# Naturally occurring wheels - do the 'mech' vs. 'tank' comparison apply to organics?

There have been several discussions about mechs, and the general consensus is - whilst cool, they're not actually a very good design.

Plausible Reasons for usage of Combat Mecha

What I was wondering though - how much of this discussion applies to organics?

First off: What ways - if any - could wheels have been evolved? As far as I'm aware there are no examples of rotary motion in a living creature - whilst we can 'windmill' our arms, that's multiple joints moving to create that effect.

Is there any evolutionary branch that might have done this?

I mean, it's only relatively recently that we've even got circular impulse - trains and the like are 'driven' via a linear piston, as are combustion engines. (Unpowered of course - wheels are pulled or pushed by things on legs anyway)

Rotary engines and electric motors are close, but they're still a linear impulse around a circular axis. So it sounds like the evolutionary key here is a 360-degree pivot because then you can translate linear to circular motion. It's also something that doesn't appear to have ever evolved.

For the sake of argument - if some fluke caused 'wheels' to evolve somehow - which branch would likely have won, evolutionary speaking? Do the same square/cube laws that make the mech worse than the tank also apply to evolved species, or is the efficiency of rotary motion a major positive factor?

• Related because rotary motion was heavily discussed in it: worldbuilding.stackexchange.com/questions/15278/… – Aify Jul 15 '15 at 15:40
• Relevant... – Mason Wheeler Jul 15 '15 at 17:29
• I feel like rotary motion evolving in a creature would require some sort of bio-magnetism...which is a seriously cool idea... – James Jul 15 '15 at 19:22
• If wheels don't work, you might consider making something like a droideka. Armadillos can curl into a ball. It seems remotely plausible that an armadillo-like creature could evolve a primitive internal shifting counter-weight to hop/roll away from predators that are otherwise willing to out-wait it. – acbabis Jul 15 '15 at 20:17
• – Kevin Krumwiede Jul 17 '15 at 2:45

Some microscopic animals have a wheel like structure that drives a flagellum.

The issue with a natural macroscopic wheel is that the wheel and axle unit is basically not attached to the rest of the animal. If it were, the blood vessels would be torn off or spun into a knot. Therefore it can't get nutrients to the wheel part, and it can't grow, or be alive for long.

• Maybe if the wheels and axle were almost a separate organism that rode in some kind of nutrient, lubricating fluid. – Zan Lynx Jul 15 '15 at 20:13
• @ZanLynx, that's a very interesting idea. Symbiotic animals where one is the wheel and one is the chassis. – Green Jul 15 '15 at 20:48
• @ZanLynx - well, in that case you could have the organism spit food into the mouth of the wheel at mealtimes. But it would be quite a case of coevolution to produce a roller and rider together and have them pair up at birth. – Oldcat Jul 15 '15 at 22:01
• @Green If you read the golden compass trilogy, in the second book there are animals which use large hard nuts shaped like wheels from trees. They coexists and transport the wheels very far away, when they break they leave them there and new trees grow. Symbiotic relationship between animal and plant. ( They also have legs for climbing, but are much faster on the wheel ) – Falco Jul 16 '15 at 9:42
• Surely the wheel could be chitin or hair or something that's grown, and then separated? Like pangolin scales or horns? – RemarkLima Jul 17 '15 at 17:18

The problem with wheels vs legs is that wheels do not cope well with uneven terrain. They sink into mud, stick on branches, fall into holes, etc. They are also very limited use for climbing, jumping, and all sorts of other similar activities.

The areas where they do have an advantage is in energy usage and speed when travelling over large flat areas. With artificial wheeled vehicles we manufacture roads and paths for them to use, in nature that is much harder to do. Even where you might have established paths for certain routes you will need to leave those paths in order to forage, feed, hide, etc.

Even large open grassy areas are unlikely to be flat enough off the path, so really you are limited to environments like deserts and beaches. Interestingly that is where you find tumble-weed and sidewinders in our world, which both have some elements of circular motion.

Basically the issue from an evolutionary standpoints are:

1. There is no advantage to having a "partial" wheel. Unless you can find a route from A to B with an evolutionary advantage (or at least no disadvantage) at each step then it is unlikely to evolve. This is the problem of local maxima.

2. Wheels are really good in some situations, but utterly useless in others. If a creature needs both legs and wheels then it might as well just use the legs.

3. Legged creatures can travel long distances efficiently (see ostrich), travel very rapidly (cheetah being the obvious example), climb (monkeys), swim (otters), run, walk and even fly. There is no clear advantage to be gained by having wheels. For example even a wheeled creature is unlikely to be able to outrun a cheetah.

One of the main reasons our vehicles can travel so far and so fast is not the use of wheels, it is the fact that they are using fossil fuels or other stored high density energy. Just using wheels alone does not solve that problem, after all a human cyclist is faster than a human runner - but they are still not as fast as a car.

