VOA标准英语2009年-Australian Frogs Change Their Tune to Find（在线收听）

By Stuart Cohen
Sydney
08 October 2009

 
Southern Brown Tree Frog sits on a log in swampy lands near Melbourne, Australia (Sept 2009 file photo)
Australia's frogs are having trouble finding love. Traffic noise and other sounds of city life, such as air conditioners and construction noise, are drowning out the mating calls of male frogs in urban areas, leading to a sharp drop in frog populations. But, in the first study of its kind, a scientist at the University of Melbourne has found that some frogs have figured out a way to compensate for human interference in their love lives.

A male southern brown tree frog sends out a mating call when he's looking for a date. It is music to the ears of a female southern brown tree frog.

But, add the sounds of nearby traffic and Dr. Kirsten Parris of the University of Melbourne says the message just is not getting out.

"The distance over which the male frog can be heard, is cut really dramatically by traffic noise from hundreds of meters, in some instances, down to maybe only 20 or 50 meters," Parris said. "We're quite concerned that … there are frogs out there that aren't getting together because the noise we're making is getting in the way."

Parris spent seven years studying frogs around Melbourne. She says some frogs have come up with an interesting strategy for making themselves heard.

"We found that it's changing the pitch of its call, so going higher up, up the frequency spectrum, being higher and squeakier, further away from the traffic noise and this increases the distance over which it can be for heard," Parris said.

The old call is lower in pitch.

The new one is higher in pitch.

Now, that may sound like a pretty simple solution. But, changing their calls to cope with a noisy environment is actually quite extraordinary for frogs. And while the males have figured out how to make themselves heard above the noise, Parris says it may not be what the females are looking for.

"When females have a choice between two males calling, they tend to select the one that calls at a lower frequency because, in frogs, the frequency of a call is related to body size. So, the bigger frogs tend to call lower," she explained. "And so they also tend to be the older frogs, the guys perhaps with more experience, they know what they're doing and the women are attracted to those."

In other words, the guys with high voices typically do not get the girls.

"That's right," Parris said. "And if it's very noisy … they can only hear a few of the males that are all calling in a group, so...the one's they can choose from is actually, the number is, is reduced. It's like if you're in a noisy cocktail bar, for example, and there are men everywhere, you can only see and hear the three that are closest to you."

Frog populations in Melbourne have dropped considerably since Parris began her research, but it is not just because of noise. Much of Australia has been locked in a 10-year drought, leaving frogs fewer and fewer ponds to go looking for that special someone.