Scientists have found a surprising explanation for snakes’ effortless slithering: A mind-bogglingly thin coat of fatty lubricant embedded on the snakes’ scales.
The findings not only explain the reptiles' eerie slickness, but they also point the way toward new kinds of industrial lubricants and coatings, not to mention improved designs for snake-inspired robots.
“You know at county fairs, when you have the greased pig contest?” says Joe Mendelson, a herpetologist at Zoo Atlanta who wasn’t involved with the study. “These guys just showed that snakes are self-greased pigs.”
As entertaining as the notion of a slippery snake may be, the finding goes a great deal toward explaining how snakes smoothly weave their limbless bodies over terrain. Snakes can climb trees, scurry across scorching-hot desert, swim, and even “fly," gliding from tree limb to tree limb. (See National Geographic's amazing snake pictures.)
But none of snakes’ amazing moves would be possible if their outermost scales—the product of millions of years of evolution—couldn’t take the beating.
One of the most obvious features of snake scales, however, has long eluded explanation: Why are belly scales so much slicker and smoother than back scales?
It shouldn’t be surprising that snake belly scales are slick and smooth, given the need to avoid getting snagged on obstacles. (See "Belly Up: Why Do Some Snakes Have Elaborate Belly Patterns?")
When researchers examined snake scales under highly powered microscopes, however, they couldn’t distinguish any structural differences between belly scales and back scales—suggesting that whatever made snake bellies slick was a surface coating of some sort. But what was it made of?
To find out, study leader Joe Baio, a chemical engineer at Oregon State University teamed up with Tobias Weidner of Germany’s Max Planck Institute of Polymers to painstakingly examine the shedded skins of a California kingsnake (Lampropeltis californiae).
The team shone lasers all over the skins' surfaces to see how the surface molecules of their scales would reflect and scatter light beams—a technique normally reserved for inspecting microelectronics.
“It was crazy,” says Baio. “All these physicists saw us bringing in snake scales, and they were like, ‘What are you guys doing over there?’”
When the team’s scans were combined with other tests, the researchers found that they had detected an extremely thin layer of lipids—hydrocarbon chains that make up fats—coating the outsides of the snake scales. (See "Snake Robots Crack Mystery of How Reptiles Climb Dunes.")
According to the study, published December 9 in Journal of the Royal Society Interface, the coating was only a few nanometers thick, tens of thousands of times thinner than the width of a human hair.
What’s more, the kingsnake seemed to be using two different lubricants on its belly and back. The lipids on the snake’s belly scales formed a much slicker, ordered layer than the ones on its back—a chemical trick that even professional engineers struggle to achieve, the authors say.
"Nature's Little Secrets"
But if snakes have been greasing their bodies for eons, how haven’t humans noticed?
Unlike creatures such as snails, which smooth their path by secreting and leaving behind trails of wet lubricant, the snakes’ lubricant stays embedded on the scales themselves, forming a durable, slick layer similar to what keeps our joints lubricated and limber.
And since the snakes’ lubricant doesn’t wipe off, people handling snakes were none the wiser. (Also see "Snakes on Gel, in Jackets Illuminate Slithering.")
The California kingsnake the researchers examined isn’t particularly unusual, so the team cautiously expects their results to hold across many different snake species.
Baio suspects that different snakes produce lubricating concoctions unique to their species.
The researchers are also excited about the prospects of mimicking the slick coating on artificial materials.
For instance, if replicated in the lab, the lubricant could help rescue robots more successfully operate or improve next-generation paints.
“Nature has figured it out over millions of years,” says Weidner. “We can try to understand its little secrets.”
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