Traffic noise could be ruining sex lives of frogs

A well-projected and energetic croak is the male frog's most important asset in the quest to attract mates to his pond, Melbourne University ecologist Kirsten Parris said.

But competition from traffic noise in Melbourne could be a reason why frog numbers have declined in Australia's second-largest city since her survey of more than 100 ponds began in 2000, she said.

"If there are a number of different males calling, the one that sounds the best often gets the girl," Parris told The Associated Press. "You have to be pretty clear about your assets if you're a male frog."

"Generally, if he's putting a lot of energy into calling - if he's calling loudly or quickly or for a long time or all those things combined - it shows he's fit and strong and generally those things tend to correlate with female choice," she added.

Parris found the distance at which a frog suitor can be heard by a potential mate is slashed by city noise.

"This makes it much harder for frogs to attract mates and this could then mean that their breeding success is reduced," Parris said.

Frog species with low-pitched croaks are most disadvantaged because they are competing against the low-pitched rumble of traffic and machinery such as air conditioners, she said.

The southern brown tree frog has adapted by raising the pitch of its croak in areas where there is traffic din, she found.

In the noisiest parts of Melbourne, the frog's usual pitch cannot be heard by other frogs beyond 21 yards (19 meters). At the higher pitch, the croaks carry an additional 16 feet (5 meters).

The popplebonk frog's call can be heard by females from 875 yards (800 meters) without background noise. That range shrinks to only 46 feet (14 meters) near busy roads.

Parris presented her research on Thursday to the 10th International Ecology Congress in the eastern city of Brisbane.

Ken Thompson, a University of Sheffield ecologist who edits the British journal, Functional Ecology, described Parris' findings of reduced mating because of traffic noise as "highly plausible."

"There is accumulating evidence that noise in urban habitats is having an effect on the behavior of animals," Thompson said.

He said his own university's research found British birds were singing at night because their habitats had become too noisy during the day.

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