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Today's episode is sponsored by Brilliant. Go to Brilliant.org/SciShow to check out their course on waves and light.
本期节目由Brilliant赞助播出。登录Brilliant.org/SciShow学习关于波和光的课程。
Frogs often have a lot to say. Just ask the residents of the Big Island of Hawai'i, who get serenaded by tiny, invasive coqui frogs every night.
青蛙通常很聒噪。问问夏威夷大岛的居民就知道了，小小的入侵物种coqui青蛙夜夜为他们歌唱小夜曲。
But if you've ever looked at a frog's head, you might have noticed that they don't have external ears.
但如果你曾观察过青蛙的头部，你可能会注意到，它们没有外耳。
So it might seem weird that they're able to hear those calls.
所以它们能够听到这些声音似乎有些奇怪。
Well, it turns out they rely on other ways of getting information about sounds to their brains.
事实证明，它们以来各种方式将声音信息获取至大脑中。
Including listening with their lungs. Amphibian hearing works a lot like human hearing,
包括用它们的肺部倾听。两栖动物的听觉和人类的很像，
except instead of eardrums buried in an ear canal, most frogs have a tympanic membrane right on their heads.
只不过大多数青蛙的耳膜不是藏在耳道中，而是在头部有一个鼓膜。
When a sound wave hits that membrane, it starts to vibrate.
当声波撞击这个鼓膜时，它会开始振动。
And those vibrations start a chain reaction that ends with fluid vibrating against special cells in the frogs' inner ear.
这些振动引发了连锁反应，最终以对青蛙内耳中的特殊细胞的不固定振动而结束。
When these cells move, they send out an electrical signal that travels to the auditory center of the brain.
当这些细胞移动时，它们会发出一种电信号，并传送到大脑的听觉中心。
Low-frequency vibrations stimulate the cells in one part of the ear, while high-frequency vibrations stimulate the cells in a different part.
低频振动刺激耳朵某一部分的细胞，而高频振动则刺激耳朵内不同部分的细胞。
So, the brain figures out what pitch the sound is by which part of the ear the signal comes from.
所以大脑通过信号从耳朵哪个部位传出而分辨声音的音高。
But when you're a female frog looking for love, you need more than the pitch of a sound.
但当你是一只求爱的雌性青蛙时，你需要的不仅仅是声音的音高。
You also need to know where it came from, and the tympanic membrane can help with that, too.
你还需要知道声音来自哪里，而鼓膜也可以帮助你来分辨。
A sound wave will actually hit the tympanic membranes on both sides of the head.
声波其实会撞击脑袋两侧的鼓膜。
And for each, it travels across the mouth and through a channel called the eustachian tube
对于每一侧的鼓膜，声波都会通过嘴部，穿过一个叫做咽鼓管的通道
which connects the mouth to the tympanic membrane on the other side.
这个通道将嘴部和另外一侧的鼓膜连接起来。
So the sound wave ends up reaching both tympanic membranes from both the outside and the inside.
所以声波最终会从内部和外部到达两侧的鼓膜。
But the pressure is usually stronger on the side a sound came from.
但声音发出的一侧的压力通常更大。
So, the difference tells the frog where her suitor is. But the problem is, sometimes this system doesn't work.
这个区别告诉青蛙她的求婚者在哪里。但问题是，有时这个系统不管用。
For one thing, when a sound is low-pitched, there actually isn't much difference between the pressure on the two sides of the membrane.
首先，当声音是低音调时，两侧鼓膜所感受到的压力区别其实不大。
And some frogs, like the coqui frog, have tympanic membranes that are too small to respond much, if at all, to low-pitched sounds.
包括coqui青蛙在内的一些青蛙，它们的鼓膜鼓膜太小，根本无法对低音调做出反应。
You see, the amount of sound energy a surface experiences is directly proportional to the area of that surface.
一个表面所经历的声能量与该表面的面积成正比。
And both lower frequency and lower volume sounds carry less energy.
低频率和低音量所携带的能量都很少。
So smaller tympanic membranes don't capture much energy from low-frequency or quiet noises.
较小的鼓膜无法从低频或大量杂音中捕获很多能量。
And if a frogs' tympanic membranes are small enough, normal volume low-frequency sounds basically don't move them at all.
并且如果青蛙的鼓膜足够小，那么正常音量的低频音基本不足以振动鼓膜。
But luckily, for these frogs, their lungs lend a hand and help them hear.
但还好，对于这些青蛙而言，它们的肺部搭了把手，帮助它们听见声音。
Sound waves passing through the body cause the lungs to vibrate like a giant eardrum.
声波穿过身体会导致肺部振动，就像一个巨型鼓膜一样。
Those vibrations pass through the vocal cords up to the mouth
这些振动通过声带来到口腔，
where they can use those eustachian tube highways to reach the tympanic membrane from the inside,
在那里它们可以利用这些咽鼓管从内部抵达鼓膜，
creating that pressure differential that localizes sound.
创造出可以定位声音的压差。
Since the lungs have a larger surface area, they can pick up the lower-frequency sounds that the tympanic membranes can't.
因为肺部的表面积更大，所以它们可以捕捉鼓膜捕获不到的更低频的声音。
And for some species, the lungs replace the tympanic membranes entirely.
对于一些物种来说，肺部完全替代了鼓膜。
We know this because when scientists studied frogs that lack tympanic membranes,
我们知道这一点，因为当科学家研究没有鼓膜的青蛙时，
the vibration of their lungs matched activity in the sound-processing parts of their brains.
它们肺部的震动和大脑中声音处理部分的活动相匹配。
And then, when they covered their bodies in a centimeter and a half of noise-dampening silicone grease
当他们给青蛙身体涂上一层1.5厘米厚的隔音硅脂
or filled their lungs with saline to limit their ability to vibrate, the neural response shrank.
或将它们的肺部填充生理盐水以限制它们振动的能力时，这种神经反应变小了。
So it was clear that the sound had to vibrate the lungs for the frogs to hear.
很明显，声音需要振动肺部才能让青蛙听见。
And researchers actually think that this kind of body-hearing is how the very first amphibians
研究人员实际上认为，在鼓膜进化之前，
might have listened to the world around them, before the tympanic membrane even evolved.
最早的两栖动物就是通过这种身体听觉来倾听周围的世界的。
So the next time you're out for a walk after it rains and you hear a symphony of frogs,
所以下一次雨后外出，听到青蛙交响乐时，
it might be fun to think about all the amazing biology going on to make it possible.
不妨想一想这些让一切成为可能的神奇的生物学原理。
But if you really want to appreciate how complex our biological world is,
但如果你真的想了解生物世界有多么复杂，
you might want to check out the courses offered by today's sponsor, Brilliant.
你或许想看看Brilliant提供的课程。
They have dozens of thought-provoking math, science, and computer science courses.
他们有几十种引人深思的数学、科学以及电脑科学课程。
And all of them use puzzles, games, or other interactive tools to make learning intuitive and fun.
所有这些都使用谜题、游戏或其他互动工具，使学习变得直观而有趣。
And you can dive deep into sounds with their Waves and Light course,
你还可以通过他们的波与光线课程深入了解声音，
which will teach you how your noise-cancelling headphones actually work!
该课程将告诉你降噪耳机的工作原理！
You'll get access to it and all of their other engaging STEM courses with a premium subscription.
你只要付费订阅，就可以获得本门课程以及所有其他有吸引力的STEM课程。
The first 200 people to sign up at Brilliant.org/SciShow will get 20% off the price!
前两百米注册Brilliant.org/SciShow的用户将获得八折优惠！
So head over there now if you want to learn more.
所以想了解更多，就去看看吧。
