[Rose] When we wanna get from point A to point B as fast as possible, we take to the air.

But humans are newbies in the world of flight. While we've had commercial flights for over a century, other animals have been flying for a millennia.

And for good reason. From an evolutionary perspective, flight is awesome. Animals do things a little different from us, though.

So today, we're going to look at the bizarre world of of animal flight, from birds, to bats to... spiders? And snails? Oh come on!

Now you're really making stuff up. Flying snails? Alright.

I'll wait for it. Let's kick things off with the undisputed champions of the air: birds. These living dinosaurs constantly dazzle us with their aerial acrobatics.

So naturally, we've looked to them for tips and tricks. For instance, fighter pilots often fly in a V shape, an idea we humans totally stole from migrating birds. But not all birds fly in Vs, no matter how far they're flying.

Why not? Here's Hank to explain. 

(Hank) Think of a flock of birds you probably think of a classic V-shape with a leader with sets of trailing birds on either side.  But not all flocks fly this way.  Starlings, for example, travel in large, three-dimensional clusters that seem to move like a wave.

So, why do some species fly in Vs, and others in clumps?  Well, it turns out it has a lot to do with the individual birds themselves.
 
Some, like geese heading south for the winter, are making long treks.  The V formation helps them stay in visual contact with each other, avoid collisions, and conserve energy.  It's the structure of their wings that lets them take advantage of the V.  

As the wing flaps, each wing tip creates a vortex that spirals up from the bottom of the wing and over the top.  This vortex trails off behind each bird as it moves forward and is encountered by the next one in the line. The trailing bird positions itself to catch just the upwash of that vortex, or the upward force, and that requires being behind and just to the side of the leading bird.  Lots of birds behind and to the side of one another creates that V shape.

Studies have estimated that birds flying in this way can save around 15% of their energy.  So, why don't all birds fly this way?  We talked to Professor Eric Green from the Univeristy of Montana Bird Ecology Lab, and he explained that this has to do with the size of the bird.

You may have noticed that birds that fly in a V, like geese, pelicans, swans, and ibises, are typically larger creatures with a long wingspan.  These species move their wings only a few degrees up and down with each flap.  This motion creates vortices that lie pretty neatly behind the bird.  

Small birds, on the other hand, tend to flap their wings all the way up and down.  The vortices created by these motions are all over the place, not consistent enough for their flockmates to actually use.  And the small birds that do flap their wings like larger ones just don't generate a big enough vortex because of their size.

For small birds, flying in groups sometimes uses even more energy, not less, but these species have another need that's even more important: protection.  In 1971, evolutionary biologist William David Hamilton proposed a theory called "the selfish herd".

It suggests that  the risk to an individual is reduced if that animal places another animal between itself and a possible predator.  Repeat this across enough individuals and you end up with a herd, or in this case, a flock.  

Other theories offer similar explanations, but whether you're talking about schools of fish or swarms of insects, it's clear that this is a pretty common survival strategy.  So the next time you see a group of birds flying by, you'll know it might be to save energy, or it could just be to stay alive.



[Rose]  I just had a weird thought.  Can you imagine if we flew planes in flocks? That would be a logistical nightmare. No wonder we don't mimic everything birds do. And actually, now that I think about it, there are other aspects of bird flight we don't usually copy, like their flat tails. Our planes have vertical ones.

Is that because we're better at flying than birds? Stefan, what do you think?

[Stefan] I don't know if you noticed, but nature is kind of awesome.  That's why so much of our technology is inspired by it. For adhesives modeled after insects to early plane wings based on birds.

But just because we try to mimic nature doesn't mean we can do it perfectly. And birds and airplanes are pretty good examples of our limits.

Take the vertical tales on most airplanes: we use them to keep our giant metal contraptions in the air and going the right way, but birds get by fine without them. That's because they're just way better at flying than we are, and for the most part we can't keep up.

