Animal's Zigzag Movements Increase Stability and Maneuverability

Animals zig-zagging along a path -- be it a lizard sashaying back-and-forth on a wall or a cockroach sidling along the floor -- have long baffled biologists because the movements seem wasteful and inefficient. Why not just move in a straight line?

A multi-institutional team of researchers has found an answer to the question: It's not nearly as inefficient as it seems; doing so allows the animals to increase both stability and maneuverability, which is something engineers often describe as impossible.

"One of the things they teach you in engineering is that you can't have both stability and maneuverability at the same time," said Noah Cowan, a Johns Hopkins associate professor of mechanical engineering who supervised the new research, which is published in the Proceedings of the National Academy of the Sciences. "The Wright Brothers figured this out when they built their early airplanes. They made their planes a little unstable to get the maneuverability they needed."

But some animals seem to bend the rules of engineering as we know it, gaining both stability and maneuverability by moving in a zigzag path.

"Animals are a lot more clever with their mechanics than we often realize," Cowan said. "By using just a little extra energy to control the opposing forces they create during those small shifts in direction, animals seem to increase both stability and maneuverability when they swim, run or fly."

By studying the movement of the tiny glass knifefish in slow motion, Cowan and his cohorts were able to learn more about the reasoning behind the non-linear movements of the fish.

"What is immediately obvious in the slow-motion videos is that the fish constantly move their fins to produce opposing forces. One region of their fin pushes water forward, while the other region pushes the water backward," said Eric Fortune, study co-author and professor of biological sciences at the New Jersey Institute of Technology. "This arrangement is rather counter-intuitive, like two propellers fighting against each other."

The team then developed a mathematical model, which suggested that the counter-intuitive movements actually increased the knifefishes' stability and maneuverability. The team used robotic fish to test the accuracy of their model.

"As an engineer, I think about animals as incredible, living robots," said study's lead author, Shahin Sefati, a doctoral student advised by Cowan. "It has taken several years of exciting multidisciplinary research during my PhD studies to understand these 'robots' better."

The researchers suggest that their study could prove useful for robotics design in the future.

"We are far from duplicating the agility of animals with our most advanced robots," said study co-author Malcolm MacIver, an associate professor of mechanical and biomedical engineering at Northwestern University. "One exciting implication of this work is that we might be held back in making more agile machines by our assumption that it's wasteful or useless to have forces in directions other than the one we are trying to move in. It turns out to be key to improved agility and stability."