How Are Snakes Able To Move In A Straight Line? (And What Can We Learn From It?)

When talking about snake locomotion, we generally just describe their movement as "slithering". Even without limbs, snakes are masters of of both horizontal and vertical movement, and are able to traverse paths that are inaccessible to other limbed animals. However, to overcome different challenges and environments, snakes have actually developed four different means of movement, each with their own advantages.

Serpentine: This is the typical side-to-side motion snakes use when traversing open, rough terrain or moving through the water (also known as lateral undulation).

Concertina: Some snakes will bunch up their bodies into alternating curves, then propel themselves forward as they straighten (almost like an inchworm).

Sidewinding: By bending in waves both side to side and up and down, some snakes are able to move swiftly while making only limited contact with the ground. This is especially useful in desert species, where such movement helps climb large dunes and limit direct contact with hot sands.

Then finally there is Rectilinear movement. This is the means by which snakes are able to move forward in a straight line, with little to no side-to-side movement. This allows snakes to enter burrows or other tight spaces that are not much larger than they are.

 "It's a very good way to move in confined spaces. A lot of heavy-bodied snakes use this locomotion: vipers, boa constrictors, anacondas and pythons." - Bruce Jayne, University of Cincinnati biologist Source

While we have come to understand the various means of snake locomotion fairly well over the past 70 years (biologist H.W. Lissmann  first studied the coordination of muscle activity and skin movement in snakes in 1950 [source]), a conclusive understanding of rectilinear locomotion has eluded us. During his studies, Lissman hypothesized that such movement was possible thanks to the snakes' powerful muscles and loose, flexible and squishy belly skin; these factors allowed the snakes to scoot along without having to bend their spine.

Biologist Bruce Jayne (University of Cincinnati) and Steven Newman set out to test Lissman's hypothesis using technology that was unavailable at the time. Using high-definition digital cameras while also recording the electrical impulses generated by particular muscles, they produced an electromyogram (sort of like an EKG), which showed the coordination between the snake's muscles, skin and body. Using boa constrictors (large bodied snakes that generally travel in a straight line across the jungle floor), the two recorded high definition video of the snakes slithering across a horizontal surface that had been marked with distance references. Small dots were also put on the sides of the snakes to mark the subtle movements of their skin.

The movement of the skin is key to rectilinear locomotion. As the reptile moves along, the ventral skin (the belly skin) flexes far more than that over the ribs and back. The skin grips the ground like a car's tire treads, and muscles then pull the skeleture forward in a fluid and seamless motion. This fluid movement is the result of the muscles sequentially activating from the head to the tail. The snake stretches its belly skin forward to gain traction, then the vertebral column is pulled forward; wash, rinse, repeat.

"The vertebral column moves forward at a constant rate.One set of muscles pulls the skin forward and then it gets anchored in place. And opposite antagonistic muscles pull on the vertebral column." -Newman Source

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Though the technology of the time limited his ability to study such moments, the study did confirm that Lissman's hypothesis was largely correct.

 "But he hypothesized that the muscle that shortens the skin was the mechanism that propels a snake forward. He got that wrong. But given the time he conducted the study, I marvel at how he was able to do it. I have tremendous admiration for his insights." -Jayne Source

The advantages of such movement for a snake is fairly obvious. Rectilinear movement enables these animals to burrow underground with ease as they don't require the room to bend their bodies. This is ideal for both taking shelter from predators and unfavorable environmental conditions and in obtaining food. For species that feed largely on rodents, this movement allows them to easily enter burrows and seek out their prey where it lives.

"Snakes evolved from burrowing ancestors. You can fit in much narrower holes or tunnels by moving this way than if you had to bend your body and push against something." -Newman Source

So, the big scientific question remains: how does this discovery help us? Actually, this discovery of fluid rectilinear movement could profoundly impact the robotics industry. Developers have already begun working on snake-like robots that move in a serpentine motion in the hopes that these robots could inspect pipelines or underwater equipment. However, if these machines were able to utilize a snake's rectilinear locomotion, they would be able to traverse these tight spaces far more efficiently, which could be a real game changer.

"This research could inform robotics. It would be a big advantage to be able to move in straight lines in small, confined spaces. They could use snake-like robots for search-and-rescue in debris and collapsed buildings." -Newman Source

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In other words, the simple understanding of how these reptiles move could be utilized to save human lives. Moving through the tightest spaces, these robots could find victims trapped among debris and allow first responders to focus their rescue efforts. It's just one more example of how these amazing animals, often so feared and hated by humans, could be key to improving human health, wellness and technological advancement.

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Article Link: https://www.sciencedaily.com/