Have you ever wondered, ‘what is the best number of legs?’
I mean, probably not.
But how many legs a species has is vital to a lifeform’s success, and it’s time someone
figured out which number reigns supreme.
So, for this entry into the archive, we’ll explore every option for leg-count that nature
makes available, excluding cases of genetic mutation or limb-loss.
Whether you’re reincarnating soon or a worldbuilder
looking to improve the realism of an alien
lifeform, this is a question you need answered…
We’ll kick things off by discussing lifeforms with one leg.
You might not think any species has a single leg as their default setting, but nature doesn’t
care what you think.
Many clams and oysters use a single muscular limb to drag themselves along.
And their gastropod relatives, a group that includes snails and slugs, have a similarly
bold approach.
Though it might not look like it, the bottom of one of these slimy fellows is actually
a single, giant foot — which they slowly ungulate to move forwards.
The only way this even remotely works is because snails constantly lubricate their undersides
with mucus, allowing them to triumph over friction — although just barely, as most
species clock in at a blazing one millimeter per second.
But if your only method of locomotion was coating your shoes in oil and shuffling your
feet, you probably wouldn’t get very far either.
Most species in this category are *herbivores, as anything faster than a leaf might outrun
them.
Perhaps on another planet, a lifeform could evolve to hop along on their one leg at greater
speeds — although the laws of physics mean that any single-limbed species will likely
struggle with uneven terrain.
Still, any lineage that, when coming onto land, looked at animals with multiple legs
and said ‘I’m good, thanks,’ is pretty hardcore…
Up next, we have two legs — a unique option with some major advantages, and some major
disadvantages when it comes to things like… keeping your balance.
The vast majority of two-legged species on earth evolved from four-legged ancestors,
which means their former front limbs are now free to be used for all kinds of weird stuff.
Birds are a massively successful two-legged group whose forelimbs evolved into wings,
allowing them to soar through the skies in a move that kind of feels like cheating.
They barely even use their legs, it’s not fair.
Not all birds take the easy route, however.
Ostriches and other large flightless birds strut about with us land dwellers, and penguins,
well…
they don’t strut, exactly, but they do their best.
Instead of wings, Penguins forelimbs have evolved into flippers, increasing their speed
in the water…
but not so much on land.
But wings and flippers aren’t the only option for what a two-legged animal does with their
front limbs.
Primates have evolved grabby hands they use in all sorts of inventive ways, like snagging
food, swinging from branches, and cleaning their fur or the fur of others.
Now, it’s worth noting that not all Primates are completely loyal to Team Two-Legs.
Great apes, for example, often switch to a semi-four-legged posture called knuckle-walking.
In fact, lots of animals only switch to two legs when it’s trendy.
Bears typically plod around on all fours, but will rear up on their hind legs to climb
trees or appear larger.
Meerkats switch to two legs when scanning for danger, the added height giving the nervous
lookouts a better view.
But let’s look at the true converts to the two-leg doctrine — humans.
Though our stability isn’t the best at first, an adult human’s sense of balance is on
another level.
With practice, a human’s flexibility on two legs becomes kind of ridiculous.
We might start off crawling on our hands and feet — and like primates we can still move
on all fours if we want — but it’s nowhere near as fast as switching to a two-legged
stance and running full-tilt.
Free from the burden of supporting our bodies, our front limbs have become advanced tools
for interacting with the world — and for showing other animals how cool we are.
Across media, lots of aliens have bipedal body plans… mainly because lots of aliens
are actors in budget-friendly costumes.
That’s not to say that two-legged life couldn’t emerge on other worlds — they just probably
wouldn’t look like us.
In the end, the two-leg setup is a strong contender for the top spot… despite some
flaws like back pain.
Next, we’ve got three legs, and right away there’s a problem.
Because no species has three legs — right?
Well... it’s complicated.
The tripodfish is a deep-sea dweller that perches on the sand using three ‘legs’
derived from elongated fins.
Whether or not these fins should be counted as legs is debatable — as it’s actually
trickier to define what a leg is than you might expect.
By most scientific definitions, a leg is any limb used especially for supporting the body
and for walking.
Strangely enough, under this definition, Kangaroos are another animal sometimes placed in the
three-leg category, because when they’re not hopping around, their tails function as
third legs that keep them upright.
What counts as a leg is a question that will only get weirder as this video continues — but
for now, it’s clear that three legs aren’t the most popular.