The parts of an airplane all work together to keep in the sky and  facing the right direction. But thanks to all kinds of invisible air currents, that isn't as simple as it looks from your window seat. The vertical tail's main job is to stabilize something called "yaw." 

Yaw measures how much the plane is pointed to the left or right of the wind, specifically the relative wind, which is created by the plane zooming through the atmosphere at hundreds of kilometers an hour.

Yaw can change in lots of situations, like during turns or in unsteady air. And, when it does, it occasionally leads to a condition called "sideslip." This is where the plane has yawed but it hasn't completed changed its direction. Instead, it's still mostly moving in the direction it was, only at a funny angle. Kind of like a sports car drifting around a turn.

Normally, this just leads to a little inefficiency. But if it's not corrected, sideslips can get our of hand. If the plane yaws too much, it starts to affect how the air flows over the wings. And if too much air flow parallel to the wings instead of perpendicular, the plane can lose lift... which is kind of important for keeping it in the sky. 

That's where the vertical tail comes in. See, as the plane starts to sideslip, the side the tail will start to face into the relative wind and become more perpendicular to it. And as the wind pushes on the tail surface, it creates a force that ultimately nudges the whole plane back to face its original direction. Birds meanwhile don't have to deal with all of this because they are way better at flying than we are. Which kind of makes sense — they've had millions of years to work on it. They don't need vertical tails for stability because they're constantly making fast, tiny adjustments to the shape and angle of their wings which lets them avoid sideslip. Right now, that's not something we can practically or easily do with airplanes, especially passenger jets. And even though there have been some planes without vertical tails, like the B2 Stealth Bomber, they come with plenty of stability challenges. So engineers are definitely working on it, but it looks like nature has us one-upped for now.

Rose: That figures-- birds have been flying for millions of years — of course, they're better at it. And there's still a ton they can teach us about flight. In fact, researchers are actively trying to apply what they are learning from studying birds and other animals to our flying contraptions. So here's Michael, with three ways nature is inspiring drone engineering.

Michael: Drones are awesome — but they could be even better. Like, they're kind of hopeless in smoke or fog, and they're just so big. I want a drone that fits on my finger like a bug. That kind of thing is exactly why engineers have been turning to nature to perfect drone tech. After all, animals have been flying for more than 300 million years, so they've got a bit of a headstart on us. And we can learn a lot from them about navigation, miniaturisation and even what the best colour would be for optimal flight. It's not hard to imagine a situation where it might be helpful for a drone to have an idea about what's around it.

Like, imagine if a drone could find its way through smoke-filled rooms in a burning building. It could help emergency workers save lives by scanning areas where people might be trapped. Smoke makes it pretty hard to see anything because the small particles bounce visible light all over the place. And it's not just smoke; fog, snow, dust, anytime you have lots of light-scattering particles, it can be pretty tricky to see where you're going. Even some radar gets stumped. So, to help drones navigate these conditions scientists are studying animals who "see" with more than their eyes. Echolocators like bats map their environment based on hwo long it takes a sound to echo back after they make it. 

Sound is a pressure wave, so it mainly moves through the air around particles instead of getting bounced. But while a couple of scientists have made drones based on bats, bats are pretty hard to mimic. They're smaller and more maneuverable than a lot of today's drones, and bats have evolved a ton of really specialized structures in their brains and bodies that are great for echolocation but aren't easy to recreate. So another team of scientists looked at oilbirds instead. These South American birds use echolocation to find their way around caves, but they're bigger than bats and their echolocation isn't quite so sophisticated. Plus, they use their talents more sparingly. When there's a lot of light, they mostly use their eyes. Then when it gets darker, they switch to their ears more and more. And that kind of switching between systems is exactly what you'd want in an autonomous drone, but it'll probably be a little while before any oilbird-inspired drones hit the market.

