Do You Know Why Snakes Slither?
Fossil evidence and genetic analyses are beginning to reveal how prehistoric snakes lost their limbs and developed such a streamlined body.
In this story, you’ll learn all about why snakes slither and how that way of moving evolved.
Just for fun, here’s a popular riddle that describes our slithery friends.
I can sizzle like bacon, I am made with an egg,
I have plenty of backbone, but lack a good leg,
I peel layers like onions, but still remain whole,
I can be long, like a flagpole, yet fit in a hole.
What am I?
And anybody who’s familiar with Harry Potter knows the House of Slytherin and that Harry can understand and speak in parseltongue.
Or maybe you remember Indiana Jones falling into a den of vipers in Raiders of the Lost Ark? Adam and Eve and the snake in the garden of Eden?
We humans have a strong love/hate relationship with snakes! They appear in many myths. They are often represented as creatures with great healing powers.
Did you know that there are over 3,500 species of snakes? Or that their ancestors actually had limbs?
So how did our modern snakes lose their limbs and develop their unusual form of locomotion?
In this article, you’ll see how fossil skeletons and evolutionary genetic changes are beginning to solve the puzzle as to why snakes slither and how that evolved.
Some general information about snakes
Just what is slithering, anyway?
From the Oxford Dictionary: to slither is “to move smoothly over a surface with a twisting or oscillating motion.”
If you’ve ever tried to catch a garter snake or other small non-poisonous snake, as I did when I was a kid, you know they can slither away pretty darn fast! And they can also be slippery.
There’s a tongue twister for you! Say slippery slithery 5 times fast!
How are snake bodies structured so that they can move in this way?
As is often the case in science, there are 2 ideas that attempt to explain how snakes lost their legs. One proposes that the precursors to modern snakes lost their limbs as a way to increase their ability to make their homes below the land’s surface, in burrows. The other one suggests that snakes lost their legs while they were still living in the oceans before they moved onto the land.
How can we distinguish between these 2 competing ideas?
It’s not a simple question. Yes, there are fossilized remains of snakes but how can you tell anything about an animal’s ecology by looking at its fossils? Especially as snake fossils are often not complete or are damaged.
Fortunately in our modern world, many disciplines can collaborate to increase the amount of information garnered from fossils. In this case, modern imaging technology and developmental biology genetics have given us a more complete story of how the bodies of snakes might have evolved.
When snake fossils were examined using high-energy x-rays, features not visible to our naked eyes were revealed.
What do snake fossils tell us?
The various ancient snakes did not lose their limbs at the same time. The first snake without limbs, Dinilysia patagonica, was present at the same time that dinosaurs were the main animal forms. That was 85 million years ago in the Late Cretaceous period. Luckily, an almost perfect fossil specimen of Dinilysia was found. It was fairly large — 6 to 10 feet, lacked any skeletal structures that would support any structures resembling legs and was found in terrestrial sedimentary deposits. That pretty much meant it had to have lived on land.
But its ecology could be established by an even more precise technique. Hongyu Yi, a scientist at the Chinese Academy of Sciences’ Institute of Vertebrate Paleontology and Paleoanthropology had developed a method that used the anatomy of a snake’s ear to distinguish modern burrowing snakes from those that lived in the water.
When Dr. Yi obtained a Dinilysia skull and analyzed it using her methods, its inner ear structure was almost identical to that of the modern sunbeam snake (Xenopeltis unicolor), a large burrowing snake. The sunbeam snake grows to about 1 metre in length and eats mainly rodents, frogs, reptiles, small mammals and other smaller snakes.
This suggested that the Dinilysia was a burrowing snake that hunted on the ground and made its shelter in loose soil.
What do the developmental genetic studies tell us?
When the genomes of many different species of vertebrates (creatures with backbones like humans, birds, fish, reptiles, etc.) were sequenced, we discovered that the actual number of genes involved in specifying the various body plans was fairly small. So the evolution of one body plan into another may only require altering a few genes or key genetic regions of the genome.
One of the main distinguishing features of snakes is their long vertebral or spinal column. We have 33 vertebrae and a typical lizard has about 65.
Snakes often have more than 300!
How do they get so many?
Vertebrae develop from structures called somites. These are blocks of cells in embryos that will develop into vertebrae, ribs or dermal tissues. The somite blocks have front and rear ends (anterior and posterior, in biology terminology).
There is a gene in vertebrates called Lunatic Fringe. This gene makes a protein that acts to establish the anterior end of the somite blocks. There is another gene, Hairy, which establishes the posterior end.
Interesting note: In humans, mutations in the Lunatic Fringe gene can lead to "Spondylocostal dysostosis, also known as Jarcho-Levin syndrome (JLS). This is a rare, heritable axial skeleton growth disorder. It is characterized by widespread and sometimes severe malformations of the vertebral column and ribs, shortened thorax, and moderate to severe scoliosis and kyphosis" (excessive outward curving of the upper back aka "roundback" or "hunchback" in severe cases). "Individuals with Jarcho-Levin typically appear to have a short trunk and neck, with arms appearing relatively long in comparison, and a slightly protuberant abdomen. Severely affected individuals may have life-threatening pulmonary complications due to deformities of the thorax." The information was taken from this article in Wikipedia.