Part of this may have to do with the physics of walking, but it also could be because no
early land animals came out of the water with three limbs — and in an alternate history,
evolution might have taken a stranger path.
But for species afraid of breaking the mold, four legs are a safe choice.
When you think land animals on planet Earth, this is probably the number of legs that comes
to mind as the default — and for good reason.
Structurally, a four-leg set-up means stability — the type of stability that lets you grow
into a kind of living flagpole if evolutionary pressures demand it.
Four legs are also great for off-roading, letting animals push through tough conditions.
They allow for an ideal running gait, and it's no accident this category is home to
the fastest land mammals.
Really, four legs are good for whatever.
But it’s not just mammals who have gotten in on the action; reptiles have had four legs
since before it was cool.
Another four-legged species are kangaroos — wait, kangaroos are here too?
I mean, they do walk on all fours sometimes, but seriously — they move on two legs, three
legs, and four legs?
Huh, I’ll allow it.
I guess.
The only real disadvantage to committing all the way to four legs and no arms is that picking
things up becomes a real challenge.
Unless you’re an elephant, a species which breaks all the rules by turning their snoots
into long, muscular appendages.
Elephants probably look at other species that had to choose between being able to pick things
up and getting really big and just laugh.
An elephant’s trunk has over forty-thousand separate muscles, more than in the entirety
of the human body, which honestly feels like they’re rubbing things in a bit.
Overall, four legs are a great basic choice, and likely would be no matter the planet.
Five legs are far less popular—the category would be pretty barren if it weren’t for
a group of star-shaped oddities.
Most species of starfish possess five limbs that somewhat stretch the definition of a
‘leg,’ as it’s actually the numerous tube-like feet below each limb that allow
a starfish to move along the sea floor.
Their appendages are sometimes called legs, sometimes called arms, and sometimes called
‘rays’ — a third, unique category created specially because starfish refuse to be normal.
The other candidate for a five-legged animal is the kangaroo — kangaroos again?
Oh, I guess they really do sometimes use all five — you know what, no.
I don’t care that science backs them up on this.
Kangaroos are officially disqualified.
Moving onto six legs — just kidding.
There’s another category we don’t want to keep waiting.
Though easy to overlook, because they’re close to the ground, many animals have no
legs at all.
Snakes are probably the most famous group in this category, able to move by using their
muscles to push their scales against terrain.
While this might sound like a huge pain, snakes and other legless lizards gave up their appendages
for good reason, as their limbless bodies are perfectly adapted to slide into burrows
and around tight corners in search of prey.
Worms are also getting along just fine without legs, moving by contracting and relaxing the
segments of their body.
Fish also technically qualify as legless animals, navigating the aquatic realm with fins alone.
Except for rebels like the Flying Gurnard, which ‘walk’ along the sea floor using
modified pelvic fins.
And some marine life like anemones and coral can spend their whole lives in a single spot,
feeding on small aquatic organisms and generally living more like plants than animals.
Many of these species could be classified as ‘sessile,’ or fixed in one place, and
should probably be in their own bonus category.
It’s easy to judge, but if you could hang out on a rock all day and have food come to
you, would you really be in a hurry to go anywhere?
All right, back to six legs, for real this time.
It’s a category that might not strike you as popular, but is actually the most widespread
of all.
This is because the majority of insects have six legs, and there are around 10 quintillion
insects on planet Earth, scuttling along often unnoticed by the large animals they vastly
outnumber.
Six legs are incredibly versatile, enabling insects to move while maintaining a supportive
tripod at all times.
Some species also evolved wings from ancient gill-like appendages, meaning they didn’t
even have to trade in a pair of limbs for flight, which seems massively unfair.
And with so many limbs to spare, some insects have gotten pretty creative.
Praying mantises are a group that have turned their front limbs into grasping appendages
while keeping their four back legs on the ground — a body plan that’s basically
as close as life on Earth gets to a centaur.
There are some inventive concepts of intelligent aliens with similar anatomy, which seems like
a layout with great theoretical potential.
While there are no large six-legged animals on our planet, the unparalleled success of
insects suggests that such a body plan might have serious advantages.
The Seven Leg category has almost no takers — unless you count the Seven-Arm Octopus
which keeps one of its eight tentacles used for fertilization retracted in a sac beneath
its right eye.
