An African Agama lizard swings its tail upward to prevent pitching forward after a slip during take-off. Photo: Robert Full laboratory, UC Berkeley, courtesy of Nature.
Leaping lizards have a message for robots: Get a tail.
University of California, Berkeley, biologists and
engineers—including undergraduate and graduate students—studied how lizards
manage to leap successfully even when they slip and stumble. They found that
lizards swing their tails upward to prevent them from pitching head-over-heels
into a rock.
But after the team added a tail to a robotic car named Tailbot, they
discovered that counteracting the effect of a slip is not as simple as throwing
your tail in the air. Instead, robots and lizards must actively adjust the
angle of their tails just right to remain upright.
“We showed for the first time that lizards swing their tail up or down to
counteract the rotation of their body, keeping them stable,” said team leader
Robert J. Full, UC Berkeley professor of integrative biology. “Inspiration from
lizard tails will likely lead to far more agile search-and-rescue robots, as
well as ones having greater capability to more rapidly detect chemical,
biological or nuclear hazards.”
Agile therapod dinosaurs like the velociraptor depicted in the movie Jurassic Park may also have used their tails as
stabilizers to prevent forward pitch, Full said. Their tail movement is
illustrated in a prescient chase sequence from the 1993 movie in which the
animated animal leaps from a balcony onto a T. rex skeleton.
“Muscles willing, the dinosaur could be even more effective with a swing of
its tail in controlling body attitude than the lizards,” Full said.
Student involvement crucial to research
Full and his laboratory colleagues, including both engineering and biology
students, will report their discoveries in Nature. The paper’s first
author, mechanical engineering graduate student Thomas Libby, also will report
the results on Jan. 7 at the annual meeting of the Society for Integrative and
Comparative Biology in Charleston,
Full is enthusiastic about the interplay fostered at UC Berkeley between
biologists and engineers in the Center for Interdisciplinary Bio-inspiration in
Education and Research (CiBER) laboratory, within which he offers a
research-based teaching laboratory that provides dozens of undergraduate
students with an opportunity to conduct cutting-edge research in teams with
graduate students. Each team experiences the benefits of how biologists and
engineers approach a problem.
“Learning in the context of original discovery, finding out something that
no one has ever know before, really motivated me,” said former UC Berkeley
integrative biology undergraduate Talia Moore, now a graduate student in the
Department of Organismic and Evolutionary Biology at Harvard University. “This
research-based lab course … showed me how biologists and engineers can work
together to benefit both fields.”
“This paper shows that research-based teaching leads to better learning and
simultaneously can lead to cutting-edge research,” added Full, who last year
briefed the U.S. House of Representative’s Science, Technology, Engineering and
Mathematics (STEM) Education Caucus on this topic. “It also shows the
competitive advantage of interdisciplinary approaches and how involvement of
undergraduates in research can lead to innovation.”
An Agama lizard next to Tailbot, a toy car with an attached tail. Sensors detect Tailbot’s orientation and swing the tail upward to keep the robot from pitching forward, similar to the way the lizard uses its tail. Photo: Robert Full laboratory, UC Berkeley.
From gecko toe hairs to tails
Full’s research over the past 20 years has revealed how the toe hairs of geckos
assist them in climbing smooth vertical surfaces and, more recently, how their
tails help to keep them from falling when they slip and to right themselves in
The new research tested a 40-year-old hypothesis that the two-legged
theropod dinosaurs—the ancestors of birds—used their tails as stabilizers while
running or dodging obstacles or predators. In Full’s teaching laboratory,
students noticed a lizard’s recovery after slipping during a leap and thought a
study of stumbling would be a perfect way to test the value of a tail.
In the CiBER laboratory, Full and six of his students used high-speed
videography and motion capture to record how a red-headed African Agama lizard
handled leaps from a platform with different degrees of traction, from slippery
to easily gripped.
They coaxed the lizards to run down a track, vault off a low platform, and
land on a vertical surface with a shelter on top. When the friction on the
platform was reduced, lizards slipped, causing their bodies to potentially spin
out of control.
When the researchers saw how the lizard used its tail to counteract the
spin, they created a mathematical model as well as Tailbot—a toy car equipped
with a tail and small gyroscope to sense body position—to better understand the
animal’s skills. With a tail but no feedback from sensors about body position,
Tailbot took a nose dive when driven off a ramp, mimicking a lizard’s take-off.
When body position was sensed and fed back to the tail motor, however, Tailbot
was able to stabilize its body in midair. The actively controlled tail
effectively redirected the angular momentum of the body into the tail’s swing,
as happens with leaping lizards, Full said.
Inertial assisted robotics
Tailbot’s design pushed the boundaries of control in robotics in an area
researchers call inertial assisted robotics, an attention-grabber at last
October’s meeting of the International Conference on Intelligent Robots and
Systems. The UC Berkeley researchers’ paper, presented by Libby and fellow
mechanical engineering graduate student Evan Chang-Siu, was one of five
finalists there among more than 2,000 robot studies.
“Engineers quickly understood the value of a tail,” Libby said, noting that
when he dropped Tailbot nose-down, it was able to right itself before it had
dropped a foot. “Robots are not nearly as agile as animals, so anything that
can make a robot more stable is an advancement, which is why this work is so
Full and his students are now investigating the role of the tail in
controlling pitch, roll, and yaw while running.