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When it comes to slithering, most snakes do it the same way: straight ahead. But for snakes that live in deserts, getting around can be a challenge.
“As we know from trying to move on sand in a beach or other places, it can be difficult to move on these materials that yield underneath you as you move forward,” said Jennifer Rieser, a professor of physics at Emory University in Atlanta.
That’s why sidewinders slither sideways. Although some snakes can move laterally under certain conditions, Dr. Rieser said, sidewinders — the common name for a group of three distantly-related vipers found in the deserts of Africa, the Middle East and North America — have raised this unique form of movement to an art. The sidewinding rattlesnake, for example, can travel at speeds of 18 miles per hour, making it the fastest snake in the world.
Now a new study by Dr. Rieser and her colleagues may have found their secret: scales packed with tiny pits, instead of the minuscule spikes found on the bottom of other snakes. Their research was published on Monday in Proceedings of the National Academy of Sciences.
The microstructure of snake bellies is important to how they move, Dr. Rieser said, because that’s how limbless animals interact with the ground. To examine the microstructure of sidewinder scales, her team used an atomic force microscope to scan naturally shed snake skins, provided by institutions such as the Atlanta Zoo. They then built mathematical models to test how the structures they saw would perform under different kinds of friction.
Although they appear smooth to the naked eye, the belly scales of most snakes have microscopic spikes that are oriented from head to tail. These create a friction between the snake’s body and the ground, Dr. Rieser said, which helps them move forward in a familiar headfirst slither.
Snakes from a wide variety of habitats and ecological roles — including close relatives of the sidewinder rattlesnake, such as cottonmouths or diamondback rattlesnakes — have these prominent spikes on their bellies.
But sidewinding species have either reduced or phased out those spikes, trading them in for belly scales that are pocked with microscopic pits that can move in any particular direction. Dr. Rieser suggests that’s because directional friction makes movement in a frictionless environment harder: “Picture a snake trying to move on linoleum or silk.”
Sidewinding instead depends on lifting large chunks of the body into the air as the animal moves. Scales that create strong directional friction, Dr. Rieser said, do very badly with this kind of movement. But if scale friction is uniform in all directions, it makes sidewinding significantly easier.
The Saharan horned viper and the sidewinding adder of the Namib desert — which are closely related — have belly scales with uniform pits and no spikes. But the sidewinding rattlesnake, which comes from a different branch of the viper family tree, still has a few vestigial belly spikes as well as pits.
One possible explanation for the difference is that the deserts of the North American southwest are only 15,000 to 20,000 years old, compared with the North African deserts, which are seven million to 10 million years old.
“So maybe there’s been less time for American sidewinders to evolve structures that might help this type of movement,” Dr. Rieser said.
While the team’s hypothesis about the precise function of the microscopic pits will require additional study, the loss or reduction of these belly spikes in distantly-related sidewinders suggests that these changes are a direct adaptation to sideways movement, they suggest.
“Given that movement is so crucial to survival, it’s reasonable to think that’s part of the reason this change has occurred,” Dr. Rieser said.
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