Here’s a puzzle: you’ve gone to all the bother to evolve fins, then limbs, and then even limbs with all these complicated joints and toes and whatnot—and then you lose them. Limbs all gone.
This seems counterproductive, to say the least. Yet it isn’t just the snakes going in for the serpentine body plan: caecilians, amphisbaenians, and legless lizards lost their legs, too, and they aren’t evolved from snakes—these limbless animals all lost their limbs independently.
To understand how being snake-shaped might be adaptive, we’ll also consider some animals that are almost—but not exactly—snake-shaped: the mustelids, or weasels.
So who is snake-shaped, besides snakes? Among the amphibians we have the caecilians, who must get really tired of being described as “worm-like,” because, come on, they’re amphibians.
To be fair, most of them burrow like worms, but still.
Also probably tired of worm comparisons are the amphisbaenians, which are reptiles.
And then there are the legless lizards, which look pretty much how you’d expect from the name.
Despite the superficial similarities, none of these are snakes. Clearly the snake shape is a popular one, though: there must be some advantages to it.
Let’s ask some mammals. Mustelids are basically furry snakes with very short legs. They have long, flexible bodies with narrow heads. What does this let them do?
Sorry, that photo was not relevant. It won’t happen again.
No, being long and narrow and flexible lets mustelids fit into small spaces like holes, crevices, and burrows.
Black-footed ferrets eat prairie dogs, which live in burrows. Other mustelids eat rabbits, gophers, and other hole-dwelling animals. Domesticated ferrets were originally tamed and bred to help people hunt rabbits by flushing the rabbits from their holes.
Fitting into holes isn’t just good for hunting: it’s also great for hiding from anyone who might want to eat you. (Unless they are also long and narrow, in which case, uh-oh.) A small enough hole will protect you from leggier predators like coyotes, bobcats, raptors, and so on.
There’s a downside, though: to be long and narrow, your head has to be small relative to your body size. This means that, considering how much food you have to eat in order to nourish your long body, your jaws are small. Mustelids take down big prey for their body size anyway, and enormous prey when you consider how little their jaws are. A hunting mustelid isn’t just trying to not starve: it also has to try to not get killed by its formidable prey.
Snakes and snake-like animals are similar, but more so. Where mustelids just have fairly long bodies and relatively short legs, snakes have gone all out. And why not? The advantages only increase: they can retrieve prey from really small holes and hide in really tiny crevices. Jettisoning the legs makes sense because the animals can move just fine without them, they’d get in the way of fitting into small spaces, and they take energy to grow. It’s a bit harder to burrow without forelimbs for digging, but the narrower you are, the easier it is to slip through the soil.
The longer you get, however, the more problematic the food issue becomes. Snakes have a lot of long body to nourish through that little mouth. The solution they are most known for is the same as the mustelids’: go after really big prey. Mustelids can tear their food into smaller chunks and so can amphisbaenians, but snakes can’t. So they do the other obvious thing and open their jaws really wide—we often say they unhinge their jaws—and have very flexible bodies that can accommodate swallowing big things, and then they hang out digesting for a week or twelve.
There’s another solution that snakes and snake-shaped creatures use. If your problem is that you are long, why not eat things that are also long? Worms and smaller snakes fit through a small mouth just fine. (This is why that young ringneck snake needs to be hiding in tiny crevices—so the bigger snakes don’t eat it.)
You want to be narrow both to catch your slightly-smaller narrow prey and to fit into spaces that your slightly-bigger predators can’t. Legs would just get in the way.
Acknowledgment: most of the ideas in this post are borrowed from lectures in Dr. Harry Greene’s Herpetology class at Cornell. I cannot recommend Dr. Greene’s classes enough. They are amazing. Also, he has just written a book, Tracks and Shadows: Field Biology as Art, that looks awesome.
*Photos obtained from Flickr and used via Creative Commons. Many thanks to these photographers for using Creative Commons!