Knifefish 'break traditional rule of engineering'

The elongated anal fin of a knifefish allows it to increase both stability and manoeuvrability, a feat that is often described as impossible in engineering textbooks.

Professor Eric Fortune has studied Glass knifefish for nearly 20 years, as he tries to understand how their tiny brains control complex electrical behaviours. But he could not help but be intrigued by the special "ribbon fin" that 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 now Fortune and a multi-disciplinary team of researchers report that these opposing forces are anything but wasteful.

"I read a Navy flight training manual that had a full page dedicated to the inherent tradeoff between stability and manoeuvrability", says Fortune, an associate professor of biology at New Jersey Institute of Technology. "Apparently the knifefish didn’t read that manual, since the opposing forces surprisingly make the fish simultaneously more stable and more manoeuvrable."

When an animal or vehicle is stable, it resists changes in direction. On the other hand, if it is manoeuvrable, 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 realise," said Noah Cowan, a professor of mechanical engineering at The Johns Hopkins University. "By using just a little extra energy to control the opposing forces, animals seem to increase both stability and manoeuvrability 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, they nevertheless achieve better locomotor performance than current robotic systems."

As part of the study, Fortune used slow-motion video to film the fish to study its fin movements. "It is immediately obvious in the slow-motion videos that the fish constantly move their fins to produce opposing forces," he says. "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, showed this odd arrangement generates stabilising 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 manoeuverable. 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 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.

The results of the study was published in the Proceedings of the National Academy of Sciences (PNAS).

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