Researchers still need to figure out how the birds' brains combine all this information to create a single picture of the world, because that's what drones will need to do before we can let them fly by ear. Now, one of the upsides to oilbirds is that they're already about the same size as modern drones. So one of the big goals of drone tech is to go small. The problem is really small drones can't fly the same way that big ones do because physics.

Today's drones mainly have fixed wings like planes, or spinning blades like helicopters. But once fixed wings and blades get down around a few centimeters long or less, they stop generating the lift needed to stay airborne. Air just doesn't really glide around small things as it does big ones. Instead, air piles up and little, random differences in pressure from one place to another push objects around, destroying any lift they get from moving through the air in the first place.

But we know small things can fly, because insects exist. And researchers are learning from them how to miniaturize. Insects don't use rotary blades for wings, for example. And their wings don't stay still either. They're constantly flapping during flight, and the smaller they are, the more they flap. Flapping wings are just better for flying at small sizes. Where flying depends on pushing against the air instead of gliding through it. So engineers have built little flappy drones modeled after little flappy insects. They're still working on efficient batteries for something so tiny.

And they're still trying to make their little mechanical insects as stable as actual insects are. But they imagine a future where tiny drones can do things like precision pollinate crops. We could try to train bees to do that, of course, but it turns out they don't always listen when we tell them what to do. And who can blame them when they look so cool in those classic black and yellow outfits. Though bees do come in different colours and so can drones.

It turns out the different colours don't just make your drone look cool; studies on animals suggest they might actually help it fly. Right now, you can split drones into two broad color groups — black ones and not black ones. But they should probably all be black. You see, a lot of animals have what's called 'countershading'. They're light on the bottom because they make them look more like what's above and they're dark on top because it makes them look more like what's below. Or so everyone thought.

But experiments with models of birds, dolphins and orcas have all found that darker backs may be more than camouflage. Both in a ir and water, they seem to let the animals move with less drag because of how dark colors absorb and radiate heat. They heat up faster from sunlight and transfer that heat to what's around them, which makes the air or water they're in thinner and easier to move through. So if you want a drone that uses as little energy as possible to fly, you should get one that's the colour we all instinctively know is coolest — black. Also, if someone tells you that you wear too much black, just tell them you want to be more aerodynamic.

Rose: Bug-size drones would be pretty cool. But I wonder if we could find a way to mimic the flight of spiders instead. Yes, spiders fly — some of them. And the way they do it is amazing. They use electricity — Hank, tell us more.

Hank: So you're walking along minding your own business when you notice something out of the corner of your eye. You look up. That's when you see thousands upon thousands of spiders on long silk balloons falling from the sky all at once. You've just witnessed one of the most incredible and terrifying natural phenomena on the planet — 'spider rains'.

For a long time scientists assumed that just like kites, ballooning spiders can fly because their silken threads generate enough lift to ride currents of air. But according to a study published in current biology this week by researchers at the University of Bristol in the UK, they don't actually need a breeze at all. It turns out, spiders can fly using the electricity in our atmosphere! Spider ballooning was first documented by an English naturalist in the 17th century, and ever since, scientists have been trying to figure out exactly what they're doing and why they're doing it. 

A lot of the time, the ballooners are baby spiders looking for a place of their own to settle down. They can reach altitudes of almost five kilometers and fly for hundreds of kilometers. Talk about putting some space between you and your parents! But instead of loading up their Volvos and moving to Montana, to take off, these spiders find somewhere high up, then stand tall, raise their rears, and emit thin meter-long silk threads in the shape of a sail. When they let go, they're pulled into the air with surprising speed, even on calm days. And that speed is one of the things that's never quite added up with the idea that these spiders ride the wind.

Biologists have seen spiders ballooning when winds are almost imperceptible or even when it's raining! And the wind hypothesis doesn't explain how the spiders eject their silk so forcefully without the help of their legs or how the strands maintain a fan-like shape without tangling. So the team from the University of Bristol decided to test something no one else had - whether the spiders can ride electricity. 

