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If wheels and rolling have proven so efficient for humans, why hasn’t evolution pushed at least some other species in that direction? Well actually, there are a few species that can get around by rolling.

Hosted by: Stefan Chin

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Sources:
https://www.researchgate.net/publication/289523168_Spiders_wheel_to_escape
https://envs.dartmouth.edu/sites/department_environmental_studies/files/dartmouth_in_nambia_2016.pdf
https://blogs.scientificamerican.com/running-ponies/its-not-funny-anymore-golden-wheel-spider/
https://www.nature.com/articles/s41598-020-59954-9
https://www.nytimes.com/2020/03/13/science/rolling-ants.html
https://www.gbif.org/species/1315215
https://www.nature.com/articles/37253
https://ukmoths.org.uk/species/pleuroptya-ruralis/
https://www.gbif.org/species/1879252
https://asknature.org/strategy/rolling-locomotion-aids-escape/
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0017746
https://www.crd.bc.ca/education/our-environment/ecosystems/coastal-marine/intertidal-zone
https://plants.usda.gov/plantguide/pdf/pg_satr12.pdf
https://www.cell.com/current-biology/pdf/S0960-9822(18)30289-6.pdf
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https://www.journals.uchicago.edu/doi/10.1086/284068
http://www2.estrellamountain.edu/faculty/hoffman/commonfiles/bio182/whydon%27tanimalshavewheels.html

Images:
https://en.wikipedia.org/wiki/File:Wasp_and_spider_02.jpg
https://www.antwiki.org/wiki/File:Myrmecina_graminicola_casent0008643_profile_1.jpg
https://en.wikipedia.org/wiki/File:Mother_of_pearl_moth_(Pleuroptya_ruralis).jpg
https://www.flickr.com/photos/usfwsnortheast/5951127039/
https://www.flickr.com/photos/plant_diversity/4050282814/
https://www.flickr.com/photos/plant_diversity/8186664746/
https://www.youtube.com/watch?v=pEbKIpI7Bp0
[♪ INTRO].

If you have somewhere to go, there's a good chance that you're using wheels to get there. Cars, trains, bikes, and skateboards… I mean, even planes have wheels.

Wheels, and rolling in general, can be an extremely efficient way to get around. But for as widespread as wheels are in civilization, they're surprisingly rare in the natural world. But that doesn't mean that there are no species that can get around by rolling.

Today let's look at five of them. Let's start on the southwestern coast of Africa in the Namib, a desert whose name literally means “vast place.” In this 800 thousand-square-kilometer sea of sand lives the golden wheel spider, whose body is only around two centimeters in size. The wheel spider is what's known as a huntsman, a family of spiders known for hunting prey on the ground instead of using webs.

And rather than living up in a tree or a plant, it burrows into the sand to form its nest. Now, you'd think that would offer good protection from its main predator, the spider-hunting wasp, but these wasps can excavate up to ten liters of sand to dig out their prey. Once it's exposed, the wheel spider has to act quickly to save itself.

And fortunately, on the steep sand dunes of the Namib, it has gravity on its side. The spider begins its escape by running downhill. If the slope of the dune is greater than 15 degrees, after it picks up a little speed, it flips onto its side and tucks in its legs to form a rough wheel-shaped structure that's able to roll.

And once these spiders get going, they can really move. One study observed them rolling at speeds up to one-and-a-half meters per second. And rolling isn't just a fast way to get away, either.

It also turns their body into a blur and makes it harder for the wasp to land a sting. Finally, after making an escape, the golden wheel spider has little trouble finding a new place to burrow in the endless sand of the Namib. Another species that can get moving quickly, at least relative to its usual pace, is a type of ant known as Myrmecina graminicola.

Myrmecina is about the size of a sesame seed, and it can be found in grasslands and forest floors throughout Europe, North Africa, and parts of Asia. In small colonies of just a few hundred members, these ants scour their habitat for food. Which is where they run into their primary predators, which include spiders and other species of ants.

But when they find themselves in trouble, they're known for making a quick rolling getaway. Like the wheel spider, their roll is gravity-powered, meaning that conditions have to be right for it to happen. When researchers recreated attacks in the lab, they found that while some ants could begin rolling on inclines of only 10 degrees, others required up to 25 degrees before they would tuck into a roll.

When it did happen, though, it could happen fast. On average, stressed ants took around half a second to go from walking to rolling. In that half-second, they would tuck their heads under their bodies and then push up and forward with their hind legs, a lot like you might do if you were trying to somersault.

Which I'm not going to demonstrate. And with the right incline, they could get going at speeds of almost 40 centimeters a second, which is nearly 80 times faster than their usual walking speed. The catch is, they don't go very far.

On a hard surface, the biologists observed them only going up to 17 centimeters in total. And on softer materials, like soil, they might only roll a few centimeters. But still, the maneuver is unique among ants, and it allows this one to move much faster than its common predators and buy itself some time to find a safe place to hide.

Now, another creature that can't roll far but can get there quickly is the mother-of-pearl caterpillar. As an adult, the mother-of-pearl moth is pretty mobile. You know, because it can fly.

But like many larvae, the caterpillar is slow-moving. It can be found munching on nettle plants all over Europe. But when it runs into trouble, the caterpillar escalates through a series of responses to back away from the threat.

