在中國(guó)發(fā)現(xiàn)了能發(fā)出“無(wú)聲”鳴叫的青蛙

Noisy waterfalls and claps of thunder can drown out even the most vocal frog.But some persistent croakers in China have a clever fix:They switch to ultrasound.

The feat,researchers say,makes the frogs the first amphibians to be placed alongside an exclusive group of mammals,such as whales and dolphins,that have ultrasonic ability.

“It shows a new example of independent evolutionary adaptation in the frogs for life in habitats filled with loud background noise,”said Albert Feng,a professor of neuroscience at the University of Illinois,Urbana-Champaign.

Researchers first heard of the unusual animals in 2000 from Kraig Adler,a biologist at Cornell University.

He tipped the scientists off to Huangshan Hot Springs,a popular tourist destination near Shanghai,where he had found frogs with recessed ears.

“I noticed the frogs sunken ears and thought they must have an odd system of communication.We had no idea they used ultrasonic sounds,”Adler said.

That year a research team led by Feng reported that these frogs,called concave-eared torrent frogs (Amolops tormotus),sang like birds.

Two years later, using wide-band recording equipment, Feng and his colleagues discovered that the frogs were also croaking in ultrasound-sound vibrations beyond the limit of human hearing.

The teams findings appear in the current issue of the journal Nature.

Ultrasonic Hearing

To test the frogs hearing prowess,Feng went to the site in Chinalast summer with devices that could play back sound in both audible and ultrasonic ranges.

The researchers played recorded sounds to eight male frogs.

When the recordings were in the audible and ultrasonic ranges,five of the frogs gave significantly more replies than when no sound was played at all.

In response to one particular ultrasonic playback,a frog shot back with 18 rapid-fire calls.

“Males not only responded to ultrasound but also approached our loudspeaker as if to confront the frog making that sound,” Feng said.

In a second test,researchers observed the brain activity of one frog during playback.The frog showed a strong response to ultrasound but no response when both its ears were blocked with modeling clay.

“This confirmed that the hearing was indeed taking place through the ear and not through sound that penetrated the skull to directly stimulate the neurons,”Feng said.

Energy-Saving Croak

Researchers theorize that there could be an evolutionary explanation for the frogs ultrasonic hearing.

Males trying to catch a females attention need to have their calls heard over the din from other suitors,as well as over background noise,Feng noted.

Shifting the call to a much higher frequency not only ensures it is heard,he said,but it also requires less energy than generating very loud audible croaks.

The secret to the frogs ultrasonic hearing appears to be in their unique ear structure.Their eardrums are superthin and sunken below the body surface to protect the membranes.

But the exact part of the ear that facilitates ultrasonic hearing is not known,Feng said.

“This is an interesting study for several reasons,”said Michael Ryan,a zoologist at the University of Texas at Austin.

“It shows the potential for some frogs to hear in a frequency range never suspected and an unusual way to adapt to background noise.”

But Fengs study begs the question as to why only males and not females have this capability,he added.

Adler,the Cornell biologist,said,“This is a fabulous paper and a carefully conducted study on the anatomy of an odd frog.”

瀑布巨大的轟鳴聲和雷電的霹靂聲都是能夠把嗓門最大的青蛙的叫聲都吞沒(méi)掉的。然而，在中國(guó)，一些固執(zhí)的青蛙有一個(gè)很聰明的補(bǔ)救辦法：它們轉(zhuǎn)而使用超聲。

研究人員們說(shuō)，超聲波能力一直是唯一的一組哺乳動(dòng)物才具有的能力，如鯨魚(yú)和海豚，它們是具有超聲波能力的，而現(xiàn)在，這些青蛙的這一技藝使它們成為第一批能與這些哺乳動(dòng)物比肩而立的兩棲動(dòng)物。

伊利諾伊大學(xué)厄巴納——尚佩恩分校的神經(jīng)科學(xué)教授阿伯特·芬說(shuō)：“在這些青蛙身上就表明了一個(gè)新的實(shí)例，那就是：在這些青蛙的棲息地，有著巨大的背景聲音，為了適應(yīng)這里的生活，這些青蛙自己做出了獨(dú)立的進(jìn)化性的適應(yīng)性改變。”

研究人員第一次聽(tīng)說(shuō)這些非同尋常的動(dòng)物是在2000年，從康奈爾大學(xué)的生物學(xué)家克萊格·阿德勒那里聽(tīng)說(shuō)的。

他向科學(xué)家們透露，那是在黃山溫泉——一個(gè)離上海不遠(yuǎn)的旅游勝地，就是在那兒，他發(fā)現(xiàn)了耳朵凹進(jìn)去的青蛙。

阿德勒說(shuō)：“我注意到這些青蛙凹陷進(jìn)去的耳朵，我想它們肯定有一套奇特的信息交流系統(tǒng)。我們那時(shí)并沒(méi)有想到它們使用的是超聲波聲音?！?/p>

