Our environments are filled with information.

Just think of all the stuff that comes at us each day, like light and sound and even objects. It’s almost overwhelming when you think about it, but animals are pretty resourceful at figuring out how to take it all in -- and weed out what’s not important.

In fact we and other animals often put our whole bodies into it. Like heads, knees, legs, abdomens, antennae, wings, they’re all body parts animals use to learn about their environment -- just through hearing! So let’s talk about what an ear even is, how ears evolved, how they work, and some of the cool ways animals have tweaked their ears to work for their lifestyle.

Perk up whatever you use for hearing because I’m going to be sending vibrations your way for the next 10-ish minutes. I’m Rae Wynn-Grant, and this is Crash Course Zoology. Like all senses, hearing involves collecting information from the environment and processing that information into signals that the brain can understand.

Specifically, hearing is the ability to interpret your environment using vibrations that move through air, water, or even solid objects. Hearing organs like ears are just one way animals can interpret the sounds in their environment. And while we vertebrates trace all our ears to one ancestor, invertebrates have independently evolved ears over and over again -- over 24 times in just insects alone!

So like vision, hearing is a spectrum: some animals have very sensitive hearing and some don’t hear at all. For those of us whose hearing organs are ears, ears have two minimum requirements to work: a sensor that turns vibrations into nerve signals, and the brainpower to interpret those signals into different sound qualities. Most animals that hear -- even if they don’t have what we’d call ears -- rely on special hair cells, which are named for the little hair-like tufts that come off them called stereocilia.

Sound vibrations cause the stereocilia to sway, which leads to the hair cell sending signals to nearby neurons that relay information to the brain. Our hair cells are in our internal ear, the part that does the actual hearing that sits behind a thin membrane or eardrum. The outside part, called the pinna, works like a satellite dish to amplify and direct sounds into our internal ear.

Pinna are very much a Mammal Thing; some species’ pinna even move to track sounds! Many other vertebrates, like most reptiles and amphibians, don’t have pinna. Their external ears are simple openings.

And some animals like fish have no external ears at all, hearing only the sounds that reach their internal ear after moving through their body. But some animals have gone a different route. Frogs and toads usually have internal ears, but there are over 200 species that have reduced or even lost many internal ear parts.

These mostly-earless toads can still hear some sounds -- like a croak! -- by feeling vibrations. Which might be all they need to chat with their friends and detect predators. Like I said before -- and like we learned about vision -- hearing and the organs we use to do it come in a whole spectrum.

And also like eyes, we can compare hearing organs based on what they can do. Most of our understanding of hearing is about how big-ish animals handle far-field sounds. Far-field sounds are sounds that have travelled far enough away from the thing making the noise that they act like a typical wave.

Very close to the sound source, the sound waves push on the air or water particles around them, so you get weird auditory effects as the particles bump into each other. Some invertebrates use this kind of sound, called near-field sound, to communicate with each other and detect predators. Though the two aren’t mutually exclusive -- some arthropods can also make and hear far-field sounds.

We can also compare different animals’ hearing ranges, which is the range of frequencies that an animal can detect. Frequencies are the number of times particles of a material vibrate in a certain time period when a sound wave passes through it. One cycle of vibration in one second is called one hertz.

Us humans have a decent hearing range of about 20 to 20,000 hertz. We experience frequency as pitch, or the low-ness or high-ness of a sound, which we can also compare. But many animals can hear sounds under 20 hertz -- which we call infrasound -- or above 20,000 hertz, which we call ultrasound.

Like wax moths can hear 300,000 hertz sounds -- squeakier than any bat! Of course, like “best eyes” or “best brain,” “best ears or hearing” isn’t really an award we can give out. Animal hearing has evolved to pick up on the type, range, and pitch of sounds that they encounter and need to respond to in their environment.

And as we’ll see, how animals hear has changed a lot in the hundreds of millions of years they’ve been around. But as for who can hear... Well, it depends what you call “hearing”, and what you call an “ear”.

For us humans, hearing usually means sounds moving in air, but there are other ways vibrations can travel. As of 2021 we know a lot of arthropods keep their ears to the ground, literally, by sensing vibrations as they move through the Earth. And that’s pretty much hearing, even though they don’t have ears that look anything like ours.

