Slithering Snakes Reveal the Secret of Limbless Locomotion

By Eliza Strickland | June 9, 2009 10:25 am

corn snakeSnakes certainly make it look easy when they slither forward, leaving perfect S-curve tracks behind them, but scientists have long been puzzled by the mechanics of their locomotion. One theory proposed that they propelled themselves by pushing off small twigs and rocks in their paths, but researchers noted that they move equally well across smooth surfaces, like flat rock or desert sand. One researcher who is studying snakes’ motions, David Hu, notes that snakes are champs at escaping across office carpet…. “One snake escaped, and we didn’t know where it was until we got a printer jam,” he says. (The snake was fine.) [ScienceNOW Daily News].

Now, after a series of experiments and some computer modeling, Hu says his team has cracked the case. A snake’s scales, Dr. Hu said, resemble overlapping Venetian blinds, and tend to catch on tiny variations in the surface they lie on. This friction is greater in the forward direction than in sideways directions, as it is with wheels and ice skates. This frictional difference results in the snake’s moving forward as it undulates [The New York Times].

For the study, published in the Proceedings of the National Academies of Science, researchers first tested the snake scale friction by sliding unconscious snakes across flat surfaces. Snakes slid easily in the forward direction, but their scale friction resisted sliding backwards or sideways [Telegraph]. They next took awake milk snakes and placed them on extremely smooth surfaces, like slick plastic, and found that although the snakes wriggled and squirmed, they couldn’t move forward. Wrapping the snakes in cloth had the same effect; in both cases, the friction was the same in all directions, which prevented the snakes from getting their forward boost. (See videos of these experiments.)

But the researchers’ mathematical models of how much momentum this friction should produce suggested that they hadn’t fully accounted for the snakes’ forward motion. Then the researchers noticed that the snakes were lifting parts of their bodies as they slithered forward on the recorded videos. Hu described it as “dynamic weight distribution” that allowed snakes to concentrate their weight on a few points and move more quickly…. “It will change the speed of snake a great deal” [LiveScience], says Hu.

So the simple, sinuous motion of a slithering snake appears to be a complex combination of factors, as it both pushes off the surface in a few specific spots and grips the ground with friction. This fits with the notion that snakes have a “remarkable capacity to manipulate their belly scales,” says Bruce C. Jayne, [who wasn't involved in the current research]. Many snakes have specialized muscles that go from scale to scale or from scales to the ribs. These muscles may change the orientation of the scales just a bit, Jayne says [Science News].

Related Content:
The Loom: How to Be a Snake has more on this study, including video from the experiments
80beats: Super-Sized Snake Ate Crocodiles for Breakfast
80beats: World’s Smallest Snake May Be the Smallest That Could Ever Exist
80beats: The Father of All Fangs—Snake Weapons Came From One Ancestor
DISCOVER: How the Snake Lost Its Legs

Image: Grace Pryor and David Hu, New York University

CATEGORIZED UNDER: Living World
  • Matt

    “A snake’s scales, Dr. Hu said, resemble overlapping Venetian blinds, and tend to catch on tiny variations in the surface they lie on. This friction is greater in the forward direction than in sideways directions, as it is with wheels and ice skates.”
    Thinking that it should read “…friction is LESS in the forward direction than in sideways…”

  • http://clubneko.net Nick

    I’m thinking Matt may be right – the coefficient of friction would have to be greater in the direction opposite the snake’s path forward, otherwise they’d move backwards – though friction is kinda a slippery subject.

    Again, I’d like to point out that snakes have had hundreds of millions (at least 150 million years of direct belly-crawling exp. according to Wiki – apparently snake skeletons don’t preserve well so we’re not exactly sure) of years of DNA evolution to work out this problem, so it’s no surprise that it’s a lot more complex than it looks to the naked eye.

    Let’s just be happy they haven’t evolved gecko-sticky for their bellies. I would not want a snake that could crawl up my walls and across my ceiling.

  • Socrates

    Matt I think that what they were trying to say is that when the snake moves, the grip in the forward direction is higher than sideways which allows them to move forward easier.

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