The slippery secret of snakes

The work may inspire new types of paints, coatings, plastics and other materials that are highly resistant to water -- or ultra slippery surfaces for other applications like robotic drone snakes. Engineers have designed fully articulated snakelike robots that can wriggle and writhe just like the real thing in recent years, but they have trouble slithering over some surfaces because their movement creates too much friction.

Anyone who has felt the soft body of a real snake knows how slick they feel -- a tactile trait that arises from its slippery scales, which are themselves covered with a fatty "lipid" molecule the snake produces. But one puzzling feature of these scales is that they essentially do not vary in size and shape over the body of the snake and yet numerous laboratory measurements confirm that snakes are more slippery on the belly than along the spine -- even though the scales look the same under a microscope.

This curiosity has defied explanation until now, but this month at the AVS 62nd International Symposium & Exposition in San Jose, Calif., researchers from Oregon State University will present data that may account for the reduced friction on the belly of the snake -- a lower "coefficient of friction" in technical terms.

Examining the surface of scales taken from both the belly and the back, the team found that the fatty lipid molecules on the reptile's underside line up like little soldiers, in tiny uniform rows and columns almost perpendicular to the surface. It is this orderly boundary layer of lubricant, the researchers say, that reduces friction for the California King Snake.

"It's crazy how well ordered this is," said Joe Baio, who led the research at Oregon State University. "It would be hard for me to believe it is random because you have to work hard to make a well-ordered monolayer."

Scientists have looked at the chemistry of snake scales before, but this was the first time anyone looked at them with techniques sensitive enough to detect the ordering of molecules on the surface. Baio's collaborators Stas Gorb at the University of Kiel in Germany and Tobias Weidner at the Max Planck Institute for Polymer Research in Mainz, Germany provided the snake skin samples, and the work is part of a larger collaboration that aims to compare the surfaces of snake scales across species.

Snakes Don't Eat Salad

We humans walk upright, so we don't have to think about sliding along the ground, but snakes do, so it makes perfect evolutionary sense that the scales on their belly would have lower friction that would allow them to slither smoothly over different surfaces, from moist tree bark to dry desert sand.

"If we did that with our skin, we would have nothing left," Baio said.

Snakes are also hunters, and they rely on making direct contact with their prey. But they have a very limited distance in which they can strike, so a lower friction belly allows them to slither silently and surprise their prey -- a huge adaptive advantage. Snakes have other features that make them good at slithering as well, like lacking appendages that would otherwise drag.

The new work suggests that controlled and ordered lipids molecules on their belly scales is another adaptation that makes it better at slithering -- and therefore hunting. Snake scales serve a number of purposes for the animal, including helping the cold-blooded creature regulate its temperature, providing camouflage and possibly regulating water retention.

When asked if you can feel the difference by running your hand along the bottom and top of a snake, Baio said he doubted it. The differences are too subtle, he said, and your hand is probably not sensitive enough to detect them -- though he admitted he's never tried.

Ironically, Baio added, he is afraid of snakes and not real big on touching them.