• Buses must be really slow then, since people are always catching them. :) – Frostfyre Jul 15 '15 at 16:21
• If both legs and wheels are useful, you should have both legs and wheels. See, for instance, the Ghost in the Shell Tachikoma, "They can move by walking, or they can drive at high speed by using the wheeled footpads on each of their four legs.". See en.wikipedia.org/wiki/Tachikoma – Rex Kerr Jul 15 '15 at 19:51
• @RexKerr But not if the extra cost of having both legs and wheels outweighs the advantages. Legs can do things that wheels cannot, so unless wheels make you much better at one thing then having both is wasteful. – Tim B Jul 15 '15 at 19:53
• @TimB - Naturally. But there are plenty of shoreline-specialists already; flightless wheeled ones would be able to escape predators and search wide regions of beach much more efficiently. So the "can't get there from here" problem seems primary, even if wheels wouldn't be a very widely-found feature. – Rex Kerr Jul 15 '15 at 20:04
• I've actually been thinking about this recently, because without roads it's practically impossible to get from A-to-B in a car/truck. Heck, even if you look at a place where there are roads, like a big interchange, it's really hard to get 100' across unless you start a mile back where you had to make your decision. But, assuming no cars turning you into Frogger, it's actually relatively quick to traverse that distance by foot. – Wayne Werner Jul 15 '15 at 21:33

The problem is creating a wheel and axle which can have no connections of blood vessels or nerves.

The work around is to create a composite organism in which each part is a separate organism. The wheel is a separate organism, the axle is another, the suspension another, the chassis another, and so on. Each organism has its own lungs, digestive system, neural system, etc. Since their systems don’t connect, they can spin relative to each other.

A good model for such a creature is the Portuguese Man-o’-War, where each apparent individual is actually a vast colony of individuals, all clones of the original fertilized egg. Each individual forms a different specialized sub-system. They breath separately, the creature has no circulation system but just kinda shoves bits of food around in an internal void. No centralized nervous system but instead a kind of cascade activation from an individual module to the ones nearby.

But, as noted, wheels aren’t really useful without smooth surfaces. Look how few pre-industrial militaries relied on wheeled weapons. Instead, they marched or rode horses. Wheeled weapons like cannons where massively unwieldy, and not only because of their weight. You’d have to have a specialized environment.

I recall a short story decades ago in which a quirk of DNA caused plants to develop curling proteins which caused all plants to form in loops and to be ground hugging. The entire planet would covered in ankle trapping loops. Animals on the planet slid like snakes or rolled like balls to avoid being constantly entangled.

Do the same square/cube laws that make the mech worse than the tank also apply to evolved species, or is the efficiency of rotary motion a major positive factor?

Actually, nature prefers large animals particularly in mammals. There is a scaling law, whose name escapes me for the moment, that says that and the energy usage of a mammal scales with 75% of the increase in mass. In other words a ton of elephant uses only 75% as much energy as a ton of mice.

The average size of mammals used to be nearly twice what it is now until we showed up with our pointed sticks. Unlike any other predator, Humans can inflict deep puncture wounds, such a wound can kill the largest of animals if it hits a major artery. Pygmies still hunt elephants this way, they just slide up and poke a 1cm hole in the jugular then follow the beast around until it drops. Since its not all that much harder to kill a elephant than a rabbit, early modern humans hunted big game almost exclusively. The only defense against humans is to grow smaller with more individuals, forcing the humans to expend more energy per unit of energy returned.

Study the giant megafauna Rhinos, they stood twice as tall and three times as long as an African elephant.

So, wheels or not, there would be considerable advantage to growing very large.

Come to think of it, a combination of wheels and legs could be very useful on giant critters. The wheels would distribute the weight and the legs would provide motive power by pushing or pulling and maybe occasionally climbing. That would avoid the problem of powering an axel.

• IIRC the "His Dark Materials" trilogy had a race which evolved something like this. They used some kind of seed pod as the actual wheel, and had developed limbs with big hooked claws to serve as the axle/suspension. They had a second set of limbs that they used for propulsion, and could get up to some pretty impressive speeds. The tree the seed pods came from benefited from the arrangement since its seeds were spread as far as the creatures traveled. – thatgirldm Jul 17 '15 at 3:28
• Kudos for the composite organism, that is exactly what I was thinking. The short story you mentioned is also a very clever way of doing it because evolution, after all, is brought about by necessity. Imagining such a creature in our own world doesn't make much sense, but in a world with such complications it becomes very believable. – thanby Jul 17 '15 at 15:36

I'm thinking that it might make more evolutionary sense for the entire creature to be a wheel.