Other invertebrates, like jellyfish are trickier. They definitely have sensors that could detect sound waves, but since they live underwater, we can’t tell if they’re responding to sound waves or like… normal water waves. So for a long time, zoologists thought that only vertebrates could hear airborne sounds, since invertebrates don’t have ears like ours.

But that got thrown out with a series of meticulously designed experiments by high school teacher. Charles H. Turner.

Let’s go to the Thought Bubble. It’s standard these days to account for extra factors that might influence research results, but in 1907 when Turner published his first hearing study, his experimental design was revolutionary. First, he insulated ant nests from ground vibrations with cotton batting, and carefully controlled the lighting in the room.

Then he used a dog whistle, organ pipes, and his own voice from different distances, and he repeated the experiment many times, and even used heat filters to keep the animals from sensing the warmth of the lights. With all those controls, the ants scuttled away when he made noise, which Turner interpreted as evidence that his ants could hear! Turner then investigated hearing in moths using a dog whistle, again taking precautions that the animals weren’t just responding to the sight of the whistle or air movement.

According to his results, moths could hear too! He reported in 1914 that the moths responded to high, but not low frequency sounds and mused that moths might still be able to hear the lower sounds, but only responded to sounds that were important. Like the high pitched whistle that sounded a bit like a bat.

So he tested this idea by jostling the moths when he made a low frequency sound. Soon the moths learned that low pitched noises were dangerous, and fluttered their wings in response to just the low frequency sound. So not only did he prove moths could hear sounds even if they didn’t respond to them, but also that moths could learn!

Thanks, Thought Bubble. Turner was a brilliant and productive scientist. He was the first African American to earn a PhD from the University of Chicago, but was excluded from professorships at white universities.

Even today, many black entomologists face similar barriers. But Turner's work with moths was one of the earliest examples of conditioning in insects and foundational to the work of many later scientists studying insect learning and behavior. In addition to Turner’s moths and ants, since 1907 we’ve learned that lots of insects and at least some arachnids, like spiders, can hear airborne sounds thanks to the work of many other scientists.

Many insects even use hairs tuned to particular frequencies, sort of like stereocilia! Other insects have a tympanal organ, in their legs, wings, abdomen, and other places, that vibrates and ends up sending signals to the brain. Invertebrate hearing evolved dozens of times in lots of different ways -- which makes sense since there are 34ish phyla of invertebrates out there.

But vertebrate-style hearing only evolved once in a fish ancestor, which was passed on to all tetrapods, or animals with four limbs like mammals, birds, reptiles, and amphibians. We know that because even though vertebrate ears vary widely, they always use the same hair cell centered mechanism. Based on fossil evidence, we think that the earliest vertebrate ears were entirely internal, like fish ears are today, and evolved over 500 million years ago.

Which is probably why fish hear low frequency sounds best -- they pass through the body more easily than high frequency sounds. Then as vertebrates moved onto land roughly 400 million years ago, things got more complicated. Like if you’ve ever been swimming at a public pool, you might’ve noticed how much quieter everything seems underwater.

That’s because a lot of airborne sounds bounce off the water surface instead of traveling through the water. The same thing happened to those early land vertebrates that only had internal ears: the sounds in the air bounced off their denser ear tissue, and didn’t reach the hair cells. So they probably only heard very loud, low frequency sounds.

That is, until tympanic middle ears, or ears with an eardrum and ossicles, evolved -- which they did independently at least three times. These external ears allowed sounds in the air to come into direct contact with the inner ear without traveling through the skull and other tissues. This made a huge difference in how well land animals could hear!

Ears spread across Metazoa because just like eyes, they’re super useful. Vertebrates probably first evolved hearing to gather more information about their environment and avoid hazards, whereas insects evolved hearing so they could keep an ear out for vertebrate predators. Except for those noisy cicadas -- they just wanted a way to talk to each other.

Some of our biggest leaps in understanding how animals interact with sound all happened very recently, and I’m sure we’ll learn even more interesting stuff soon! Next episode we’ll get into the really old and weird ways animals sense their environment! Thanks for watching this episode of Crash Course Zoology which was produced by Complexly in partnership with PBS and NATURE.

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