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While they’re not the only land-lubbers that lack legs,
snakes are probably the first creatures with that description to come to mind.
But if you look at a snake closely, you will notice
there is another anatomical feature where their legs should be.
Two of them.
It’s penises. We’re talking about penises.
So is it fair to say that snakes developed penises instead of legs?
Well, to answer that, we need to understand the history of limb loss in snakes
and have a closer look at why snakes have phalluses to begin with,
cause they’re more closely tied to legs than you might think.
Let’s start with our understanding of when snakes became full time slitherers,
and how, despite getting along just fine without legs,
they’re still clinging on to their ability to grow them.
According to the fossil record, snakes managed to evolve themselves
out of having front legs over a hundred million years ago.
And although there is a gap in the fossil record, which makes the timeline
a bit muddy, the back legs eventually made their leave, too.
So it would make sense that over such a long period of time,
snakes would also lose the genetic instructions tied to growing legs altogether.
A use-it-or-lose-it type deal, happening on the level of individual genes…
the segments of DNA that express specific traits.
But fossil evidence has also revealed that some extinct snakes
may have re-developed the ability to grow legs…toe bones and all…
after their ancestors became legless.
And recovering your long lost legs is much easier if those instructions
are still stashed somewhere in your genetic makeup,
but just aren’t doing that particular job anymore.
After many years of finding these limbed snake fossils,
researchers started looking at snake genes to figure out what,
if any, leg growing abilities modern species had managed to keep.
And it made sense to start with the boas and pythons, because despite their
outward appearance, they’re the leggiest snakes still around today.
As embryos, they actually start to develop barely-there hindlimbs.
But you are, of course, unlikely to see a bipedal boa anytime soon.
Their embryonic buds only develop for the first 24 hours,
and then their growth completely halts.
Still, it’s enough growth that as adults, they’ve got tiny leg bones, and even claws,
hiding under all those scales and muscles.
If you look closely, you can actually see them
as little spurs poking out of their bodies!
Though, for clarity, take some safety precautions before you get that close.
Now, the reason why you don’t have to worry about getting run down by a python
before it strangles you to death all comes down to transcription factors.
These are proteins that help specific genes get turned on or off.
Turn on the gene and the body’s gonna follow those instructions.
Turn it off, and it’s like forgetting to add baking powder to your favorite cake recipe.
The whole thing falls flat.
In order for a vertebrate to grow something long and complex like a leg,
transcription factors can’t just flip a single switch at the start that just says
“Two legs to go, please!”.
It’s a highly coordinated effort of a number of genetic factors,
building hip bones all the way down to claws.
These limb-related transcription factors are also helped by limb enhancers,
short stretches of DNA that help determine
where and when these genes kick into high gear.
But in snakes, the transcription factors assigned to leg growth
have mutated over time, and are missing some key pieces.
Those missing pieces cause the transcription factors to stop instructing the genes
to build out limbs, so the snake embryos can’t finish what they started.
And these losses seem to get progressively more dramatic as snakes evolved,
with boas holding on to more factors than, say, cobras.
That explains why snakes can’t grow legs anymore,
despite having the genetic instructions to do so.
But it does not explain why those genes would still be kicking around,
even if they’re not exactly functioning optimally.
When researchers began looking into snake genomes,
we started to uncover a lot of what was going on beneath their legless exterior.
For one, it was revealed that snakes and other reptiles
keep around two-thirds of the limb enhancers that are found in mammals.
And by comparing what these genetic helpers do in the embryos of snakes,
anole lizards, and mice, we learned that they have overlapping roles.
They don’t just help control the development of legs,
both the front and back variety, but the development of genitals, too!
So it looks like evolution kept some leg-growing genes in a slightly mutated form
because they’re still very important for modern snakes to,
not, like, get around, but to get around.
But before we can address the snakes-have-two-penises of it all,
let’s take a step back and ask why snakes have any penises.
And that requires asking the question why any amniotes…the large animal group
which includes reptiles, birds, and mammals…have them.
A lot of it comes down to evolution bringing life from the sea up onto land.
To help with that dramatic change of scenery, our ancient amniote ancestors
evolved more complex eggs, complete with their own fluid-filled sacs.
