When it comes to sliding, most snakes do the same thing: straight ahead. But for snakes that live in deserts, getting around can be a challenge.
“As we know from trying to move about on sand on a beach or other places, it can be difficult to move on these materials that emerge below you as you go,” said Jennifer Rieser, professor of physics at Emory University in Atlanta.
That’s why cross winds slide sideways. Although some snakes can move sideways under certain conditions, cross winds – the common name for a group of three distantly related vipers found in the deserts of Africa, the Middle East, and North America – have made this unique form of movement an art . For example, the crosswind rattlesnake can travel at a speed of 18 miles per hour, making it the fastest snake in the world.
Now maybe a new study by Dr. Rieser and her colleagues found their secret: sheds full of tiny pits instead of the tiny spines on the bottom of other snakes. Her research was published in Proceedings of the National Academy of Sciences on Monday.
The microstructure of snake bellies is important to their movement, said Dr. Rieser, because this is how limp animals interact with the ground. To study 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 created mathematical models to test how the structures they saw would behave under different types of friction.
Although they appear smooth to the naked eye, the ventral scales of most snakes have microscopic spines that line up from head to tail. These create friction between the snake’s body and the ground, said Dr. Rieser, who helps them move forward in a familiar upside-down glide.
Snakes from a variety of habitats and ecological roles – including close relatives of the Sidewinder rattlesnake like cottonmouth or diamond-backed rattlesnakes – have these distinctive spines on their belly.
But crosswind types have either reduced or tapered these peaks, trading them for ventral scales, which are littered with microscopic pits that can move in a specific direction. Dr. Rieser suggests that directional friction makes movement difficult in a smooth environment: “Imagine a snake trying to move on linoleum or silk.”
The cross wind, instead, relies on large parts of the body being lifted into the air while the animal moves. Scales that create a lot of directional friction do very poorly with this type of movement, said Dr. Rieser. However, when the scale friction is even in all directions, the cross wind is greatly eased.
The Saharan horned viper and the crosswind adder of the Namib Desert – which are closely related to each other – have ventral scales with uniform pits and without spines. But the crosswind rattlesnake, taken from another branch of the Viper family tree, still has a few traces of belly and 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 to the North African deserts, which are seven to 10 million years old.
“American sidewinders may have less time to develop structures that could help this type of movement,” said Dr. Rieser.
While the team’s hypothesis about the exact function of the microscopic pits requires additional study, the loss or reduction of these ventral tips in distantly related curlers suggests that these changes are a direct adjustment to sideways movement.
“Given that exercise is so vital to survival, it is reasonable to believe that this is part of the reason for this change,” said Dr. Rieser.