Slithering Snakes, Swimming Lizards: Researchers Study Reptile Locomotion for Insights into Future Robotics

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By studying movement of reptiles, researchers could develop more options of locomotion of robots, especially in confined areas.

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Written by Abby Vogel

Reptiles use unique forms of locomotion to get around in the world. Legless reptiles use their entire bodies for movement, while some reptiles with legs choose between using legs or their bodies – depending on the environment.

Georgia Tech researchers recently published studies detailing how lizards and snakes move across and through different environments. Insights from this research could give the developers of future generations of robots more options for locomotion, especially in confined areas.

How Sandfish Swim

A study published in the July 17, 2009, issue of the journal Science details how sandfish – small lizards with smooth scales – move rapidly within desert sand. In this first thorough examination of subsurface sandfish locomotion, Georgia Tech researchers found that the animals place their limbs against their sides and create a wave motion with their bodies to propel themselves through granular media.

“When started above the surface, the animals dive into the sand within a half second. Once below the surface, they no longer use their limbs for propulsion – instead, they move forward by propagating a traveling wave down their bodies like a snake,” says study leader Daniel Goldman, an assistant professor in Georgia Tech’s School of Physics.

With funding from the National Science Foundation and the Burroughs Wellcome Fund, the research team used high-speed X-ray imaging to visualize sandfish – formally known as Scincus scincus – burrowing into and through sand. The team used that information to develop a physics model of the lizard’s locomotion.

The sandfish used in this study inhabits the Sahara desert in Africa and is approximately four inches long. It uses its long, wedge-shaped snout and countersunk lower jaw to rapidly bury into and swim within sand. The sandfish’s body has flattened sides and is covered with smooth shiny scales, its legs are short and sturdy with long and flattened fringed toes and its tail tapers to a fine point.

To conduct controlled experiments with the sandfish, Goldman and graduate students Ryan Maladen, Yang Ding and Chen Li built a seven-inch by eight-inch by four-inch-deep glass bead-filled container with tiny holes in the bottom through which air could be blown. The air pulses elevated the beads and caused them to settle into a loosely packed solid state. Repeated pulses of air compacted the material, allowing the researchers to closely control the density of the material.

“Because loosely packed media is easier to push through and closely packed is harder to push through, we thought there should be some difference in the sandfish’s locomotion,” says Goldman. “But the results surprised us because the density of the granular media did not affect how the sandfish traveled through the sand; it was always the same undulatory wavelike pattern.”

By tracking the sandfish in the X-ray images as it swam through the glass beads, Goldman was able to characterize the sandfish’s motion – called its kinematics – using a single-period sinusoidal wave that traveled from the head to the tail.

“The large amplitude waves over the entire body are unlike the kinematics of other undulatory swimming organisms that are the same size as the sandfish, like eels, which propagate waves that start with a small amplitude that gets larger toward the tail,” explains Goldman.

After collecting the experimental data, Goldman’s team developed a physics model to predict the speed at which sandfish swim. The model allowed the researchers to partition the body of the sandfish into segments, each of which generated thrust and experienced drag when moving through the granular environment.

To establish the equations for drag through sand, the researchers measured the granular thrust and drag forces on a small stainless steel cylindrical rod, thus allowing them to predict the wave efficiency and optimal kinematics. They found that the faster the sandfish propagate the wave, the faster they move forward through granular media – up to speeds of six inches per second. This speed allows the animal to escape predators and the heat of the desert surface, and to quickly swim to ambush surface prey they detect from vibrations.

“The results demonstrate that burrowing and swimming in complex media like sand can have intricacy similar to that of movement in air or water, and that organisms can exploit the solid and fluid-like properties of these media to move effectively within them,” notes Goldman.

Understanding the mechanics of subsurface movement could reveal how small organisms like worms, scorpions, snakes and lizards can transform landscapes by their burrowing actions. This research may also help engineers build sandfish-like robots that can travel through complex environments.

“If something nasty was buried in unconsolidated material, such as rubble, debris or sand, and you wanted to find it, you would need a device that could scamper on the surface, but also swim underneath the surface,” Goldman says. “Since our work aims to fundamentally understand how the best animals in nature move in these complex unstructured environments, it could be very valuable information for this type of research.”