It would roll along, using muscles to deform the wheel shape to provide thrust. It would have some kind of mouths on the outside of its wheel to feed itself.

Alternatively, it could be a more regular kind of creature that rolls itself into a wheel and unrolls to eat.

Something like a large version of a pill-bug.

A lot of answers have focused on the problem with attachment of blood vessels, etc; I'm not sure that's a big problem.

I am envisaging a freewheeling creature that grew a wheel out of dead antler-like material around an axle; the growing point would periodically thin enough to snap, at which point the wheel would start spinning.

The wheel would be used until it wore down and sloughed off, at which point the process starts again.

The process could be linked to a seasonal event, such as a regular drought that exposed a very flat lakebed every year; possibly as a time-critical route to a spawning ground or something?

It could scoot itself along quite nicely, or if you wanted to go totally out there then creatures that started to grow off-centre nodules on the wheel would be able to pump them like a unicycle and get along even faster, so could be selected for.

There are quite a number of types of wheels; I've been able to come up with five types:

1. Free-motion wheels: these are round wheels with no driving force of their own. Instead, the wheel spins freely on a fixed axle. The rear wheels on a front-wheel-drive vehicle are like these. Multiple wheels would spin independently.
2. Locked wheels: like the free-motion wheels, these have no driving force, but rather than spinning on a locked axle, these wheels are directly connected to the axle, which spins freely. Many children's toys have a system like this. Multiple wheels on a single axle would all turn at the same rate.
3. Axle-drive wheels: these are the driven version of the locked wheels. Rotational force (torsion) from the axle causes the wheel(s) to turn.
4. Direct-drive wheels: each individual wheel has a drive system internally. Thus, each wheel moves completely independently of other wheels. Large earth-moving equipment often has wheels like this.
5. Treads: rather than a fixed round shape, trades are a flexible strip which is "pushed" by an internal drive system, usually wheels.

## Free-motion wheels

This option is fairly possible, though would result in some strange creatures (see diagram). The actual drive system could be legs, claws, or even a slithering motion; the wheels would simply reduce sliding friction, and possibly remove the creature from a dangerous environment, like caustic or hot surfaces.

The creature would need a fixed bone that protrudes from each side. The bone would terminate in a sphere. An organ just above the bone would excrete a cementing substance, similar to the oyster's pearl-forming system. It would coat the sphere, gradually growing larger. The sphere (and attached bone) would excrete a lubricating liquid that would also keep the area clean.

Here is a creature, viewed from the front (so cute!):

 ___    ___    ___
/   \  /o o\  /   \
) O==={  _  }===O (
\___/  \/ \/  \___/


Small amounts of damage would be repaired, while large damage would result in the wheel breaking off and being reformed.

## Locked wheels

This may be even easier to design; a creature with a soft, slippery underbelly would find round branches, rocks, or anything else shaped like a cylinder. It would lay on top of the object, pinching it slightly, and excrete a slippery substance. As it dragged itself forward, the cylinder would roll on the ground. These wheels would allow the creature to have a small, low-energy means of propulsion, but still grow quite large; it could also float, assuming it uses trees rather than rocks. Of course, it doesn't work well on land that isn't terribly flat.

 _/
/,\     _________________
|_ \___/  __       __    \
\ ____//  \\___//  \\__/
//     \__/     \__/


As it grew, it would have to find bigger and bigger cylinders, which may limit growth. Then again, a few well-placed fallen trees, and this creature may end up the size of a house!

## Axle-drive wheels

This was the drive method I had the most trouble with. The best I can come up with is a system like the locked wheels above, except that the "feet" that push the creature along push on the center of the wheel, instead of simply sliding. That would allow the creature to have tiny feet and a flabby body, but still get around with ease.

 _  _______ ________
/o\/ ____  V  ____  \
>   ||||||   ||||||  \
\__//  \\___//  \\__/
\__/     \__/


## Direct-drive wheels

These are actually pretty easy to imagine; a slug-like creature finds (or builds) a large hollow cylinder. It then climbs inside and "walks", pushing the wheels forward. The wheel not only protects it against predators, it also allows it to achieve very high speeds going down hills!

    ====
//    \\
//     _\\
|| i_  |  ||
||/  \_/  ||
\\      //
\\====//


Similar to the direct-drive wheels, this creature is fairly slug-like. It excretes a thin, slippery lubricant. When it has no wheel around it, the lubricant dries into a sticky, rubbery film. As the slug moves, it pulls the film from its back, across its front, and underneath itself. The film picks up rocks, sticks, and other debris as it is pushed onto the ground. At the read end of the slug, it flips the film up, where if sticks and becomes a rubbery tread. The slugs can use this tread for protection, both from enemies and from a hostile environment (hot or dry ground, salt or other traps, etc.).

