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
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