A tail is a handy thing to have — it can aid in flight, swat insects, display showy plumage, grasp onto tree limbs or scare away predators with a brisk threatening twitch.
One of the tail’s basic functions is thought to be movement and balance. After a careful analysis of leaping lizards, researchers have figured out how the creatures use their tails to angle themselves in midair — and demonstrated the process in a tailed robot, christened Tailbot.
The experiment, reported in the journal Nature, was not as simple as it may sound. Figuring out what a tail actually does is difficult when an animal — a lizard, say — is on the ground running, with as many as four feet hitting the surface, helping to stabilise the animal. The one time feet aren’t getting in the way is when the animal is in the air.
So study senior author Robert J Full of the University of California, Berkeley and co-workers coaxed red-headed Agama lizards to run on an acrylic track and over a box, which acted like a vault. From the vault, the lizards leaped onto a vertical wall in front of them. The researchers videotaped the lizards in midair.
To land properly on the wall, the creatures would have to tilt their bodies upward somehow — no small feat, especially in the instances when the researchers wrapped the vault in slippery-smooth card stock, rather than sandpaper.
This is where the tail came in. The scientists saw that as the lizards jumped, they flipped their tails upward. As a result, their bodies would tilt up too, rather like a V closing its arms in. The more slippery the vaulting surface the more the creatures bent their tails in order to get their bodies to rise upward.
Arching the tail and bending it toward the body caused the body to arch up toward the tail in reaction, Full explained.
The strategy is analogous to a tightrope walker holding a balancing stick across his body, said Robert McNeill Alexander, a biomechanist at the University of Leeds in England who was not involved in the study. By tilting the balancing stick’s right side upward, the walker can make his body lean toward the right side — thus, conserving angular momentum and remaining in balance.
The researchers tested their tail-balance theory in a mathematical model and then plugged their models into Tailbot, the wheeled, lizard-sized robot they built.
They had the robot speed up a ramp and into the air, as if on a ski jump. When Tailbot’s tail was fitted to be a rigid part of the body, the robot took a nose dive.
But when the robot was programmed to sense its angle to the ground and lift its tail accordingly, its nose stayed titled upward, allowing it to land on its feet — or wheels — when it hit the ground.
Perhaps the tails of some dinosaurs were used in a similar way, the authors suggested. Though the behemoth meat-eater Tyrannosaurus rex was too big to effectively use its tail for jumping, a tail might have come in very handy for smaller dinosaurs such as Deinonychus, which may have jumped onto the backs of much larger prey.
And when they modelled the tail’s influence on another dinosaur, a Velociraptor, using a two-dimensional model, the authors calculated that this dinosaur may have been able to use its tail to stabilise its body even more effectively than the Agama lizards were.
A jumping robot would be of great use on rough terrain or as a search-and-rescue robot in emergency situations where it was too dangerous to send in people, Alexander said. “Suppose you were designing a robot that had to go over really rough country, across crevasses and ice ... you’d like to have a robot that could jump across gaps,” he said.