How did the lizard cross the road? It depends.
In a new study published in the prestigious Proceedings of the National Academy of Sciences, Philip Bergmann, professor of biology, investigated how a lizard’s movement is related to its body form — fully limbed and short-bodied, limbless and long-bodied, or somewhere in between. In “Coordinating tiny limbs and long bodies: Geometric mechanics of lizard terrestrial swimming,” Bergmann and his co-authors, Daniel Goldman, Baxi Chong, Eva Erickson, and Tianyu Wang of Georgia Tech, conducted multidisciplinary research to examine how the evolutionary process has impacted lizard locomotion.
“Snakes evolved from lizards, and so their ancestors used to have four limbs and a short body,” Bergmann explains. “At some point, it’s thought that they became burrowers — and in the process, they elongated their bodies and lost their limbs. And that’s one of the key hypotheses behind how snakes evolved.”
To study body–limb coordination in diverse lizards, Bergmann and his colleagues used biological experiments, a geometric theory of locomotion, and even a lizard robot.
There are key differences in the ways each animal moves, Bergmann says. Fully limbed lizards use a side-to-side motion or “standing wave” of their body, while their limbs propel them forward. Limbless lizards and snakes use their bellies to push themselves along, and the motion moves along their body from front to back, in what’s known as a “traveling wave.”
However, Bergmann adds, there are “in-between” species that have elongated bodies and barely any limbs. These are the animals he and his colleagues studied in order to understand how they move.
“What we found was that, as you move from that pure lizard-like or quadrupedal body form to that snake-like body form, you get a transition in how the body bends … as they put more of their weight on the body. It’s suddenly a situation where the body and the limbs share the weight of the animal. They’re transitioning more and more to that traveling wave that a snake uses. Belly involvement, the propulsion from the body or the belly being in contact with the ground, means that the traveling wave enhances their locomotion and allows them to move more quickly.” The researchers refer to this movement as “terrestrial swimming.”
Geometric mechanics, a type of mathematical modeling, was used to predict how the animals should be moving based on what they look like — predictions that were then tested using a lizard robot. The mathematical modeling matched the robot’s movements and confirmed what they had observed among real animals in nature. The snake-like traveling wave movement was the optimal method of forward motion when the body was in contact with the ground.
“As the evolutionary biologist on the team, it’s exciting to be able to support these evolutionary stories through hard data,” Bergmann says.
In addition, the research has potential applications in the field of robotics, given what they discovered about the most efficient means of motion on different surfaces.
Finding a common language between the biology and physics areas of the study was among the most challenging aspects of the project, Bergmann says. The work of observing rare species was completed years ago in remote parts of Australia or the Philippines.
The Proceedings of the National Academy of Sciences (PNAS), a peer-reviewed journal of the National Academy of Sciences (NAS), is one of the world’s most-cited multidisciplinary scientific journals. Having their paper accepted by the journal is a notable accomplishment. “It publishes research across all of the sciences, not just biology, so they’re very selective in terms of who gets published there, or the significance of the research that they include,” Bergmann says. “It’s a pretty high bar.”