That description is close to the kind of locomotion that Dr. Jayne and his graduate student, Steven J. Newman, analyzed in a recent issue of the Journal of Experimental Biology.
Unlike most snake forms of movement, this one involves moving in a straight line, with no bending. It is not swift, as Kaa’s progress was in Kipling’s “Jungle Book,” but it is eerily liquid. Slyly, slowly, the snake flows without bending.
The motion had not escaped students of snakes. A scientist named Hans Lissmann described it about 70 years ago and produced a hypothesis about what muscles the snakes used to propel themselves, and how those muscles acted.
Dr. Jayne said, “We had an extremely good idea of all of the movements,” and Lissmann hypothesized how the muscles might work. “But what we were really lacking were any direct observations of how the muscles work,” Dr. Jayne said.
To do those observations, Mr. Newman inserted fine wire electrodes in the snakes’ muscles to record their activity, something like the way sensors record heart activity in an electrocardiogram.
What the scientists found was that Lissmann was mostly right, but not exactly so. Here’s how it works:
As the snake is moving forward, a muscle in its belly skin shortens the skin and stays tensed. Then a muscle running forward from the tip of one rib to the skin tenses to pull the skeleton and body forward over the skin.
Another muscle that runs toward the tail from the middle of a rib to the skin pulls the skin forward.
Lissmann did not expect the skin muscle to stay tensed, and he thought the skin muscles did the work of pulling the body forward, whereas it is the muscle running from the tip of a rib to the skin.
Why do the details matter? Scientists and poets alike want to get things right, whether it’s a matter of the precise word or the electrical activity of the muscle.
And in terms of practicality, the makers of soft, snakelike robots meant to find their way through nooks and crannies without troublesome legs to snag on obstacles, might find the exact nature of the motion useful, as snakes have for millenniums.
Interestingly, Dr. Jayne said, the slow, liquid crawl he and Mr. Newman studied seems to be used more by the heavier snakes — boas and pythons.