Why be shaped like a snake? (Also, weasels)

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.

You're just a head and a tail now. Why, corn snake, why?

You’re just a head and a tail now. Why, corn snake, why?

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.

Least weasel. Photo by Sergey Yeliseev*

Least weasel. Photo by Sergey Yeliseev*

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.

Yellow-striped caecilian. Photo by Kerry Matz*

Yellow-striped caecilian. Photo by Kerry Matz*

To be fair, most of them burrow like worms, but still.

They do kind of look like worms... Photo by Teague O'Mara

Not at all worm-like. Besides the shape, and the sliminess, and the segments, and… But really, NOTHING like worms.
Photo by Teague O’Mara

Also probably tired of worm comparisons are the amphisbaenians, which are reptiles.

Not a worm! Photo by Gustavo Durán*

Not a worm! Photo by Gustavo Durán*

And then there are the legless lizards, which look pretty much how you’d expect from the name.

Regular lizard head...

Regular lizard head…

...but no legs! (This is a Scheltopusik.)

…but no legs! (This is a Scheltopusik.)

Common scaly-foot. Photo by eyeweed*

Common scaly-foot. Photo by eyeweed*

Juvenile California legless lizard. Isn't he a pretty copper color? Photo by squamatologist*

Juvenile California legless lizard. Isn’t he a pretty copper color? Photo by squamatologist*

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?

It lets them be so cute! (Black-footed ferret kits; photo by Kimberly Tamkun of the US Fish and Wildlife Service*)

It lets them be so cute!
(Black-footed ferret kits; photo by Kimberly Tamkun of the US Fish and Wildlife Service*)

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.

Adult black-footed ferret fits in a burrow just great. Photo by Ryan Hagerty of the US Fish and Wildlife Service*

Adult black-footed ferret fits in a burrow just great. Photo by Ryan Hagerty of the US Fish and Wildlife Service*

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.

Morna the domestic ferret showing off her catch

Morna the domestic ferret bringing back her catch

Unfortunately, Zap the domestic ferret cannot quite make himself fit through the crack under the door.

Tragically, Zap the domestic ferret cannot quite make himself fit through the crack under the door.

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.

Stoat chasing a rabbit. Photo by cobaltfish*

Stoat chasing a rabbit. Photo by cobaltfish*

Stoat with its (vole?) prey. Photo by Marek Stefunko*

Stoat with its (vole?) prey. Photo by Marek Stefunko*

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.

Rough green snake is very long.

Rough green snake is very long.

Very small ringneck snake could fit anywhere.

Very small ringneck snake could fit anywhere.

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.

Rock python swallowing an ungulate. Photo by Alex Griffiths*

Rock python swallowing an ungulate. Photo by Alex Griffiths*

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.

Corn snake doesn't know why you needed to read a whole long thing about why being snake-shaped is cool. Obviously it is.

Corn snake doesn’t know why you needed to read a whole long thing about why being snake-shaped is cool. Obviously it is.

Oops. I did it again. (Black-footed ferret kits. Photo by Kimberly Tamkun of the US Fish and Wildlife Service*)

Oops. I did it again.
(Black-footed ferret kits. Photo by Kimberly Tamkun of the US Fish and Wildlife Service*)

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!

Advertisements

7 thoughts on “Why be shaped like a snake? (Also, weasels)

  1. Great post.
    Why be a long snake though, unless you need that length to strangle your prey? It must cost energy.
    Also I always assumed that a recently fed big snake would be disabled, ungainly and with energy diverted to the gut. Why are they not prey for something else at that time?
    Any ideas?
    I never thought of a weasel as a snake. Strange how they overpower much larger prey.

    • Some of the benefits of being long are probably movement-related. A lot of snake movement depends on a certain length: it would be hard to climb a tree or sidewind if you were short and stubby. Defensive/offensive movements like striking from a coil also require a minimum length. The longer you are, the more muscle you have *and* the more leverage you have.

      But the main reason for length is probably size. Size protects you from predators (long skinny snakes would be easy to consume and eat *once dead*, but in most cases subduing a larger/longer snake is harder than subduing a smaller one), makes you more competitive against others for mates, and lets you store more energy, allowing you to go longer between meals and/or produce more offspring at one time. If you want to be big but also skinny (to fit in small spaces), you have to be long. There’s nowhere else to put that mass.

      It’s an interesting question why recently-fed snakes aren’t just easy prey. Snakes aren’t completely disabled by eating: they can still move, although they prefer not to and aren’t so agile. And those teeth/strong coils/venom sacs don’t go away. A snake that would be vulnerable whether it had eaten or not (e.g. a small snake, a snake without good defenses) will want to hide after it’s eaten, but it would want to hide anyway. Also, under pressure, snakes can regurgitate their meal to free themselves up for fast agile movement. (There are many accounts of harassed snakes regurgitating a prey animal that then ran away!)

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s