Tiny limbs and long bodies: Coordinating lizard locomotion
Credit: Georgia Institute of Technology

The body movements of snakes and lizards are different. The lizards bend from one side to the other. Snakes move like waves that travel down the body. Scientists have wondered about the purpose of the lizards that have long, snakelike limbs. Understanding how these lizards move could shed light on why an evolutionary transition from lizard to snakelike motion occurred.

Using biological experiments, robot models, and a geometric theory of locomotion from the 1980's, researchers at the Georgia Institute of Technology investigated how and why intermediate lizard species use their bodies to move. The research team studied body-limb coordination in lizards. Their multidisciplinary approach uncovered a previously unknown spectrum of body movements in lizards. Their findings were published in the June issue of the Proceedings of the National Academy of Sciences.

Goldman was interested in how intermediate lizards use their bodies and limbs in different environments. More capable wigglingrobots can be inspired by this question.

There is a multi-disciplinary approach.

At a presentation by Philip Bergmann, an associate professor of evolutionary biology at Clark University, a PhD student in Goldman's lab became interested in the short-limbed, long-necked lizard. He reached out to Bergmann because he thought he could help explain how the lizard moved. Goldman's lab received footage of the lizards in the wild.

Eva Erickson, a recent graduate from Georgia Tech and Goldman's lab, used new artificial intelligence techniques to analyze the lizards' body movement in Bergmann's videos. Neural network tracking uses artificial intelligence to identify features of images such as legs and bodies.

It has been thought that organisms use no body bending at all. The researchers were able to see a wide variety of snakelike waves and lizardlike movements in the footage.

"Markerless animal pose estimation software has been greatly improving, allowing much greater insight into the kinematics of organisms." Tracking videos with the program DeepLab cut, we found that these species perform a variety of wave patterns.

How to make sense of the waves was the next question. While there are many ways to think about the waves and what they mean, the information is so complex that it is nearly impossible for humans to comprehend.

Credit: Georgia Institute of Technology

Particle physicists and control theorists have developed a mathematical technique. The concept of legs was added to the theory of geometric mechanics to study idealized locomotion.

The body-limb coordination data was visualized in diagrams using geometric mechanics. As the primary thrust generation shifts from the limbs to the body, they were able to show the advantage of snakelike waves in short- limbed lizards.

The advantage of geometric mechanics is that we don't have to think about every possible way to move.

Findings from the neural network tracking and geometric mechanics allowed the group to form a theory that the style of lizard movement is related to limb size and body length.

The theory was tested with lizards.

They tested the theory in two different ways. They changed the environment and put sand-dwelling lizards in it. The lizards with strong legs were forced to wiggle their way out of the water. The existence of a spectrum of locomotion patterns was supported by the ability to trick lizards into moving.

The advantages of lizardlike and snakelike body movements in the intermediate lizard species were investigated by the team. The robot is a model of a living system that can be used to vary parameters such as limb length and how the lizard's body drags on the ground. They have the ability to test the predictions of their theoretical model and also understand the biological system.

The advantages of lizardlike and snakelike body movements in the intermediate lizard species were investigated by the team. The robot works as a model of a living system. They can use the robot to test the predictions of their theoretical model while also learning more about the intermediate lizard.

The lizard robot can be changed with the addition and removal of a belly plate. The robot was used to conduct similar experiments in sand.

The advantage of snakelike body movement in getting lizards where they need to be was found by the researchers.

The degree of body and limb reduction in lizards is related to how body and limb movements are coordinated. The researchers found that the limbs were used to lift the lizards' bellies off the ground.

There are insights.

They were able to conclude that evolution was acting to shorten limbs and lengthen bodies in a very coordinated and functional way.

Bergmann said that their work helps explain why intermediate species are able to compete with each other for millions of years. They are functional species with their own ecological roles.

Researchers' work can be applied by roboticists. The findings from Goldman's lab have led to the creation of snake, lizard, and amphibian-inspired robots that could one day be used in search and rescue operations.

Goldman said that the next generation ofrobots could have to crawl around in rubble or move around in extraterrestrial environments.

An important part of the study was its multi-disciplinary approach. Each student brought their own expertise to bear on the research question by taking videos from an evolutionary biologist, using artificial intelligence, and building a model to test their hypothesis.

Goldman said that the project was an awesome student led one.

More information: Baxi Chong et al, Coordinating tiny limbs and long bodies: Geometric mechanics of lizard terrestrial swimming, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2118456119 Journal information: Proceedings of the National Academy of Sciences