But, it turns out that the same genes involved in leg growth are also involved in growing penises.And since snakes still need to reproduce, those genes are still being used.So, in a way, snakes have kept their ability to grow legs, but instead of using it to make legs, they’re using it to make penises.And that’s why snakes have penises instead of legs.
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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.
But, it turns out that the same genes involved in leg growth are also involved in growing penises. And since snakes still need to reproduce, those genes are still being used. So, in a way, snakes have kept their ability to grow legs, As snakes evolved, their losses became increasingly more dramatic, with boas retaining more factors than cobras. This is why snakes can no longer grow legs, even though they possess the genetic instructions to do so. Researchers looked into snake genomes and found that they still have two-thirds of the limb enhancers found in mammals and that these enhancers have overlapping roles in the development of legs and genitals. It appears that evolution kept some of the leg-growing genes in a slightly mutated form as they are still very important for modern snakes. To understand why snakes have any penises, one must ask why any amniotes have them. This comes down to the evolution of more complex eggs that needed to be fertilized internally. This led to the development of the phallus, a sperm delivery system. To explain why modern squamates have two penises, some researchers propose that the halves of the early penis failed to fuse together. Hemipenes look like tiny medieval weapons and are often used to tell apart closely related species. They are retractable and male snakes can use them one at a time, alternating back and forth between the left and the right. Females have one vagina, but two hemiclitores, which were not described in the literature until 2022. 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.
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