Tail-assisted pitch control in lizards, robots and dinosaurs

• Published in: Nature (London) Vol. 481; no. 7380; pp. 181 - 184
• Main Authors: Libby, Thomas, Moore, Talia Y., Chang-Siu, Evan, Li, Deborah, Cohen, Daniel J., Jusufi, Ardian, Full, Robert J.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 12-01-2012; Nature Publishing Group
• Subjects: 631/57/2272/2275; 631/601/18; 639/766/25; 639/766/747; Active control; Animals; Appendages; Biological and medical sciences; Biological Evolution; Biomechanical Phenomena; Computer Simulation; Dinosaurs; Dinosaurs - anatomy & histology; Dinosaurs - physiology; Feedback, Sensory - physiology; Fundamental and applied biological sciences. Psychology; Humanities and Social Sciences; Inertial; letter; Lizards; Lizards - anatomy & histology; Lizards - physiology; Microelectromechanical systems; Models, Biological; multidisciplinary; Ordinary differential equations; Posture - physiology; Predators; Regression analysis; Robot control; Robotics - instrumentation; Robots; Rotation; Science; Science (multidisciplinary); Stabilization; Tail - physiology; Vertebrates: body movement. Posture. Locomotion. Flight. Swimming. Physical exercise. Rest. Sports; Locomotion; Animal model; Vertebrata; Experimental test; Sauria; Agama agama; Stabilization; Kinematics; Tail; Reptilia; Dynamic model; Robot
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature10710

Comparison of real lizards with a robotic version and a dinosaur model shows that lizards use their tails to control body pitch in aerial motion by means of transfer of angular momentum from the body to the tail.
Leaping lizards use tails for stability
In 1969 — a few years after unearthing the first
Velociraptor
fossil — John Ostrom speculated that theropod dinosaurs used their tails as dynamic stabilizers during active or irregular movements. A study combining computer modelling, video observation of leaping agama lizards (
Agama agama
) and the construction of a robot with a lizard-like tail provides support for Ostrom's hypothesis. The results show that, using sensory feedback, active tails can stabilize body attitude mid-air by transferring angular momentum from body to tail. The inertia of swinging appendages has also been invoked as a stabilizing factor in primates and other animals, so these findings are relevant to our understanding of appendage evolution in general. They may also provide biological inspiration for the design of highly manoeuvrable search-and-rescue robots using tails.
In 1969, a palaeontologist proposed
1
that theropod dinosaurs used their tails as dynamic stabilizers during rapid or irregular movements, contributing to their depiction as active and agile predators. Since then the inertia of swinging appendages has been implicated in stabilizing human walking
2
,
3
, aiding acrobatic manoeuvres by primates
4
,
5
,
6
,
7
,
8
and rodents
9
, and enabling cats to balance on branches
10
. Recent studies on geckos
11
,
12
,
13
suggest that active tail stabilization occurs during climbing, righting and gliding. By contrast, studies on the effect of lizard tail loss show evidence of a decrease, an increase or no change in performance
14
,
15
. Application of a control-theoretic framework could advance our general understanding of inertial appendage use in locomotion. Here we report that lizards control the swing of their tails in a measured manner to redirect angular momentum from their bodies to their tails, stabilizing body attitude in the sagittal plane. We video-recorded Red-Headed Agama lizards (
Agama agama
) leaping towards a vertical surface by first vaulting onto an obstacle with variable traction to induce a range of perturbations in body angular momentum. To examine a known controlled tail response, we built a lizard-sized robot with an active tail that used sensory feedback to stabilize pitch as it drove off a ramp. Our dynamics model revealed that a body swinging its tail experienced less rotation than a body with a rigid tail, a passively compliant tail or no tail. To compare a range of tails, we calculated tail effectiveness as the amount of tailless body rotation a tail could stabilize. A model
Velociraptor mongoliensis
supported the initial tail stabilization hypothesis
1
, showing as it did a greater tail effectiveness than the Agama lizards. Leaping lizards show that inertial control of body attitude can advance our understanding of appendage evolution and provide biological inspiration for the next generation of manoeuvrable search-and-rescue robots.