Thanks to Brilliant for  supporting this SciShow video!

As a SciShow viewer, you can keep  building your STEM skills with 20% off an annual premium subscription and a 30  day free trial at Brilliant.org/SciShow. [♪ INTRO] 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.

That’s why Brilliant has  an entire course devoted to Random Variables and Distributions. Brilliant is an online learning  platform with courses in science, computer science, and math. And their course on random statistics gives you the chance to understand things like chance.

Through interactive lessons and  puzzles, you can finally learn the true meaning of words you use  every day, like “normal” and “random.” Brilliant can help you understand  the random world you live in with a 30 day free trial and 20% off an  annual premium Brilliant subscription. And you get that special deal  because you watch SciShow! Find those savings at the link in the description down below or at Brilliant.org/SciShow.

And thank you for watching SciShow. Thanks to Brilliant for supporting even this one! [♪ OUTRO]