就在那年，一個(gè)由芬領(lǐng)導(dǎo)的研究小組報(bào)告說(shuō)，這些名叫凹耳湍蛙（Amolops tormotus）的青蛙是像鳥(niǎo)一樣鳴唱的。

兩年之后，芬和他的同事們通過(guò)使用寬頻帶的錄音設(shè)備發(fā)現(xiàn)，這些青蛙也用超聲?shū)Q叫，而超聲是指超過(guò)了人的聽(tīng)力范圍的聲音振動(dòng)。

這個(gè)研究小組的發(fā)現(xiàn)發(fā)表在當(dāng)前一期的《自然》雜志上（譯者注：本文是2006年3月中旬發(fā)表的文章，所講的《自然》雜志可能指3月份的這一期）。

超聲波聽(tīng)力

為了檢驗(yàn)這些青蛙杰出的聽(tīng)力本領(lǐng)，芬在去年夏天去了中國(guó)，到這些青蛙的棲息地進(jìn)行研究，他帶去了專門的設(shè)備，這些設(shè)備可以在聽(tīng)得見(jiàn)的聲域和超聲波聲域這兩個(gè)聲域播放聲音。

研究人員對(duì)八只雄性青蛙播放了早已錄制好的聲音。

當(dāng)播放的聲音是在聽(tīng)得見(jiàn)的聲域和超聲波聲域時(shí)，其中的五只青蛙，與在根本沒(méi)有聲音時(shí)相比，其反應(yīng)要強(qiáng)烈得多。當(dāng)聽(tīng)到一個(gè)播放的異常的超聲時(shí)，一只青蛙猛地向后跳去，同時(shí)發(fā)出18 聲快而尖的鳴叫。

芬說(shuō)：“這些雄性青蛙不僅對(duì)超聲做出反應(yīng)，它們還靠近我們的揚(yáng)聲器，好像是要勇敢地面對(duì)那只發(fā)出這個(gè)聲音的青蛙。”

在第二次測(cè)試中，研究人員觀察一只青蛙在播放聲音時(shí)的大腦活動(dòng)。這只青蛙對(duì)超聲做出的反應(yīng)很強(qiáng)烈，但是，當(dāng)它的兩個(gè)耳朵都被做模型用的黏土堵起來(lái)的時(shí)候，它卻沒(méi)有什么反應(yīng)。

芬說(shuō)：“這就證實(shí)，聽(tīng)這一活動(dòng)實(shí)際上是通過(guò)耳朵完成的，而不是通過(guò)聲音完成的，即不是通過(guò)穿透顱骨，然后又刺激神經(jīng)的聲音完成的?！?/p>

節(jié)約能量的蛙鳴

研究人員們推論，對(duì)于這些青蛙的超聲波能力，應(yīng)該是能有一個(gè)進(jìn)化論方面的解釋。

芬指出，那些想要引起雌蛙注意的雄蛙，需要使它們自己的蛙鳴聲超過(guò)別的求偶者，也超過(guò)棲息地嘈雜的背景聲音，讓雌蛙聽(tīng)到。

他說(shuō)，把蛙鳴聲轉(zhuǎn)換到一個(gè)更高的頻率不僅能保證這聲音能被聽(tīng)到，而且發(fā)出這種聲音，與發(fā)出很大的聽(tīng)得到的蛙鳴聲相比，其所需要的能量要少很多。

這些青蛙的超聲波聽(tīng)力的秘密好像就是它們的耳朵的獨(dú)特構(gòu)造。它們的耳鼓特別薄，而且深陷在身體表面之下，以保護(hù)鼓膜。

但是，芬說(shuō)，能使青蛙具有超聲波聽(tīng)力的耳中的確切部位，目前人們還不清楚。

得克薩斯大學(xué)奧斯丁分校的一位動(dòng)物學(xué)家邁克爾·賴安說(shuō)：“這項(xiàng)研究很有趣，說(shuō)它有趣，原因有好幾個(gè)?！?/p>

“它表明，一些青蛙是有潛力能聽(tīng)到一個(gè)頻率范圍內(nèi)的聲音的，而這一頻率范圍內(nèi)的聲音，人們從來(lái)沒(méi)有想到青蛙能夠聽(tīng)到。這項(xiàng)研究也表明，對(duì)于周圍嘈雜的背景聲音來(lái)說(shuō)，這是一種非同尋常的適應(yīng)方式?！?/p>

但是他也補(bǔ)充說(shuō)，為什么只有雄性青蛙具有這一能力，而雌性青蛙卻沒(méi)有，對(duì)于這個(gè)問(wèn)題，芬的研究是用未經(jīng)證明的假定來(lái)辯論的。

康奈爾大學(xué)的生物學(xué)家阿德勒說(shuō)：“這是一篇極好的論文，而且也是一項(xiàng)對(duì)一只奇特青蛙所進(jìn)行的仔細(xì)認(rèn)真的解剖學(xué)上的研究?！?/p>