The idea that electrostatic forces provide the necessary lift has been around for centuries, but no one ever really looked at it. Then, in 2013, a physicist from the University of Hawaii worked out some of the theoretical details. He released his paper as a pre-print that was never officially published, but the authors of the new study thought it was worth investigating.

The whole thing hinges around the fact that no matter what the weather is, there's a difference in electric charge between the ground and the sky that creates an electric field. So if the spider's silk picked up some of that static charge, those threads could be pushed by the electric field. Since like charges repel one another, the charge of the ground (or whatever the spider is standing on) would repel the silk out and up, and enough pushing could fling the spider into the sky.

But since that 2013 paper was purely theoretical, the new study's authors decided to put it to the test. They took ballooning spiders and placed them on a small cardboard pedestal in a special chamber designed to have no electric field or air movement.

Then, they induced electric fields of different magnitudes and watched what the spiders did. Even in the complete absence of wind, the spiders began to get into that rump-raising position that sets them up for ballooning. And with a strong enough field, they started to spin silk and even flew.

Once airborne, the researchers could make the spiders rise or fall just be turning the electric field on or off. An earlier study published just last month in PLoS Biology noted that these spiders seemed to test the wind with their legs before they start to spin their silk sails. And this week's study found that the hair on the spiders' legs moved in response to changes in electric fields too.

But those hair movements were different from the way they moved in response to wind. Which means the spiders might be feeling around for both of those things. Riding electricity could explain some of the weirder aspects of their flight, like how they take off on seemingly windless days, or in the rain, but most of the time, air isn't completely still, so the spiders probably use a combination of electricity and wind to fly.

There are still some parts of this left to figure out, though, like how the spiders' silk becomes charges in the first place, or whether they can control their flight to decide where to land. Learning more about how spiders fly can help biologists predict when they're going to do it, and get a better understanding of their ecological needs. And maybe it will make it easier to predict those rare episodes of spider rain, because I don't know about you, but if 10,000 spiders are going to land in my neighborhood, I would prefer to know that that's going to happen before it happens. 

Rose: Well, I know what I'm having nightmares about. Speaking of scary things, let's talk about flying mammals for a moment. We've already learned how their ability to echolocate could help us make smarter drones. Well, echolocation probably evolved after flight.

And after birds exploded in diversity and pushed bats to hunt at night. And it turns out that flight plays a pretty central role in another aspect of their lives: their immune system. Here's Stefan to explain what flying has to do with immunity. 

Stefan: One fifth of all mammal species today are bats. And that's pretty awesome because they help us out in all sorts of ways. Like, they pollinate a lot of plants, help regrow forests and control pests, and their poop is a pretty excellent fertilizer. Plus, they're just really cool.

Some of them can sense magnetic fields or use sound to find their food, and of course, there's one thing that they all have in common: They can truly fly. They're the only mammals around capable of powered flight. Without the help of machines.

But they're also somewhat notorious for something else: Being flying sacks of germs. Now you might have noticed that part, what with all the talk about zoonotic diseases that's been happening lately. Thsoe are diseases that are passed to humans from other animals.

And while bats aren't to blame for everything, they have played ar ole in the transmission of at least 11 viruses. Probably 12, counting SARS-CoV-2. These diseases aren't the bat's fault, of course.

If anything, they're ours. Research shows that disturbing animal habitats is usually what causes the transfer of a zoonotic disease to humans. Still, bats in particular do carry a lot of viruses.

And that's because they have unique immune systems. Which means we can learn a lot about these pathogens and their effects by studying bats. In fact, bat immune systems are so special that what we learn from them could someday help us treat a wide variety of conditions, from cancer to diabetes.

And the kicker is, bats probably have weird immune systems because they fly. Unlike gliding, flapping flight requires a huge amount of energy, so bats have evolved ways to kick their cellular fuel production into high gear.  Mainly by putting their mitochondria into overdrive.

Those are special compartments within cells that turn food into fuel. But there's a catch.

