2020托福听力练习：嘈杂环境中青蛙发出视觉信号（下载）

　　Ask anyone what sound a frog makes and they'll probably say ribbit.

　　But there's only one frog that makes that sound:the Pacific chorus frog.

　　It may have become the go-to frog call because it just happens to live near sound artists in Hollywood.

　　Herpetologists have long known that frog calls are more diverse than just ribbit.

　　But their communications are more diverse than just calls—they also signal each other by sight.

　　Say you're a male of a species called the Brazilian torrent frog.

　　And you live alongside a fast-flowing stream in the Amazon jungle.

　　You happily chirp away, advertising your availability to whatever female torrent frogs are around.

　　But when another male comes too close, your chirps give way to peeps and squeals.

　　So you wave your red and brown arm in front of your cream-colored face.

　　The ensuing color contrast turns your body into a warning light.

　　The intruder gets the hint and hops away to bother someone else.

　　Brazilian zoologist Fabio P. de Sa witnessed Brazilian torrent frogs using eighteen different visual displays, five of which had never been seen before in other frogs.

　　He and colleagues published their findings in the journal PLOS ONE.

　　The frog semaphore system probably developed because streams are noisy places.

　　Instead of shouting over the sound of rushing water, these frogs communicate by jumping, head bobbing, arm waving or even wagging their toes.

　　The set of signals is the most sophisticated ever documented for a frog, but de Sa thinks that other frogs could have similarly complex repertoires, if only researchers would take the time to look.

　　One thing is certain: it's not easy being green, but it's simple to be seen.