How Fish Swim: Scientists Reveal Secret Behind Undulatory Swimmers

Fish are efficient swimmers, gliding through the water peacefully or quickly darting to escape predators. Yet actually finding out how they do it is more complicated than you might expect. Now, scientists have discovered exactly how fish swim, which may help develop biologically inspired machines that could cruise the world's oceans.

Scientists have long looked to animals in order to design machines. The very first attempts at flight were actually inspired by the flapping motion of bird's wings. These constructions, though, were unsuccessful--and for a good reason. The mechanics that allow a bird to fly--or a fish to swim--are far more intricate than they look at first. Every motion needs to be taken into account in order to create a vehicle or machine that's inspired by biological movement.

"If we could play God and create an undulatory swimmer, how still should its body be? At what save frequency should its body undulate so it moves at its top speed? How does its brain control those movements?" said Neelesh Patankar, professor of mechanical engineering at Northwestern, in a news release. "Millennia ago, undulatory swimmers like eels that had the right mechanical properties are the ones that would have survived."

It would have been difficult to create actual models of a fish to test them, trying to find the right movement; that's why the researchers turned to computers. They used computational methods to test assumptions about the preferred evolutionary characteristics of undulatory swimming. For example, species with low muscle activation frequency and high body stiffness were the most successful.

It turns out that the optimal stiffness for undulatory swimmers is the same as the experimentally determined stiffness of undulatory swimmers with a backbone. In fact, the research seems to suggest that precursors of a backbone would have given rise to animals with the appropriate body stiffness. This, in turn, would have been mechanically beneficial for the emergence of swimming vertebrates.

That's not all they found, though. After analyzing the curvature of a fish's undulations, the researchers found that a simple movement pattern gave rise to a complicated-looking deformation.

"This suggests that the animal does not need precise control of its movements," said Patankar.

These findings reveal not only how a fish swims, but also unifies the concepts of both active and passing swimming. The research could give rise to fish-inspired vehicles or robots that could explore the deep ocean more efficiently.

The findings are published in the journal PLOS Computational Biology.