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To keep building your STEM skills beyond this video, check out Brilliant.org/SciShow. That link will give you 20% off an annual premium subscription! [♪ INTRO] When you run to catch the bus, go for a swim and need a breath of air, or do pretty much anything else, your vestibular system is there to help you keep your balance and figure out which way is up.

Without a functioning vestibular system, your daily activities would be impossible. Take it from the pumpkin toadlet that can’t land its jumps because its vestibular system doesn’t quite work. So the way that your body senses itself in the world is really important.

That sense, for humans, mainly has to do with the direction that gravity pulls you. You have three semicircle shaped tubes in your ears, each at a 90 degree angle to the others on its own x, y, or z axis. They’re called semicircular canals, and they have fluid in them.

As you move around, the fluid moves through the tubes. Where the fluid goes, based on the tug of gravity, is what tells you how your body’s leaning. For a more detailed description of the human vestibular system, watch our previous video on the senses you didn’t know you have.

But this video isn’t about you, because animals also know what’s up. Some of them manage to walk and swim just fine without the kind of vestibular system that you have. Many even fly without it.

These animals can do all of that because the natural world has several ways of solving the same problem. As it turns out, there’s more than one way to sense gravity. Here’s what they use instead, and what happens to their sense of orientation in space when they’re literally in space without the strong pull of gravity.

Jellyfish can sense gravity using entirely different parts on the opposite end of their bodies from us. Ever wonder what those tassels are at the bottom of the lamp shade that is the adult jellyfish body? That’s their version of semicircular canals.

Instead of tubes in their ears, they have fringe along their bell. That fringe is a bunch of sensors called rhopalia, and the entire sensory structure bends with gravity rather than just the fluid inside. Now, you’ve probably been swimming or taken a bath and noticed that gravity doesn’t feel as strong when you’re floating in water.

But it’s still there, and jellyfish use their rhopalia to sense it. And we know that gravity is important to keep rhopalia working properly because a 1991 experiment sent young jellyfish to space where gravity has even less influence than when they’re in water on Earth. Rhopalia have three main types of cells involved in sensing gravity’s pull.

Type I cells sense movement like the cells in your semicircular canals. Type II cells send that movement information to brain cells. And type III cells do a little bit of both.

What’s more, rhopalia go beyond just sensing gravity. They also help jellyfish move around by communicating with their muscles. But that muscle communication seemed to go haywire when jellyfish went to space.

The three types of rhopalia cells still developed like they do on Earth, but they didn’t quite function the same. They weren’t moving as much as they usually do. And when they did, it was more of a vibration than the smooth movement that they would otherwise make.

Not to mention, the jellyfish had muscle spasms when they came back. Researchers think those vibrating rhopalia might have been to blame. So the way jellyfish move depends on their ability to sense gravity; not just the way they swim around, but also the way they control their muscles in general.

Jellyfish may look like aliens, but they don’t seem to do so well in space. Rhopalia aren’t the only gravity sensing organs below sea level. Snails have something called a statocyst, which has been compared to a system in your inner ear.

Besides the semicircular canals, I mean literally beside them, you have otolith organs, which are additional gravity sensing organs made of compartments lined with hair cells and full of crystals. They’re another part of your vestibular system. When you cock your head to the side, the crystals move to the side, too.

It’s still based on where gravity pulls them, and helps you know how your head is positioned. And snails have a similar organ. Of course, the statocyst in snails that crawl is more similar to your system than the one in snails that swim.

So let’s start with the creepers and crawlers. You can think of their statocyst like one of those lawnmower toys that kids used to play with. You know the ones with the popping balls inside the dome?

The foundation is a sac of stones rather than crystals or balls. And the sac is filled with gel and lined with sensory hairs, so the stones don’t just sit there at the bottom. They get juggled around by moving hairs.

When the snail moves forward quickly, the acceleration sends the stones to farther back hairs, which churn and toss the stones back into the center of the statocyst. Those hairs are short enough that the stones only touch some of them at a time. So when the snail isn’t moving, or when it’s just moving slowly, the stones touch the hairs at the bottom of the statocyst because that’s where gravity puts them.

And that tells the snail which direction gravity is pulling on it, because the hairs that throw the stones back into the center of the statocyst communicate to the brain that they were the ones to touch the stones. Swimming snails, along with crustaceans and mollusks, have a slightly different setup with longer hairs and sometimes only a single stone that doesn’t settle to the bottom of the statocyst. The hairs are long enough that they keep the stone suspended in the center of the statocyst like toothpicks suspending an avocado seed above water.

