Great leaping lizards. Studying how lizards lift their tails when they jump may lead to more-agile robots.

T. Libby, E. Chang-Siu, and P. Jennings/PolyPEDAL Lab & CiBER/UC Berkeley

Tails Guided Leaping Dinosaurs to a Safe Landing

In Jurassic Park, there was no escaping the agile, bloodthirsty Velociraptor. And in real life, this fearsome two-legged dinosaur was probably as quick on its feet as on screen, moving its large tail up and down to keep itself upright as it leaped from boulder to boulder in pursuit of prey. That's the conclusion of a new study that used high-speed videos of lizards and robotic cars to show just how important a movable tail can be for agile, airborne maneuvers.

The work suggests that tailed vehicles might offer a new option for navigating rough terrain. "It provides valuable insights for the next generation of mobile robots," says Mark Cutkosky, a mechanical engineer at Stanford University in Palo Alto, California, who was not involved in the research.

Tails are often an enigma; many creatures have them, but scientists know little about their function, particularly for extinct species. Dinosaur tails are no exception. Researchers have speculated that some species' tails were used in fighting, whereas others for stability. "Studies that use a more theoretical approach coupled with experimental work on animals are the only way we are going to work out what a dinosaur used its tail for," says William Sellers, a zoologist at the University of Manchester in the United Kingdom, who also was not part of the study.

Scientists at the University of California, Berkeley, combined those two approaches. Biomechanicist Robert Full and colleagues videotaped red-headed Agama agama lizards, which are about the size of a small rat, as they ran and vaulted off obstacles with surfaces ranging from smooth paper to gritty sandpaper. Slippery surfaces sometimes would cause a lizard to lose its footing as it jumped, potentially sending it head over heels. But the more perilous the takeoff, the more the animal lifted its tail to compensate for the stumble, keeping it from tumbling out of control (see video), the team reports online today in Nature.

Watch the tail. As it leaps off a slippery surface, the lizard uses its tail to stabilize itself.
Credit: Nature

The researchers followed up this finding by pushing an Agama lizard-sized robotic car off a ramp. The car took a nosedive when it had no tail or a tail that was not movable (see video). But adding a movable tail controlled by a gyroscope that sensed the car's fall kept the vehicle's nose up, allowing it to land on its tires (see video).

Crash. A tail that isn't adjustable doesn't help the robotic car keep its nose up.
Credit: Nature
Safe landing. A movable tail helps stabilize this lizard-sized robotic car.
Credit: Nature

"This made the interpretation of the observed tail motion in lizards very convincing," says Susan Larson, a comparative anatomist at Stony Brook University in New York. "We have long suspected that having a tail helps leaping," Sellers adds, "but this study tells us exactly how much it actually helps, and that's what you really need to know if you want to make use of this information."

The Berkeley researchers developed a mathematical model from their lizard and car data that predicts how a tail affects maneuverability. By plugging in the dimensions of a Velociraptor, taken from fossils, they calculated that the dinosaur's tail might have been better than the Agama lizard's at making aerial acrobatics possible.

"This wonderful study clearly demonstrates how swings of a tail can prevent or produce rotations of the body," says David Carrier, a comparative biomechanicist at the University of Utah in Salt Lake City. However, he points out that big tails can have a downside, as they can slow down an individual that needs to turn around fast to escape or attack a predator. That limitation may explain why airborne mammals, such as bats and many birds that need to twist and turn frequently, often have reduced tails compared with lizards and dinosaurs.

The Berkeley researchers think that actively controlled tails might be a useful addition to search-and-rescue robots that have to maneuver over extremely rough terrain. Current mobile robots don't go fast enough to take advantage of what a tail can offer, but eventually they will, Cutkosky says. Sellers agrees in principle but notes that engineers might not follow nature's lead exactly: "We may not see cars with tails, but we may see other structures [on cars] that perform the same job."