When mitochondria convert nutrients into energy, they also create by-products called reactive oxygen species. So basically, mitochondrial exhaust fumes in the form of really reactive molecules which contain oxygen. Now, these aren't all bad.

The immune system uses them to rouse cells to action and kill bacterial invaders. But they can also cause a lot of damage. They can weaken cell membranes, mess with proteins, and even break DNA.

And because of that, they play an important role in diseases like cancer and arthritis. Cells can try to keep them in check with antioxidants, compounds that essentially neutralize these overeager molecules, but those can only do so much. And when the balance gets out of whack, cells experience a condition called oxidative stress.

This is when most of the DNA damage happens. So, bats' supercharged mitochondria mean extremely high levels of oxidative stress, which in turn means constantly high levels of DNA damage. But since bats don't immediately get super-cancer after their maiden flight, researchers have long suspected that they've evolved ways to protect themselves from all this flight-related damage.

And a few years ago, genetic studies found mutations which boost their ability to detect and repair damaged DNA. Essentially, they've also turbocharged the mechanisms that prevent genetically damaged cells from replicating. Which may also explain why they don't seem to get cancers very often.

So bats produce tons of energy without damaging their cells. Sounds pretty awesome, really. There's just one small problem.

DNA damage can also be a sign of a viral infection. Because viruses need to hijack the cells' genetic machinery to reproduce, and that process usually involves some strategic snipping. So naturally, DNA damage triggers an immune response-- inflammation.

Essentially, when cells detect DNA damage or other signs of infection, they chemically call in white blood cells. These cells kill and destroy pathogens using a variety of genetic and chemical tools. And they also help control how the inflammation unfolds.

Like by bringing in additional white blood cells, or by switching some of them from germ-killing to tissue repair once the invasion is over. This immediate, or acute inflammatory response, helps to get rid of the invaders and promotes healing.

But remember, thanks to their supercharged mitochondria, bat cells experience constant DNA damage. They can repair this damage, thanks to those advanced DNA repair tools, but the damaged DNA should still light an immune flare before it's fixed. So bats would experience super inflammation all the time.

And prolonged, chronic, and systemic inflammation isn't so great. White blood cells, and the processes they set in motion, can be really destructive to the body's own tissues. In short, too much inflammation can lead to organ failure and even death.

So, flight should be a death sentence, except bats have evolved some neat ways to knock down inflammation too. For one thing, they dampen the activity of STING proteins. These proteins are one of the ways mammalian cells trigger an inflammatory response when a virus is detected.

Also, a genetic analysis of multiple bat genomes showed that they're the only mammals that completely lack genes for PYHIN proteins, another set of inflammation triggering sensors, activated by damaged DNA. And those are just part of the story. It will still be a while before we completely understand how bats prevent or dampen inflammation in their bodies.

Because it seems like every time they look, researchers keep finding more of these adaptations. So, to recap: We know that to make sustained flight possible, bats have ramped up fuel production and DNA damage detection, while dialing inflammation down to a 1. But we know that inflammation is one of the big ways the immune system fends off intruders, so doesn't that leave them open to all kinds of actual pathogens?

And the answer is yes. Around the turn of the 21st century, scientists discovered that bats act as a reservoir for a lot of viruses that are extremely dangerous to humans. This includes filoviruses, which cause hemorrhagic fevers like Marburg or Ebola, and henipaviruses, like Hendra and Nipah, both of which can cause fatal brain infections. And of course, coronaviruses.

Like SARS, MERS, and most likely, the notorious new coronavirus that started the COVID-19 epidemic. And there's mounting evidence that bats were involved in transmitting these diseases to humans, either directly or indirectly, like by infecting farm animals.

And bats are also suspected of having given us other diseases in the past, like mumps, measles, and hepatitis B. But here comes another magic thing about bats. Even though they're widely infected with notoriously deadly viruses, they don't actually seem to get sick from them.

