their distinctive appeal sounds like a low-rhythm sound signal. Female frogs sound like pillows.
Every evening at sunset, the 1-inch male brown frogs crawl into serenade puddles. The female frog chooses a partner, largely based on his love song.
Researchers find that urban frogs call faster, more often and give more embellishments – a series of static "chucks" at the end of the initial whining – compared to those in the woods.
These fantastic urban love songs are three times more likely to attract ladies, as scientists have learned, by playing records of urban and forest frogs for an audience of female frogs in a lab. Thirty-forty female frogs jumped over to the speaker, playing urban frogs, calls researchers in the journal Nature Ecology & Evolution.
Whether female frogs emerged from the city or the forests themselves, they displayed the same preference for a fast, sophisticated kroning that combines high and low tones into quick arrangements.
Research co-author Michael J. Ryan, a Texas biologist who has been studying frogs for more than 30 years, said high and low notes are likely to stimulate female and female frog cameras in a pleasant or interesting way.
Why then do the tropical forest frogs not sing the same way?
Scientists are trying to confirm their hypothesis that frogs that add extra high "cartridges" attract not only more friends but also more problems with the bats eating the frog and the parasitic blades. Using camera traps and sticky paper, researchers have demonstrated that extended frogs greatly increase the risk of attracting predators.
In rain forests, frogs have to balance two goals: to attract a friend and stay safe.
There are no lures in the city to eat the frog, and far less snakes and blades. Male frogs are freer to turn their hearts off.
"The city man can take greater risks," said lead author Wutter Pollacker, an ecologist at the University of Wille in Amsterdam.
Urban frog should also find it harder to find a companion because ladies' frogs are less common in town. "Competition for women is increasing," said Halfwerk. "The best adaptation is to be the most appealing, with a complex love song."
Corinne Lee Zawaki, a biologist at the University of Pittsburgh, who did not participate in the study, said the researchers' methodology confirms that urbanization is the cause of call changes.
"I love the choice of a school system," she said. "Many studies have been done on this frog so we can clearly see how urbanization changes the interaction between natural and sexual selection" or the trade-off between survival and courting objectives.
But not all amphibians are as happy as the dark frog of Panama.
"Amphibian population declines globally, mainly due to the destruction of habitats," said Erews Blauestein, an ecologist at Oregon State University, who was not involved in the study. "This is a rare case – and a very interesting case – of an animal that adapts itself rapidly to new circumstances in an evolutionary way."
Follow Christina Larson on Twitter at @larsonchristina.
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