That configuration gives the swimming snail information about acceleration in any direction, which they’re much more likely to need than the crawling snails. You can’t crawl straight up or down. It’s always forward, backward, or side to side.

Because of how they move, different snails need to know different things about how gravity is acting on them. And their different gravity sensing setups help them move in those diverse ways. Snails might have a variety of ways to move, but none of them can travel quite like flies.

It takes a special kind of understanding of where you are in space to fly through the air without injury. That might be why flies have so many gravity sensors on their bodies. One way that flies sense gravity is through chordotonal organs.

Those are little sensors on their antennae that measure the pressure pushing against them from wind and gravity. Each one is made up of smaller groups of sensory cells, ranging from one to several hundred, depending on the location on the fly. And that upper range can be found on antennae.

Now, that’s a lot of cells, but they don’t all do the same thing. Some of them are in charge of sensing gravity, while others sense sound. So it’s a multifunctional sensor.

In that way, it’s kind of like your ears that also sense both sound and gravity through different cells. Flies also have little balancing rods called halteres behind their wings. That’s one of the tools that makes them so hard to swat, as we described in a previous video.

So these animals are master movers in their natural habitat. But when they were put in a space simulation in 2008, it seemed to send them into hyperdrive. The flies moved around significantly more in microgravity than Earth’s gravity.

While other animals were observed having movement problems without their natural gravity, flies were going strong. The caveat here is that they were only able to walk, not fly, so that their movement could be more accurately measured. We still don’t know if they would have had more trouble flying in space.

And if a fly goes to space but doesn’t fly, was a fly ever really in space at all? Speaking of insects, crickets are so much smaller than you that their gravity sensors have to be way more sensitive than yours. And, in what may be the most on-brand fact of this video, some crickets and cockroaches detect the pull of gravity through pendulums swinging from their butts.

Check out the SciShow Tangents butt facts if you don’t know what I mean. They have special structures on their rears covered with hairs of different thicknesses and lengths that all serve their own functions. Long hairs are sensitive to incredibly small amounts of movement, while short hairs detect fast movement.

And these different hairs also correspond with brain cells that are sensitive to those specific stimuli. They’re kind of like tree branches. The longer branches farther out from the trunk are often the first to sway in the wind.

They’re more sensitive to movement than the shorter branches. But what if that movement happens in space? In a 2002 experiment entitled “Crickets in Space,” the insects were pretty much the polar opposites from those space jellyfish from before.

When crickets went to space, their brain cells changed but their behavior didn’t. That experiment studied the other end of the cricket body and concluded that crickets who went to space seemed to move their heads the same way that Earth-bound crickets would. But space travel still had an effect on the little guys.

Some of their brain cells quieted down, and others grew smaller than they would have on Earth. The researchers running the experiment explained the disconnect as best as they could by proposing that the crickets were sampled at the wrong stage of development or that they may have looked at the wrong kinds of brain cells. Either way, it’s nice to know that the crickets seemed to bounce back after their wild ride.

In some ways, other animals may have better sensors for space travel than your vestibular system. But it’ll take more than that to keep humans from exploring. But one more thing!

Having a pendulum on your butt is a weird enough concept that it’s worth spending some more time on. So to learn about how pendulums work, you can go to Brilliant’s course on Classical Mechanics. Brilliant is an online learning platform that offers guided courses in math, science, and engineering, all with a focus on problem-solving.

This course includes 49 interactive lessons that you can complete on your own time scale whenever it works in your schedule. Even when you’re traveling, you can access this Brilliant course offline too. And pendulums are all over this course.

Starting in the introduction lessons, you’ll get to solve a pendulum clock puzzle. Then you’ll learn how pendulum clocks work in the first place. And once you get the hang of those little pendulums, although they’re not as little as the ones on cricket butts, you’ll level up to large scale pendulums.

You can find all of that pendulum goodness and more at the link in the description down below, which gives you 20% off an annual Premium Brilliant subscription. And before you commit to a year of Brilliant, you can try it for free using that link or by visiting Brilliant.org/SciShow. Thanks to Brilliant for supporting this SciShow video and thank you for watching! [♪ OUTRO]