I once saw something that continues to puzzle me. A snake slithered down the bank of a pond and continued right on across the surface of the water, seeming to move in just the same way as on land. But how is that possible? If I'm not wrong, snakes use their ribs to push against the ground when crawling, but that wouldn't be much use on the water surface would it? And of course they don't row or paddle like a swimming duck. Can you shed any light on how the mechanics of this would work?

Hi L.T.,

snakes actually move in a variety of ways, depending on the species and the terrain. David Attenborough's most recent series "Life in Cold Blood" actually has a well illustrated section on various forms of snake locomotion (it is useful to see, since text description of movement are often very cumbersome). Five types of snake locomotion are discussed here: http://www.ucs.louisiana.edu/~brm2286/locomotn.htm

When swimming the snake uses lateral undulations of the body that start at the head and continue down the length of the body (the same motion that is used to move on a smooth surface). By throwing its flanks into an S-shape that propagates from the front, the snake exerts force against the water, which results in an opposite forward force. Obviously the water is not solid, so the force against it is not like force against rock, so the efficiency of this swimming technique is not optimal. Sea snakes have flattened sides to increase the efficiency of this swimming style and Eels use a very similar method.