BAT Flight vs BIRDS, with SLOWMO, robots, swimming and treadmills - Smarter Every Day 87

Alright, so several months ago we took a deep dive and learned exactly how bird wings work. And it was pretty cool, so go check it out if you want. But, a couple of you had the audacity to ask me how bat wings work, and I didn't know the answer.

So you hear that behind me, right? It's a waterfall. It's called Blowing Wind Cave, and there are thousands of bats overhead right now, flying to the river to eat for the night. You see those little dots? They're all bats... All of them. That's pretty crazy, isn't it?

So here's the deal. I'm gonna send you to the Peruvian Amazon rainforest with ignorant Destin from several months ago and a Phantom high-speed camera. So go make observations, and when you get back, me and you are gonna talk about bat-wing biomechanics by looking at data from Brown University's bat wing wind tunnel. It's amazing. I'll see you on the flip side. This is worth your time.

I'm going to the cave. So bats... Amazon rainforest, they live everywhere. They live in the top of hollow trees, like this one, which I can understand because right now I'm about, oh I don't know, 30-40 feet off the ground? Here's Phil.

(Phil) We are in a strangler fig, which means that basically the figs start off as a vine and then strangle the tree that used to live in this hollow space.

(Destin) Here, I'll hand this off to you.

(Phil) Cool.

• There's some high-speed video we're gonna shoot of bats that live near a lake nearby, so we're gonna go check that out. Right now, let's just try to get video of this. Getting Smarter Every Day.

See 'em?

• Oh yeah, nice!

• There he is... You don't like me being here, do you? Oh man! The joker just peed in my mouth. Stop. I need to clarify, that was a lie. The bat didn't actually poop in my mouth. He pooped towards my mouth, and it was deflected by my moustache and then it hit me on the mole here. So, good thing I had a moustache. That's cool.

Our guide Gerson grew up in the area so he knew the exact log that bats like to hang out on. So that's where the bat is, huh?

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So how do bats fly differently than birds? That's the question I want to answer here. So when I started researching the topic, several papers led me to the real-life dynamic duo Dr. Kim Breuer, who's an engineer, and Dr. Sharon Swartz, who's a biologist. They work together at Brown University's school of engineering animal flight wind tunnel to unravel the mysteries of bat flight.

Now most people's knowledge of how bats fly stops at echolocation, but I want to go much further than that. If you look back at the high-speed footage, you'll notice that when the bat strikes the other bat in the head, his entire body starts to roll to one side. Now at first blush, it looks like he might be countering the aerodynamic lift of that open wing by rolling his body.

But if you look at this footage that I captured of macaws, you can see when one of the macaws strikes another one in the head, he doesn't roll his body like the bat does.

Now what's the difference between the bat and the macaw? The answer is mass. Bats have 12-20% of their entire body weight in each one of their wings. This means that inertial forces play a much bigger role in bat flight than they do in bird flight, because birds are much lower than that.

If you think about it, the ability to react against their own wing mass allows bats to do maneuvers that birds can't. For example, go from full-speed flight to an upside-down and sideways twist in order to gently grab the top of a cave.

Don't let this freak you out. There's this place I like to go where owls will drop their prey and they decompose. What's so cool about that is the soft tissue will decompose in the same position as it usually is relative to the bones. Now on this duck, that means we can check out the wing structure really well. Look at this.

You can see the major bones here. What I think's cool is you can see that there's no bones that go down into the wing. Bats have an entire hand built into the wing. That means a bat has a lot more precision control over the shape of the wing relative to a bird who basically just has a few major joints and the ability to pitch each individual feather.

So when you use a wind tunnel, typically you put a model right in the middle, and then you'll measure the airflow over the surfaces of the model. But what do you do if your model is actually a bat and it can fly wherever it wants? How do you know where to measure?

Well, the answer is you put dots on the model, or in this case a bat. Using multiple camera angles and a little math, researchers track the position of each individual dot and can recreate a simulation of how the finger joints in the wing open and close the wing.

This doesn't answer our airflow question though.

(Son speaking, unintelligible)

That's right. Particle image velocimetry is a pretty interesting way of measuring a flow field. So right now, it's springtime in Alabama and there's pollen all over my driveway. So as my son pours this water out, if you look, you can see this pollen moving along with the water.

It's pretty awesome because it lets you see things in the flow that you couldn't see before, like this little vortex. This is exactly what the researchers are doing with the bats. They put particles in the wind tunnel and then they illuminate the particles with a laser. They then take these images and calculate the local vorticity, omega.

By plugging omega in and integrating over a defined area, you can get circulation, or gamma. Gamma goes into one last equation, and there you have it. The lift generated by a bat wing while in flight. Imagine how useful this could be if you're doing something awesome like, oh... I don't know... developing a robotic bat wing. Yep, that's happening.

So did I answer your question? Bats have more mass in their wings, so they maneuver using inertia. They're more agile than birds; they can even hover. They control their wing geometry better, so on the backstroke instead of delaminating the wing, they umbrella fold and slide the wing along their body.

Heck, they even fly before they can fly. They're mammals; their moms carry them. Here's a question for you in the comments. Some bats have tail membranes and some don't. So how do you think that affects flight?

So big thanks to Dr. Breuer and Dr. Swartz for letting us use your research video. I think a good way of ending this would be to couple that with some music that Gordon wrote that reminds me of Castlevania. Quick warning though, there's a little nightmare fuel in this.

Anyway, I'm Destin. You're getting Smarter Every Day. Please subscribe. Have a good one.

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[Captions by Andrew Jackson] captionsbyandrew.wordpress.com Captioning in different languages welcome. Please contact Destin if you can help.