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Snakes: Do The Locomotion (Part 1)

Posted September 08, 2009 12:01 AM by Vi Pham

How do snakes get from place to place without limbs? Without arms and legs, humans would experience great difficulty moving about. So how do snakes move so easily (and so very quickly, too)?

From Head to Tail

Many people believe that a snake is a head attached to a tail, but this is incorrect. In fact, a snake's tail, relative to the snake's entire body, is very short.

Between the head and tail is an elongated trunk. A snake can have anywhere from 200 to over 400 vertebrae. All of these vertebrae have two ribs attached, with the following exceptions: the atlas (two vertebrae at the beginning of the vertebral column), one to three precaudal vertebrae, two to ten lumbar vertebrae, and the caudal vertebrae (the vertebrae in the tail, numbering less than 20% of total vertebrae).

These large numbers of vertebrae give snakes the maneuverability they need for terrestrial, fossorial, aquatic, arboreal, and aerial locomotion.

The Five Modes

There are five basic modes of terrestrial locomotion.

Lateral undulation

The most common mode is lateral undulation. It is used on most forms of terrain. Unlike limbed locomotion, there is no point of the snake's body that remains static on the substrate. Rather, the snake moves continuously. Horizontal waves (that decrease in amplitude) propagate down the snake's body and generate a force against fixed points in the environment. These points can include rocks, sticks, and trees.

The snake exerts a force against the point at an angle greater than 90 degrees from the direction in which the snake is moving (see image A). The points exert an equal and opposite force back, which is at an angle less than 90 degrees from the direction in which the snake is moving (see image B). The inward forces cancel each other out, leaving a force that propels the snake forward. To see a video of lateral undulation, click here.

Slide-pushing

Over flat, low-friction surfaces, there are no fixed points. Therefore, when lateral undulation is not an option, most snakes employ slide-pushing.

Horizontal waves propagate down the snake's body, creating a sliding friction that propels the snake forward. Slide-pushing looks similar to lateral undulation, but is much faster and does not seem as well-controlled. Snakes seem to be flailing about and, despite all of the energy they are using, forward motion is slow.

Concertina

Concertina is a mode of terrestrial locomotion that snakes use when traversing through low-friction rock crevices, burrows, and any other terrain that is long and relatively narrow with high walls or that is tubular in shape.

The snake's body is formed into tight curves and presses against the walls of the structure to provide static friction. The front portion of the snake's body extends and moves forward. It then forms more curves and remains stationary, while the back portion of the body is drawn forward to become the stationary part. The pattern is then repeated.

Concertina is also used by large snakes to climb trees. Instead of forming into tight curves, however, the snakes make tight coils around the tree trunk or branch.

Rectilinear motion

Rectilinear locomotion is utilized generally by heavier-bodied snakes such as boas, pythons, and vipers to move in a straight line. Travel requires no side-to-side movements. Therefore, when observed from above, it is difficult to see how the snake is moving forward.

Rectilinear locomotion primarily uses two sets of costocutaneous muscles that, in turn, run from the ribs to the ventral (underbelly) skin. The costocutaneous superior pulls the ventral skin forward. Once the ventral scales anchor the skin to the ground, the costocutaneous inferior pulls the ribs (and the rest of the body) forward to meet the ventral skin. The pattern continues, alternating between moving the ventral skin and the rest of the body.

Sidewinding

Over loose substrates like sand, gravel, or mud, snakes move by sidewinding. Research has shown that most snakes are capable of sidewinding; however, there are only a few species that are specialized sidewinders. These include the sidewinder rattlesnake (Crotalus cerastes) and the sidewinding adder (Bitis peringueyi).

Alternating sections of the body are lifted, moved forward, and set down. Usually, there are only two points of the snake's body that are in contact with the ground. This is very useful since the desert sand (sidewinders are typically found in the desert) can reach very high temperatures. The contact points shift down the snake's body, resulting in tracks that are at an angle to the direction of travel. Here is an Animal Planet video clip of a sidewinder.

What's left for Part 2?

Next week, I will discuss non-terrestrial locomotion, including swimming, burrowing, and more!

See you then!

Other Blog Entries

New Animal Attractions
Reptiles: A Scaly Introduction
Snakes: They're All Around
Snakes: A Family Affair (Part 1)
Snakes: A Family Affair (Part 2)
Snakes: Clever And Deadly Behaviors
Snakes: Do The Locomotion (Part 2)

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Guru
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#1

Re: Snakes: Do The Locomotion (Part 1)

09/08/2009 1:07 PM

Is that a movie of your snake? The music was a real toe-tapper!

This was a very cool entry and I can't wait for the second part!

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#2
In reply to #1

Re: Snakes: Do The Locomotion (Part 1)

09/08/2009 2:13 PM

Yeah, the snake in the first movie is one I have here at home! Her name is Storm and she's a leucistic texas rat snake.

I'm glad you enjoyed the video and the blog entry. The next one will be running next week!

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Guru

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#3

Re: Snakes: Do The Locomotion (Part 1)

09/09/2009 5:14 AM

All the varieties of tropical snakes that I have seen in India (quite a few used to come into the house too) seemed to move mostly in the lateral horizontal undulation mode.

Bioramani

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#4

Re: Snakes: Do The Locomotion (Part 1)

09/10/2009 1:32 PM

As much as snakes creep me out, I love how they move - so gracefully!!!

And very cool video! It helped me appreciate their abilities without being in the same room as one...

I look forward to the next entry!

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