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Frogs increase pitch of calls to overcome Melbourne’s traffic noise

Melbourne’s worsening traffic isn’t just posing a problem for motorists — even frogs are being drowned out by the noise.

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Frogs are cranking up their croaks to overcome Melbourne’s noisy traffic.

The amphibians not only increase the pitch of their calls but make them more frequent.

They’re vital if male frogs are to find a mate.

Researchers from Monash University placed acoustic ­recorders alongside a busy road at Notting Hill in the city’s southeast to monitor the calls of brown tree frogs.

Lead researcher Vanessa Higham said rather than increasing the volume of calls, frogs raised the pitch so they could be heard over the noise of cars and trucks, typically lower on the sound register.

The behaviour was observed as far as 300m from the roadside. “When brown tree frogs were closer to the source of the vehicle noise the pitch of their calls was greater,’’ Ms Higham said.

“And the closer they were to the road, the more frequently they were calling.

“The increase in call pitch appears to be an immediate response to vehicle noise events.

“Rather than trying to be louder, the brown tree frog’s call is higher pitched. It could be thought of as a screech.”

The brown tree frog increases its pitch to be heard over Melbourne’s noisy traffic, a study has found.The brown tree frog increases its pitch to be heard over Melbourne’s noisy traffic, a study has found.

Acoustic recorders were placed at five locations of increasing distance from Blackburn Road at Jock Marshall Reserve — a popular home for frogs — as part of the study.

Ms Higham said opting to increase the pitch of a call rather than the volume was typically more energy efficient. But it was a calculated risk because female frogs were usually ­attracted to mates who had a deeper call.

“Females preferentially select males with a lower pitch call because it generally signals a larger body size which is ­associated with higher fitness,’’ Ms Higham said.

“It’s a trade-off between wanting to be heard by their potential female partner at lower energetic cost versus increasing the volume.

Frogs recorded in the study changed their calls in response to traffic noise. Picture: David CroslingFrogs recorded in the study changed their calls in response to traffic noise. Picture: David Crosling

“But utilising higher-pitched calls has implications for their potential reproductive success because they may be perceived as less attractive by female mates.

“But they will be heard. So it’s definitely a bit of a gamble.

“Given they rely heavily on acoustic communication for social and reproductive behaviours, it highlights the potential implications that noise pollution might have on the ecology of these frogs in this environment.

“Within the urban areas of Melbourne, a lot of the remnant habitat patches that they remain in are quite close to sources of noise pollution.”

The study was published in the Journal of Zoology.