How Snakes Slither

Snakes use both friction generated by their scales and redistribution of their weight to slither along flat surfaces, researchers at Georgia Tech and New York University have learned. Their findings, which appeared June 8, 2009, in the journal Proceedings of the National Academy of Sciences, run counter to previous studies that have suggested snakes move by pushing laterally against rocks and branches.

Insights from the research could give developers of future generations of robots more options for locomotion, especially in confined areas.

“We found that snakes’ belly scales are oriented so that snakes resist sliding toward their tails and flanks,” says the paper’s lead author, David Hu, an assistant professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering. “These scales give the snakes a preferred direction of motion, which makes snake movement a lot like that of wheels, cross-country skis, or ice skates. In all these examples, sliding forward takes less work than does sliding sideways.”

The study, conducted while Hu was a postdoctoral researcher at New York University’s Courant Institute of Mathematical Sciences, centered on the frictional anisotropy – or resistance to sliding in certain directions – of a snake’s belly scales. While previous investigators had suggested that the frictional anisotropy of these scales might play a role in locomotion over flat surfaces, the details of this process had not been understood.

To explore this issue, the researchers first developed a theoretical model of a snake’s movement. The model determined the expected speed of a snake’s center of mass as a function of the speed and size of its body waves, taking into account the laws of friction and the scales’ frictional anisotropy. The model suggested that a snake’s motion arises through the interaction of surface friction and its internal body forces.

“The animals propel themselves using their muscles to move their bodies in a wave. As the wave travels backwards through its body, the snake’s scales catch the ground, generating a frictional force that propels it forward,” explains Hu.

To study the model’s accuracy in describing the movement of real snakes, the researchers measured the sliding resistance of snake scales and monitored the movement of snakes through a series of experiments on flat, inclined, smooth and rough surfaces. They employed video and time-lapse photography to gauge movements of the snakes.

First, the research team measured the ability of milk snakes to slither on rough cloth and a smooth plank. The snakes had trouble moving on the smooth surface, but could move more easily on the cloth-covered one. However, the snakes ran into movement difficulties again when researchers fitted them with a cloth jacket, which eliminated the scale frictional anistropy.

Hu also anesthetized snakes and placed them head-first, backwards and sideways over inclined smooth and rough surfaces. On the smooth surface, friction was fairly evenly matched in all directions, whereas on the rough surface, snakes slid easily in the forward direction, but their scale friction resisted sliding backwards or sideways. The researchers found that it was twice as hard to move the snakes sideways as it was to slide the animals forward.

“The friction was caused by the orientation of the snakes’ scales, which are arranged like shingles on a roof to resist such movements,” notes Hu.

That test provided a friction coefficient that could be studied with the computer model. With that value included, the theoretical snake followed roughly the same path as the real snakes. However, the speeds predicted by the model were lower than those the researchers observed in the snake experiments.

To find out why, Hu’s team placed moving snakes on a photoelastic gelatin that lit up when force was applied. They found that the snakes lift parts of their bodies slightly off the ground when moving. This helps reduce unwanted friction and applies greater pressure to the parts of the body wave that are pushing the snake forward. While friction accounts for about 65 percent of the forward movement, this weight redistribution by the snake accounts for the other 35 percent, according to Hu.

After factoring this into the model, the results showed a close relationship between what the model predicted and the snakes’ actual movements. The theoretical predictions of the model were generally consistent with the snakes’ actual body speeds on both flat and inclined surfaces.

“In the future, understanding snake locomotion might help engineers design better snake robots, which can be used to maneuver into tight spaces,” Hu adds.

The study’s other co-authors were Jasmine Nirody and Terri Scott, both undergraduate researchers at New York University, and Michael Shelley, a professor of mathematics and neural science and the Lilian and George Lyttle Professor of Applied Mathematics at Courant.

The information on the sandfish is based upon work supported by the National Science Foundation (NSF) under Award No. PHY- 0749991 and the Burroughs Wellcome Fund. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the researcher and do not necessarily reflect the views of the NSF. James Devitt, deputy director for media relations at New York University, contributed to the portion of this article relating to snakes.

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