The virus can be found in their bodies, but they don't have any symptoms. As for how that's possible, well we still have more questions than answers. But researchers have discovered a few evolutionary quirks about bat immune systems that help make that happen.

In part, that's because active viral infections in bats tend to be pretty short lived. Thanks to their hyper-vigilant interferon production systems. Remember how we said DNA damage is a signal of infection?

That's because viruses try to reprogram cells to create copies of themselves. But cells aren't sitting ducks during this process. In addition to calling out for help, which is that whole inflammation bit, that we discussed, cells have an internal defense mechanism.

They can make a protein called interferon alpha, which activates genetic and chemical tools to reduce the virus's ability to multiply and spread. Every other mammal we know of switches their interferon system on when an infection occurs. But genomic studies suggest bat cells always have their interferon alpha genes activated.

This drastically cuts down the time is takes to react when a virus is present, so it allows bats to nip the infection in the bud before it becomes a full blown disease. And, of course, there's more. All mammalian cells contain an enzyme called ribonuclease L, which when activated, chops up viral RNA to stop a pathogen from spreading.

But in us and most other mammals, activating this enzyme takes a complex chain of steps, so it's not super quick. But bats on the other hand can activate it directly with interferons, drastically speeding up infection containment. Scientists also think this fast activation of ribonuclease L helps bats outsmart viruses that have evolved to inhibit the enzyme before it can be switched on.

Something HIV does in humans, for example. So basically, when bats do get viruses, they're able to quickly clear them out for the most part.

They can still fall prey to at least a few, like the rabies virus. And even quick suppression of a virus can mean it stays around in a group of bats. Bats tend to huddle close together when they roost.

Plus they fly around in the same areas as bats from other colonies, and they're constantly spraying snot and saliva everywhere when they echolocate. So there's a good chance that during that window when a bat has a virus that is actively replicating, it will spread it to another bat. That means that even if each bat only hosts a virus for a short time, it can linger in the population.

Even this probably isn't the full picture. Mathematical models indicate that by themselves, bats' social habits don't completely explain why they host so many viruses. Instead, research suggests that the viruses themselves have figured out how to lie dormant and undiscovered.

Like in the bats' lungs, spleens, or intestines. And then, when the bat gets stressed, like when it's roused from hibernation, that stress temporarily dampens its anti-virus systems, allowing any hidden viruses to emerge. This leads to another period of increased viral replication and shedding, so again, the bat can transfer the infection to other animals.

Then, its anti-viral systems get back up to speed, and the viruses are eliminated or driven back into hiding. Still, even when a virus is replicating and being shed, bats don't generally seem sick. Not like a person would with the same virus.

And that's likely because their inflammation dampeners are still running. So they're still suppressing a lot of the immune responses that would make them noticeably sick. And that may also be why viruses that are deadly to us aren't lethal to them.

It turns out that the most severe symptoms of illnesses like MERS and Ebola, the symptoms usually responsible for the lethality, aren't caused by what the virus itself does to the body.  Instead, they're the result of the destructive, catastrophic inflammation the virus triggers.

This includes an extreme systemic reaction called a cytokine storm, where an over-release of pro-inflammatory signaling proteins turns a person's own immune system into their worst enemy. And some researchers think that our aggravated inflammatory response may actually be because these viruses have evolved to dodge the super-refined immune systems of bats.

Basically, they evolved to survive in a host that constantly and aggressively attacks their ability to replicate. So when that assault is suddenly weaker in a new host, they go wild and produce a lot of little virus babies that send the host's immune system into panic mode. This kind of overreaction can also happen to bats, it's just not usually in response to viruses.

Instead, it seems like their unique immune system may leave them vulnerable to non-viral invaders. The most infamous example of this is white nose syndrome, a fungal pathogen which has devastated bat populations in North America. Some researchers think the bats' dampening of inflammation, especially during hibernation, makes it easy for the fungus to infect the bats.