 /#########\
|#/,\_/    \#\
\#\_____/\__\#\
\############/


## Would it work?

The biggest disadvantage to wheels in nature is that wheels sink, get stuck, or otherwise slow you down. Wheel are terrible for climbing. Walking, slithering, and burrowing are all more reliable means of transportation. If wheeled creatures "won out," there would need to be a good reason. Large, flat spaces, with ample materials and good reason to have wheels. Of those I listed, the slug-in-a-tube is most likely the best possibility. It's not hard to tunnel out a branch or wrap a leaf into a tube, and if it protects its inhabitant, all the better!

• Very good job on these examples, and I love the ASCII illustrations to assist :) I hadn't even thought of an organism building its own shell or tread but it definitely sounds plausible, even in a world not too different from our own. – thanby Jul 17 '15 at 15:56

# Lizards

My speculative guess is that it could evolve from the sub-group, Lacertalia, which is its own evolutionary grade. Some lizards' mode of locomotion resembles wheels already, such as the racerunner (but only visually), and they have provide the proper joints for a full circular mode of movement.

How (and why) can they evolve to have wheels?

Pure speculation, but I could see a more successful (faster and more agile) lizard eventually getting multiple legs from the same joint until their four appendages appear to look like wagon-wheel spokes without the rim (just the spokes with feet at the end.

Eventually this from this new species evolves a webbing of the feet, in order to navigate moist climates and even walk on water. As these webbed feet evolve ("grow") into each other, you now have four wagon wheels. Feel free to close in the gaps between each spoke of the wagon wheel until you have one cool lizard species.

You wouldn't see "branches" with one winning or the other. Evolution is not proud. It will do whatever it needs to.

It is remarkably difficult to create a wheeled or treaded system with organics, primarily because it is hard to connect the two parts sufficiently for organic growth. As in the answer Aify linked, you would end up with two symbiotic halves of a creature or group of creatures, each of which woudl need its own digestive tract, sensor mechanisms, etc. In this case, there would be a strong pressure to be able to survive without this delicate symbiosis. Both halves would likely seek to be mobile on their own, without the wheel. Thus evolution would combine both patterns into one comprehensive tool which handles both modes of locomotion.

I would argue that nature may have actually done this... if you argue our factories exude tires and hubs, and the most intelligent species on the planet uses them for 99% of their distance traveled! It all depends on how you look at it! ;-)

Wheel is not easily compatible with another device heavily used by nature - a pipe. Blood vessels are pipes, but also nerves.

A pipe cannot connect a stationary chassis to the freely rotating wheel, with both ends being fixed. Such pipe would get twisted (one end turned 360 degrees with relate to another as soon as the wheel makes a full revolution). While humans are able to resolve this technical problem (some military vehicles supply compressed air to they rotating tires, a measure against small punctures), this requires mechanically rather complex device, evolutionary challenging to evolve.

This is not a problem if the whole organism is a wheel (there is no chassis). Such organisms exist. Alternatively, a wheel could consist of dead tissue (like hairs or nails), no longer connected to the organism by vessels. Such maybe could be useful but without regeneration, the duration of the service is limited. Ok, it could be a short-lived organism. Maybe such will appear in the evolutionary future.

to create circular motion you don't need actual wheel shape body parts . a simple joint that lets conical motion is sufficient. actually lizards use this kind of motion already. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1273693/

it wont sink ,wont get stuck,and wont be single purpose. actually wheel is used by us because its so simple to make and maintain. i see the future of our land vehicles as legged machines.

The Mulefa from His Dark Materials are an example of a life form evolving to use wheels. In their case, the wheels are not part of themselves, but rather are formed by another symbiotic organism.

The Mulefa evolved with a diamond body plan, one limb in front, one in back and one on each side. On their world, there were numerous lava flows, creating long smooth surfaces. There were also large trees that had ~1 foot diameter circular seed pods. These seed pods had a hole in the center from where they were attached to the stem of the trees.

At some point the Mulefa found that they could hook the claws on their front and back limbs into the holes in the seed pods and then use their side limbs to kick themselves along the smooth lava flows. When a seed pod cracked, the Mulefa would discard it, and new the seeds would sproute.

As harder seed pods would be used longer, and would be dropped farther away, the genes that produced harder seed pods were spread farther and produce more seed pods. Over time the seed pods became so hard that they required many months of use before they cracked and could sproute.

The Mulefa also became reliant on the wheels, and developed a complex culture based around them.

As Whelkaholism mentioned, the lava flows could be replaced with a dry lake bed, or some other smooth hard surface.

Edit: The wikipedia page on Wentelteefje linked to Rotating locomotion in living systems, which contains quite an extensive analysis of the constraints of wheel based locomotion.

Also, Category:Rolling animals.