That meant instead of dispersing them through the water,
eggs could remain inside the body for much longer,
staying safe from hostile environmental conditions that might dry them out,
and predators that would eat them up.
This also meant that sperm had to find a way to come to them.
Enter… the phallus, a handy sperm delivery system.
Having sexual organs on the outside of our bodies allowed for internal fertilization
to occur, which really changed the game for amniotes
in terms of the habitats we could call home.
So yeah, phalluses are truly beneficial,
in more than just a pee-while-standing-up kind of way.
But what the earliest phalluses looked like remains a bit of a mystery.
Soft tissues don’t tend to show up very often in the fossil record.
Looking at the evidence, some researchers have proposed that
early penis evolution started with a single penis.
But to get that single penis, an embryo starts growing two separate structures
on each side of the body, until eventually they fuse together in the middle.
So if evolution had a hiccup and those halves failed to fuse,
that might explain why modern squamates…
a group of reptiles that includes snakes and lizards…have two penises.
Okay, technically, actually, they’re called hemipenes.
A lot of them look like tiny medieval weapons.
And there is so much diversity in hemipenes’ shape and size that they’re
often used as the main means of telling apart closely related species.
They’re also retractable, meaning they only need to come out when in use.
And boy do they get used!
Each hemipenis gets its own testis, so they can be used one at a time,
alternating back and forth between the left and the right.
That means male snakes can squeeze in more matings,
and not have to wait between rounds.
Which is such an advantage in nature’s game to procreate profusely before you
perish, it makes sense that the hemipenis trait would win, evolutionarily speaking.
Oh, and if you’re wondering if any of this
anatomical weirdness extends to female snakes, it does!
Kinda. While they only have one vagina, they do have two hemiclitores!
The science of those is still pretty new.
They weren’t described in the literature until 2022.
But scientists have been all over the hemipenes for much longer.
I’m honestly not that surprised by this.
Now, even if hemipenes happened to develop from some evolutionary fluke,
it’s not a complete coincidence that snakes have
two penises right around where they would otherwise have two legs.
In a study from 2014, researchers found that squamate embryos
develop hindlimbs and genitals from very nearly the same group of cells.
They’re pretty much right on top of one another.
But this is actually different from what happens in other amniotes.
In birds, and even more so in mammals, their embryonic genitals
don’t form from embryonic limb buds.
They grow from their /tail/ buds and other nearby cells.
So there’s a fundamental difference between how a human penis forms and how
squamate hemipenes do, even though the genes regulating that growth are similar.
There’s also a notable difference between
how a snake hemipenis forms compared to other squamates.
Since lizards still grow legs, their genitals start off as
a secondary bud that grows off their initial leg buds.
But snakes don’t grow fully functional legs, so they can recruit
any leftover leg bud cells to put them all towards hemipenis production.
I mean, it makes sense.
Those limb growing cells have already lost their other job.
Snakes might as well put them to good use.
And even in pythons that do grow wee little remnant leg bits,
their hemipene development really takes the cake.
But are we allowed to say that snakes have hemipenes instead of legs?
Not really, since other squamates that walk around on all fours
are using a lot of the same hemipenis growing tactics.
But we do know that snakes are taking advantage of their
remaining leg-growing cells to let their penises reach their full potential.
Even though hemipenes are the norm in squamates,
snakes really showcase the amazing range of shapes and sizes they can take.
And that’s led researchers to wonder if, by losing their limbs,
evolution let snakes completely reshuffle their appendage-growing priorities
and free up their capacity for greater penis diversity.
But, as we often say, more studies are needed.
Scientists have a lot more puzzles to solve
when it comes to all these appendage growing genes.
For example, one study found some snake
limb-penis enhancer activity in the…nasal region.
But limb enhancers that could have helped snakes grow legs have also been
shown to influence an animal's health in ways that have nothing to do
with appendages, like helping to develop the kidneys.
In other words, maybe these genes aren’t just
sticking around just to grow hemipenes.
There are still a lot of questions, but one thing is clear:
snakes are putting their appendage-growing genes to spectacular use.
The different ways that snakes use their genes might seem a little random,
but they show us that random can still be useful.
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