Ordinarily, it walks with the classic “inchworm” movement, and when it encounters a mild threat, it just reverses its direction, lifting its hind segments first to go backwards. If the threat is more urgent, it can start doing what one study described as “galloping” backwards. Basically, it just speeds up the process of walking by thrusting its whole middle section up at once.

But in response to the most extreme stresses, the mother-of-pearl larva can curl up and begin to roll. The act of rolling begins the same way a gallop does, with the caterpillar flinging much of its body into the air. But then it curls its tail underneath itself, rolling up until only its front legs are still attached to the ground.

Finally, those legs let go and it starts rolling backward. It uses the momentum from throwing itself backward to get the roll started. That means that, unlike the spider and the ant in the last two examples, the mother-of-pearl doesn't need an incline to get rolling.

But it also means that it doesn't go very far. In fact, most rolls end after only around six revolutions, which, for an animal of this size, is only around a few centimeters. But it crosses that distance in a hurry, traveling nearly 40 times faster than it could normally walk.

Now, another insect larva that can roll without the help of gravity is that of the Southeastern beach tiger beetle, which is about a centimeter long and has evolved to roll with the wind. These larvae are native to the East Coast of the U. S., where they live under the sand in the intertidal zone.

That's the part of the beach that's underwater at high tide, but exposed to the air at low tide. And like the wheel spider, scientists suspect tiger beetle larvae are threatened by a species of wasp that can dig down into their sandy burrows. And they're pretty vulnerable, because while tiger beetles themselves are some of the fastest insects out there, their larvae are definitely not.

They have a few tricks for escaping predators, but in really dire circumstances, they can initiate a special escape sequence to get away. They start by arching their head backwards towards their rear to form a tiny loop. By flexing this loop, they can spring themselves into the air to take advantage of something that's almost always present in their seaside habitat: the wind.

Studies seem to show that if the wind catches them midair, they can start to roll once they hit the ground. But even once they're in motion, the larvae aren't entirely passive:. They can hold their legs out to the side to improve their balance and nudge the ground with their tail to keep up their momentum.

Still, it's the wind that's the driving force and it comes with a neat side effect:. Young tiger beetles can travel uphill. That's almost unheard of in species that roll, but it's especially important for these larvae since beaches often curve up and away from the intertidal zone.

And these larvae can really get going! With moderate wind, they commonly travel 10 to 25 meters, but in windier conditions, one study observed them going as far as 60 meters. And some even reached a speed of several meters per second.

Finally, we have a rolling species that's a little bit different from the others. It relies on rolling more than anything else we've seen today, but it only does so after it's dead. Of course, I'm talking about tumbleweeds, which aren't actually a single species but a group of organisms that have all evolved a similar strategy.

One of the most successful examples is the Russian thistle. Although it probably originated in Russia, it's been accidentally introduced in the Americas, Africa, Asia, and Europe. And in most of those places, the thistle has thrived.

Like, after making its way to the U. S. in contaminated grain shipments in the 1870s, it has since spread to every state except Alaska and Florida. Basically, anywhere there's a flat, dry environment, you can probably find a Russian thistle.

And that has a lot to do with the fact that these plants can move. The tumbleweeds begin as plants that grow in a round-ish shape up to a meter high and two meters across. As they die in the late summer and early fall, the part above ground dries out.

And the base of the stem contains a special layer of cells called abscission cells, which can break easily and cleanly detach the plant from its root system. That severed bushy part can be covered in hundreds of thousands of seeds. And now that it's not anchored to the ground, when the wind catches it, the dead plant begins its iconic roll.

All the while, the seeds, which are shaped like tiny hooks, can catch on things and break off. So by slowly spreading their seeds over long distances,. Russian thistles usually end up spread out enough that they don't compete with one another for resources, which makes them both extremely successful and also kind of a nuisance, since they can quickly crowd out native plants.

So if you're ever on the prairie and see a bushy plant where it doesn't seem like it belongs, you might have a unique roll to thank for it. The common thread among all these kinds of life is that, while they can roll, that's not their main way of getting around. Which seems a little bit strange.

I mean, if wheels and rolling have proven so efficient for humans, why hasn't evolution pushed at least some other species in that direction? Well, the key is that wheels aren't always that efficient. Imagine, say, trying to push your shopping cart through the grass, or riding your bike on a sand dune.

You're probably not having a very good time. Wheels work for us because we've built roads, rails, and sidewalks to make them efficient. Most other species aren't moving around on those types of surfaces.

And there are physiological problems to deal with, too. If you're an animal with a wheel, that wheel, and the axle it spins on, would probably be made of some kind of living tissue. But how could they be connected with blood vessels to provide the cells with energy and remove their waste?

Those blood vessels would be twisted up in no time. Besides, it seems like it would take a pretty large evolutionary leap to get from legs to wheels. If it is possible for a bunch of small adaptations to eventually result in a wheel as a primary mode of transportation, well, it doesn't seem like evolution ever found its way there.

In a way, all of this makes the rolling species that we do have seem even more impressive. So rolling might not be a common way of getting around, but now and then, a surprise move can be a good survival strategy. And what's more surprising than a spider doing cartwheels?

Thanks for watching this episode of SciShow! And thank you to our patrons on Patreon for making videos like this possible. If you'd like to join the amazing community of supporters helping us make science education free on the internet, you can find out more at patreon.com/SciShow. [♪ OUTRO].