For the likes of tuna or sharks, making their way through the ocean is all about speed and stamina. But for select group of marine species like the Persian carpet flatworm, cuttlefish and the black ghost knifefish, the key to navigating their murky and cramped worlds is agility.
In a study published in PLOS Biology, Northwestern University researchers have demonstrated that all three species evolved the same technique to get around - their elongated fins both undulate, or ripple, lengthwise and oscillate from side to side.
"Why do you see the same traits, such as the camera-lens eye or wings, in animals that are so different and have no common ancestor with that trait?" Malcolm MacIver, one of the the study's senior authors, said. "It is because there is a finite number of ways to really do something well. In our study, we have quantified how an unusual group of swimming animals optimizes force and, therefore, speed."
Now, he and other researchers are studying video footage of these fish in motion, creating computer simulations, and have created a robotic fish named "Ghostbot" to understand how they get around. And in doing so, they hope to one day inspire the next generation of underwater vehicles.
"There is a real need for underwater vehicles that are more maneuverable than the current things we use for disaster recovery or inspection of structures underwater," MacIver said. "They are very nonmaneuverable, which results in things like with the BP oil disaster, the robots banging into oil well heads or what have you."
When MacIver and his colleagues broadened their study to look at 22 animals that demonstrate the undulation-oscillation technique, they found the length of one undulation during swimming divided by the mean amplitude of the sideways movement was always a ratio of around 20. They believe the same observation may be found in up to 1,000 species.
Neelesh A. Patankar, the other senior author on the paper, said the study illustrates an example of how movement was critical to the fish's survival and how physics helped select the most effective, efficient technique.
"Chance offers many possibilities as to how a fish can swim, but physics and the animal's environment puts constraints on these possibilities," he said. "In this case, the selection pressure is very high, pushing the animal to one particular solution, and necessity triumphs."
Now, he and MacIver hope to translate nature's innovation into cutting edge technologies.
Looking for inspiration in the physics of fish movements is just one example of how biomimetics - devices that mimic nature to solve human problems - is revolutionizing the designs of robots.
Boston Dynamics has developed a robot that runs like a cheetah while a New York University scientist has demonstrated a jellyfish-like flying machine. There is even a robot inspired by flapping baby turtles.
And just last week, researchers Johns Hopkins University, Columbia University and the University of Maryland unveiled a study in Cell describing some of the secrets behind the bat's acrobatic maneuverability - details, they say, that could help develop a better drone.
In their study, Johns Hopkins neuroscientist Cynthia F. Moss and her colleagues show how sensory receptors in wings of the big brown bats send information about airflow to neurons in the brain. That enables the bat to make split-second flight control adjustments, allowing them to maneuver through dense forests or swoop down to catch flying insects.
Moss said there is an interest to developing technology that duplicates "the sensing and control used by the bat" to build something that not only "senses air flow but stretches and can change shape as it powers its flight."
"Amazon is talking about drones delivering packages. Well, a drone would have to be pretty maneuverable to get to someone's doorstep unless it's dropped with a little parachute," she said. "If they wanted to maneuver through trees to get a particular location, having some of the capability that bats exhibit would enable that kind of control."
But in many cases, mimicking the movements of birds, animals and insects is far beyond the reach of current technology. And as they learn more about these movements, researchers are coming to appreciate how complex they are.
"What they can do far exceeds what current aerial vehicles can do," Moss said of bats and other creatures, adding that it partly is due to a limitation of the materials used in aerial vehicles and the constraints that exist in transferring what is known about biological systems into engineering systems.
"We study these animals and we learn bits and pieces of how they work but we don't have complete understanding of the entire system," she said.
Still, researchers are closing the knowledge gap. In the case of the fish, MacIver said their work in quantifying their movements has informed the design of a free-swimming robot that moves in all directions and, more broadly, could produce a class of robots that operate more efficiently and thus uses less battery power.
"The problem with the biomimetics robots of the past is that we didn't really understand the motion well enough to sort of hand it over to an engineer and say program your robot like this and this is how it is going move," MacIver said. "What we are getting to with this work - because it's oriented around understanding the basic mechanisms - is that we are getting to the point where we can hand it off to a manufacturer and say put it on and dial up the parameters like this."