Unlocking the secrets of Animal Locomotion
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Robert Full with centipede
Photo ©2000 Peter Menzel, from Robo sapiens: Evolution of a New Species (MIT Press)

VIDEO: See crabs, cockroaches, and centipedes in action in the lab.

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In the PolyPEDAL Lab, animals show the way

Squeamish students might want to avoid Robert Full's PolyPEDAL Lab, where the test subjects are cockroaches, crabs, beetles, ants, centipedes, millipedes and various lizards.

"I personally think cockroaches are repulsive," says Full. "But you don't have to like them to find them interesting." The lab seeks to understand how many-footed animals move from many different perspectives: what kind of control system they use, how their skeletons and muscles factor into the motion, and how they interact with their environments to overcome or avoid obstacles.

"We follow the August Krogh principle, which says that for many problems, there is an animal you can study as the best model," explains Full.

Jell-O running tracks

As the name PolyPEDAL suggests (it stands for many-footed — the "poly" part — plus Performance, Energetics, Dynamics of Animal Locomotion), Full and his student assistants study every facet of animal movement. They employ instruments such as treadmills and force platforms that are sensitive enough to measure even a running ant's side-to-side energy. To capture the force exerted by a single tiny foot, they built a special platform with photoelastic material sandwiched between two polarizing filters then beamed light through all three. Where an animal sets a foot down, light penetrates the filters and is captured by a camera.

(The initial version of the photoelastic material was store-bought Jell-O; when the insects proved to be more interested in eating the material than walking on it, Full switched to plain gelatin.)

Alternating tripods and pogo-stick principles

The first surprising discovery Full and his students made about multi-legged animal locomotion was that rather than moving like a wheel — raising and lowering each leg in turn, as previous observers had supposed — instead they follow the same bouncing principle as a running human. Two legs on one side and one leg on the other form a tripod that acts like one of our legs, while the opposing leg-tripod swings forward. Arthropods with more than six legs, such as centipedes, also follow this principle, with groups of legs from the front, mid, and rear sections forming the tripod.

Multi-legged animals move like their biped cousins in other ways.

Aided by force platforms and high-speed cameras, Full's team found that crabs exerted the same force patterns when running as humans do. When animals run, says Full, their legs become like pogo sticks equipped with shock absorbers and springs. "Crabs may look completely different — they run sideways, they have different numbers of legs, they have their skeleton on the outside, and they're cold-blooded — but they bounce along just like humans do," says Full. As it turns out, crabs, insects, lizards and other animals also generate the same amount of mechanical energy per pound of body weight as humans do when we walk or run.

cockroach running
And he's off: a bipedal sprinting roach

(Another wacky congruence between the bug world and ours: At top scuttling speeds, the roach actually rears up on its hind legs — which are longer than its front and middle pairs — and runs upright, like a human sprinter. The roach is also the world's fastest land insect, able to run the human equivalent of 200 miles per hour, as Full's students relished informing the Guinness Book of World Records.)

To learn how insects correct their motion after being knocked off-stride, Full and his students mounted a tiny cannon on the insect's back. The bug-backed cannon fired a pellet, giving the researchers an exact known perturbation to measure against the insect's recovery. They discovered that the animal self-stabilized, regaining its equilibrium without any feedback from the brain. This, they deduced, was from the built-in springiness of the legs: push to one side, and the legs recover on their own.

Lesson learned: by incorporating springiness into robot legs, Full's collaborators were able to simplify the units' control systems.
A roach with a tiny, pellet-firing cannon mounted on its back to measure how it reacts to perturbation As the cannon fires, the roach stumbles. Within a few strides after the disturbance, the roach's springy legs self-correct.
A roach with a tiny, pellet-firing cannon mounted on its back to measure how it reacts to perturbation. As the cannon fires, the roach stumbles. Within a few strides after the disturbance, the roach's springy legs self-correct.

Don't fence me in

green gecko

To study how animals interact with their environment, says Full, "we chose geckos, because they can run on any surface and go anywhere."

Geckos can run up a wall effortlessly, as fast as a meter per second, and they can hang from the ceiling supported by just one foot. As Full and his fellow researchers studied the gecko's feet, they discovered something amazing. Its toes operate like those party favors that uncurl as you blow, then retract into a circle. When the gecko's toes unfurl, they stick to surfaces; to move on, they peel them off, just like removing tape from a wall ... only 30 times per second.

How do they do it?

Full found that the millions of tiny branched hairs on gecko toes — imagine if your split ends had split ends that had split ends— can generate enormous amounts of force. Not by suction, or interlocking into cracks, or glue, but by using the weak intermolecular forces that hold atoms together. His engineering collaborator here at Berkeley, Ron Fearing, has managed to synthesize the tip of one of these hairs (for details, read the news release). Soon, a gecko toe hair could inspire the first-of-its-kind, self-cleaning adhesive.

small cockroach