Today's episode is sponsored by Brilliant.

Go to Brilliant.org/SciShow to check out their course on waves and light. [ ♪INTRO ]. 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. But if you've ever looked at a frog's head, you might have noticed that they don't have external ears. So it might seem weird that they're able to hear those calls.

Well, it turns out they rely on other ways of getting information about sounds to their brains. Including... listening with their lungs. Amphibian hearing works a lot like human hearing, except instead of eardrums buried in an ear canal, most frogs have a tympanic membrane right on their heads.

When a sound wave hits that membrane, it starts to vibrate. And those vibrations start a chain reaction that ends with fluid vibrating against special cells in the frogs' inner ear. When these cells move, they send out an electrical signal that travels to the auditory center of the brain.

Low-frequency vibrations stimulate the cells in one part of the ear, while high-frequency vibrations stimulate the cells in a different part. So, the brain figures out what pitch the sound is by which part of the ear the signal comes from. But when you're a female frog looking for love, you need more than the pitch of a sound.

You also need to know where it came from, and the tympanic membrane can help with that, too. A sound wave will actually hit the tympanic membranes on both sides of the head. And for each, it travels across the mouth and through a channel called the Eustachian tube which connects the mouth to the tympanic membrane on the other side.

So the sound wave ends up reaching both tympanic membranes from both the outside and the inside. But, the pressure is usually stronger on the side a sound came from. So, the difference tells the frog where her suitor is.

But the problem is, sometimes this system doesn't work. 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. And some frogs, like the coqui frog, have tympanic membranes that are too small to respond much, if at all, to low-pitched sounds.

You see, the amount of sound energy a surface experiences is directly proportional to the area of that surface. And both lower frequency and lower volume sounds carry less energy. So smaller tympanic membranes don't capture much energy from low-frequency or quiet noises.

And if a frogs' tympanic membranes are small enough, normal volume low-frequency sounds basically don't move them at all. But luckily, for these frogs, their lungs lend a hand and help them hear. Sound waves passing through the body cause the lungs to vibrate like a giant eardrum.

Those vibrations pass through the vocal cords up to the mouth where they can use those Eustachian tube highways to reach the tympanic membrane from the inside, creating that pressure differential that localizes sound. Since the lungs have a larger surface area, they can pick up the lower-frequency sounds that the tympanic membranes can't. And for some species, the lungs replace the tympanic membranes entirely.

We know this because when scientists studied frogs that lack tympanic membranes, the vibration of their lungs matched activity in the sound-processing parts of their brains. And then, when they covered their bodies in a centimeter and a half of noise-dampening silicone grease or filled their lungs with saline to limit their ability to vibrate, the neural response shrank. So it was clear that the sound had to vibrate the lungs for the frogs to hear.

And researchers actually think that this kind of body-hearing is how the very first amphibians might have listened to the world around them, before the tympanic membrane even evolved. So the next time you're out for a walk after it rains and you hear a symphony of frogs, it might be fun to think about all the amazing biology going on to make it possible. But if you really want to appreciate how complex our biological world is, you might want to check out the courses offered by today's sponsor, Brilliant.

They have dozens of thought-provoking math, science, and computer science courses. And all of them use puzzles, games, or other interactive tools to make learning intuitive and fun. And you can dive deep into sounds with their Waves and Light course, which will teach you how your noise-cancelling headphones actually work!

You'll get access to it and all of their other engaging STEM courses with a premium subscription. The first 200 people to sign up at Brilliant.org/SciShow will get 20% off the price! So head over there now if you want to learn more. [ ♪OUTRO ].