Then, once the bat awakes, its immune system does react, only it goes too far. Which can lead to the bat developing a life-threating form of systemic inflammation. So basically, the reason they die from white nose is why we die from viruses like MERS and Ebola.

That may mean that the key to saving bats from this fungus, and us from the deadly viruses they carry, may lie in further research on bats. And also, if we're being selfish, we have a lot of other diseases characterized by inflammation like heart disease and diabetes. So studying their inflammation system may lead to treatments for out chronic conditions.

And the same goes for studies on the ways that bats keep viruses at bay. Right now, we only have effective antiviral drugs for about 10 of the more than 200 viruses that can infect humans, and very few broad-spectrum antivirals. If we can discover more of bats' tricks, we might be able to use them to develop therapies against the disease they host and other dangerous viruses. Plus, all this research might help us live longer.

Many researchers think bats' immunological adaptations are also behind some of their other superpowers. Like how they seem to rarely get cancer, or how they live incredibly long lives for animals of their size.

Usually, little animals live fast and die young. But Brandt's Bats can live for over 40 years, even though they only weight 4 to 8 grams. So instead of looking at bats as species zero, we should think of them as flying keys to longevity and resilience.

And in the end, they're going to be our allies in health, not our enemies. 

Rose: We've now talked about bats, birds, and even weird ballooning spiders. But there are snails that fly too. Underwater, anyway. But it's still really cool, and totally counts as flight.

Since both air and water are fluids. Hank here can explain.

Hank: Birds fly, fish swim. We learn this when we're two. But of course, things in nature are never quite so simple. One creature that muddies the water, so to speak, is Limacina helicina, an Arctic-dwelling species of sea butterfly-- a type of mollusk.

Rather than swim like most other aquatic life, this creature seems to fly through the water with tiny wings. And scientists studying these wing movements noticed something interesting: they looked remarkably similar to the wing movements of hummingbirds and fruit flies. But why would you need to fly in water?

Isn't flying different than swimming for a reason? Well, the answer lies in a concept from the field of fluid dynamics called the Reynolds number. The Reynolds number is used to predict how a fluid behaves, based on the speed of a movement and the physical properties of the fluid.

A high Reynolds number describes fast, turbulent movements, such as humans swimming through water, while a low Reynolds number describes slow, smooth, or laminar movements like that of this sea butterfly. But what's really interesting is that the Reynolds number for these sea butterflies flying in water closely matches the Reynolds number of fruit flies flying in air. Because while water is denser and more viscous than air, sea butterflies are bigger and slower than fruit flies, so from a fluid-dynamics point of view, the numbers work out about the same.

This means that the water and the air behave similarly when these two creatures generate lift through their flying movements. But, why fly at all?

Why not just swim like most of the other self-respecting ocean dwellers? Well, like many things in nature, this unusual behavior may be motivated by the need to feed. Sea butterflies feed by deploying a large, spherical mucus web that captures plankton.

As you do. While feeding, this sea butterfly is neutrally buoyant, it will continue to drift in the ocean currents, neither floating nor sinking. However, if disturbed, it will retract into its shell and sink.

It's no longer neutrally buoyant, so it can't just swim back to where it needs to be the way many fish would. To make its way back to where the food is, it flaps its wings to generate lift, until it can re-deploy its web. This is a great example of convergent evolution, when two different creatures with only distant relation to one another develop similar behaviors to adapt to similar environments.

In this case, sea butterflies and fruit flies both evolved to use flight mechanisms that manipulate the fluid around them in similar ways. Even if the fluids themselves are completely different. Maybe we should start calling sea butterflies "sea fruit flies." Then again, maybe not. 

Rose: Could we play that clip of the flying snails again? They're just so adorable. I could watch those little flappers all day. But that's it for our dive into the weird world of animal flight.

Thanks for watching this SciShow compilation. If you enjoyed it, and you want to keep learning about stuff that flies, check out our compilation about the coolest birds ever. Or, our whole playlist.

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