So how does this all work in snakes?
Somites in snakes accrue in the tail end of the embryo. Once the right number of cells accumulate, a somite is formed and moves up the body. Picture adding beads to a string. The genes that make these somites are called a somitogenesis clock because they turn on and off at regular intervals. Since the total number of somites produced is also dependent on the speed of the oscillation; the faster the clock goes, the more somites that are produced from the same pool of cells.
When Dr. Celine Gomez measured the expression of the Lunatic Fringe gene in corn snakes and compared it to lizards, she found that it was expressed more frequently in the snakes. More frequent expression leads to more somites and more somites produce more vertebrae.
This is also the case with the ribs in snakes. In other creatures like the mouse, the ribs are only associated with the thoracic (chest) vertebrae. In snakes, all the vertebrae except the three closest to the head and a few in the tail also bear ribs.
For the longest time, it was thought that snakes evolved their vertebrae-rib column from the same form as the mouse and reptiles like the alligator. Their trunk skeletons have neck and waist vertebrae that are distinct from the chest vertebrae. So maybe snakes took that form and just lost the limbs. But recent fossil evidence is questioning that scenario.
Investigators Jason Head and P. David Polly suggested that the fossil evidence actually indicates that snakes have the same number of distinct regions in their vertebral columns as lizards do. They also noted that fossils showed that ancestral four-limbed mammals and alligators also had ribs associated with the neck and waist. This meant that the loss of these in modern birds, alligators and mammals evolved independently and was not inherited from a common ancient ancestor.
Yet another set of genes, the Hox genes, also plays a dominant role in the differentiation of vertebrae in animals. One idea was that different patterns of expression of the Hox genes in snakes occurred to bring about their elongated vertebral column that lacked ribs. When Drs. Head and Polly compared the pattern of expression of the Hox genes between snakes, mammals and alligators, they found no significant differences.
So when we look at snakes and the other four-limbed animals we can see that the snakes must have inherited the rib pattern from these animals and only the elongated vertebral column is unique to them.
How did snakes lose their limbs?
It looks like yet another bit of genetics may be responsible for how this came about. And this is a bit trickier so bear with me.
As you know because you read this article,
Everything You Need to Know About Your Genes
Learn these 6 basic things and you’ll be well on your way to feeling pretty comfortable about your DNA and genes.
genes are discrete stretches of DNA that make up your genome. What you didn’t learn in that article was that most of our genes have what are called regulatory regions. These are usually short stretches of DNA either before, after or within the actual gene’s sequence. They specify landing sites for one or more proteins that tell the cell when to turn the gene on or off. Because of this ability to turn genes on and off, these regions are referred to as regulatory genetic “switches”. The switches that turn the genes on are called “enhancers” and those that shut them off are called “suppressors”.
So, Evgeny Kvon and his colleagues at Lawrence Berkeley Laboratory were studying limb development in snake and mouse. And they found a genetic switch that enhanced the formation of limbs. This enhancer is called ZRS. When they “cut out” the ZRS enhancer DNA from snakes and “pasted” it into where the mouse’s ZRS enhancer resides, they got a mouse with a normal body but truncated limbs.
This really emphasizes how careful you need to be when you move pieces of DNA around between different organisms! But this was a carefully controlled experiment. And even more interesting was the fact that the snake ZRS enhancer DNA they used differed by only a single mutation from the normal mouse sequence!
Think about it. If the enhancer gets mutated in mice, there’s a very good chance that they wouldn’t be able to survive or if they did, very little chance that a mate would accept them. So in mice, this piece of DNA is what we call “highly conserved” and rarely, if ever, will we see it altered.
But in snakes, that’s not the case. The ZRS limb enhancer has lots of different versions. And these versions correlate quite nicely with the snakes that we see today. For instance, primitive snakes like the python and boa constrictors have a short ZRS limb enhancer and both these snakes have rudimentary forms of limbs. Here’s a picture from Wikipedia showing the “limbs” on a python’s skeleton. They’re called spurs.
In contrast, the corn snake, which we referred to previously, has completely lost its ZRS enhancer region of DNA. And that is why there are not even hints of limbs on it and other more evolutionarily advanced snakes.
So that’s why most of the snakes we see don’t have any limbs. They’ve lost or completely mutated their ZRS enhancers.
We’ve explored the fossil evidence and genetics of the evolution of the snake body plan with its modern elongated vertebral column.
Combining those two scientific avenues have given us a pretty good idea about how snakes might have lost their legs and were thereafter relegated to a life of burrowing and slithering. Not that the snakes are complaining, mind you. They seem to be quite happy to oscillate their way around on both land and water.
I hope you enjoyed learning about this as much as I did.
Until next time,
References for the original articles that inspired this post.
How Snakes Lost Their Legs, Hongyu Yi, in Scientific American, January (2018), p71–75. (pay for access)
Progressive Loss of Function in a Limb Enhancer during Snake Evolution, Evgeny Z. Kvon et al., in Cell, Volume 167, Issue 3, 20 October 2016, Pages 633–642.e11 (open access)
Evolution of the snake body form reveals homoplasy in amniote Hox gene function, Jason J. Head and P. David Polly, in Nature 520, 86–89 (2015). (pay for access)
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