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A Fish That Pushes in the Wrong Direction Solves a Mystery of Animal Locomotion

Photo credit: Will Kirk

For nearly 20 years, Professor Eric Fortune has studied glass knifefish, a species of three-inch long electric fish that lives in the Amazon Basin. In his laboratory he tries to understand how their tiny brains control complex electrical behaviors. But he could not help but be intrigued by the special "ribbon fin" that knifefish use to swim back and forth. The fin oscillates at both ends, allowing the fish to move forward or backward. Biologists have long wondered why an animal would produce seemingly wasteful forces that directly oppose each other while not aiding its movement.    

But in the Nov. 4-8 online edition of Proceedings of the National Academy of Sciences (PNAS), Fortune and a multi-disciplinary team of researchers report that these opposing forces are anything but wasteful. Rather, they allow animals to increase both stability and maneuverability, a feat that is often described as impossible in engineering textbooks.

“I read a Navy flight training manual that had a full page dedicated to the inherent tradeoff between stability and maneuverability, says Fortune, an associate professor of biology at NJIT. “Apparently the knifefish didn’t read that manual, since the opposing forces surprisingly make the fish simultaneously more stable and more maneuverable.”

When an animal or vehicle is stable, it resists changes in direction. On the other hand, if it is maneuverable, it has the ability to quickly change course. Generally, engineers assume that a system can rely on one property or the other—but not both. Yet some animals prove an exception to the rule.

“Animals are a lot more clever with their mechanics than we often realize,” said Noah Cowan, a professor of mechanical engineering at The Johns Hopkins University and the senior author of the multi-disciplinary research team. “By using just a little extra energy to control the opposing forces, animals seem to increase both stability and maneuverability when they swim, run or fly.”

And Fortune suspects that the study will inspire young engineers to approach mechanical design in novel ways. “Despite the fact that the knifefish break a traditional rule of engineering,” says Fortune, “they nevertheless achieve better locomotor performance than current robotic systems.”

To conduct its study, the team used a combination of careful observations of the fish, mathematical modeling and an analysis of a swimming robot. Working in his NJIT lab with students and in collaboration with his colleagues at Johns Hopkins, Fortune used slow-motion video to film the fish to study its fin movements: What the videos revealed was startlingly counterintuitive. 

“It is immediately obvious in the slow-motion videos is that the fish constantly move their fins to produce opposing forces,” says Fortune. “One region of their fin pushes water forward, while the other region pushes the water backward. This arrangement is rather counter-intuitive, like two propellers fighting against each other.”

A mathematical model designed by Shahin Sefati, a graduate student at Johns Hopkins and a lead author of the research project, showed that this odd arrangement generates stabilizing forces. But the model also suggested that the opposing forces simultaneously improved the ability of the animal to change its velocity, thereby making the animal more maneuverable. The team then tested this model using a robot in the laboratory of Malcolm MacIver at Northwestern University; the robot mimicked the fish’s fin movements.

One exciting implication of study is its possible application to robotics systems, including the design of sophisticated robots and aircraft. Designers and engineers might make simple changes to propulsion systems, such as tilting engines or motors so that some of the thrusts oppose each other. Such an arrangement might waste some energy, but this cost may be more than offset by making a robot or aircraft simpler to operate and thus safer.

Fortune joined the NJIT faculty last year as part of a university initiative to increase interdisciplinary research.   

“This study is a good example of how engineers can look to nature with the tools of biology to inspire new approaches to solving fundamental design challenges in engineering,” says Fortune.  “In the other direction, biologists benefit from the application of the analytic tools and quantitative approaches that are routine in engineering. NJIT, with its focus on interdisciplinary research, is just the ecosystem we need to translate these ideas into new technologies.”   

One of the nation’s leading public technological universities, New Jersey Institute of Technology (NJIT) is a top-tier research university that prepares students to become leaders in the technology-dependent economy of the 21st century. NJIT’s multidisciplinary curriculum and computing-intensive approach to education provide technological proficiency, business acumen and leadership skills. With an enrollment of 11,400 graduate and undergraduate students, NJIT offers small-campus intimacy with the resources of a major public research university. NJIT is a global leader in such fields as solar research, nanotechnology, resilient design, tissue engineering and cybersecurity, in addition to others. NJIT ranks 5th among U.S. polytechnic universities in research expenditures, topping $121 million, and is among the top 1 percent of public colleges and universities in return on educational investment, according to PayScale.com. NJIT has a $1.74 billion annual economic impact on the State of New Jersey.