Seems a bit like cheating, but I’ll let it slide.
Certain starfish also have seven legs, although once again, that depends on your definition.
You might have noticed that there’s a lot of animals with an even number of legs, yet
scarcely any with an odd number.
This has to do with bilateral symmetry — the tendency of animals’ right and left halves
to be roughly mirror images.
This trend is likely because bilateral symmetrical species are faster and more direct than a
radially symmetrical species like a starfish, whose lack of a distinct front or back makes
moving in a straight line a real challenge.
The best chance for an odd-legged organism to hit the big leagues is probably if a radially
symmetrical lifeform later evolved bilateral symmetry, which seems to be the origin of
the seven-limbed aliens in Arrival.
Back to even numbers, eight legs are the domain of insects’ cool older siblings — the
arachnids.
The poster children of this group are spiders and tarantulas, most of whom have turned their
eight legs into all-terrain web climbing machines.
Some people find an arachnid’s legs overkill, but they might look at us and think we’re
seriously underequipped.
The other most famous eight-legged contender is the octopus, who typically possess eight
multi-purpose tentacles that can both carry them across the sea floor and function as
arms.
Some have argued that the octopus is technically a biped, as they’ll sometimes use only two
tentacles to move across the sand.
This depends on the species, however, and there’s a strong case to be made that an
octopuses’ tentacles meet the criteria for both arms and legs.
Proving that you can’t put an octopus in a box; they’ll just ooze back out again.
Both octopuses and spiders have been observed functioning just fine with missing limbs—because
when you have eight legs, you’ve got enough to spare.
The nine-leg category is another unpopular odd-number section, with the closest contenders
being the Nine-Rayed Starfish, or the exceedingly rare nine-armed octopus — although its extra
limb only happens when a severed tentacle regrows as two.
But with ten legs, things really get crazy as we enter the domain of crustaceans.
Many species of lobster and shrimp have a full ten legs, and with that many legs available,
things can get weird.
Crabs are crustaceans that have gone the centaur route and lifted their front limbs off the
ground to use as a pinchers.
Some crabs have even turned their back legs into flippers to, uh, ‘swim’ — because
when you have ten limbs to work with, you might as well.
And as we know, the crab model is a favorite of evolution, as numerous other crustaceans
have converted to the way of the crab — creating multiple ‘false crab’ lineages that jealously
copy the crabs’ winning formula.
While more legs aren’t always better, the stunning popularity of crustaceans suggests
that they’re onto something with their high limb-count — and that life on other planets
might just be a bunch of crabs.
Beyond the event horizon of ten legs, we enter a black hole of ever-increasing digits.
Giant, deep-sea isopods have a staggering fourteen legs.
Caterpillars have a rough average of sixteen total legs — with six primary limbs in the
front and multiple rows of pseudo-feet to help them move their elongated bodies.
Centipedes can have anywhere from sixteen to over a hundred legs, and finally, some
millipedes can have an even higher number than that.
And the species with the absolute highest number of legs is the Eumillipes persephone
which has an astonishing one-thousand and three-hundred legs in total.
Though an impressive total count, such a number is only needed to help the species push through
the soil deep below ground, and would be an absurdly impractical number for the vast majority
of life
on earth.
So, what’s the Answer? Well, it depends. And yes, I know that’s not the most fun
answer — but I promise it’s the one that’s accurate. There are some stand outs, but
as easy as it would be to arbitrarily assign two legs, or six legs, or ten legs as the winner — it
would be ignoring the complexity of nature. If there is one thing we’ve learned, though,
it’s that nature shows an astonishing amount of variability when it comes to leg numbers;
should you ever design a lifeform, know you have a lot of creative freedom in terms of total limbs.
In the end, we should strive to appreciate the leg counts of all animals. …Except for kangaroos.
And speaking of numbers, I’d like to thank all my subscribers for bringing Curious
Archive past the five-hundred thousand milestone, plus a little extra. That’s
over four-hundred and ninety-eight thousand more people than the Eumillipes has legs,
which is absolutely astounding to me. I really can’t express my gratitude enough
to every one of you — this channel wouldn’t be possible without your continued support. Thank
you all so much for watching. And if you’re new to the channel, this is the part where
I say that if you enjoyed this entry, you can lend your support by liking, subscribing, and
hitting the notification icon to stay up to date on all things Curious. See you in the next video.