In 1969, a palaeontologist proposed1 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 walking2,3, aiding acrobatic manoeuvres by primates4,5,6,7,8 and rodents9, and enabling cats to balance on branches10. Recent studies on geckos11,12,13 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 performance14,15. 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 hypothesis1, 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.
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We thank P. Jennings for video editing and figure production, T. Full for digitizing the lizard video and K. Padian for his advice on the dinosaur reconstruction and capability. We thank O. O’Reilly, S. Sponberg and N. Sapir for advice on analysis. This work was supported by a US NSF FIBR grant to R.J.F., a MAST CTA grant to R.J.F, an NSF IGERT under award DGE-0903711 and a Swiss NSF Fellowship to A.J.
The authors declare no competing financial interests.
This file contains Supplementary Tables 1-2, Supplementary Figures 1 and legend and legends for Supplementary Movies 1-5. (PDF 254 kb)
This move shows a side view of a lizard, A. agama, leaping to a vertical surface from a high-friction vault – see Supplementary Information file for full legend. (MOV 3774 kb)
This movie shows a side view of a lizard, A. agama, leaping to a vertical surface from a low-friction vault - see Supplementary Information file for full legend. (MOV 2814 kb)
This movie shows a side view of the robot driving off an inclined ramp without feedback control - see Supplementary Information file for full legend. (MOV 4143 kb)
This movie shows a side view of robot driving off an inclined ramp with an active tail - see Supplementary Information file for full legend. (MOV 4365 kb)
This movie shows a side view of a lizard, A. agama, leaping to a vertical surface from a high-friction vault - see Supplementary Information file for full legend. (MOV 3484 kb)
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Libby, T., Moore, T., Chang-Siu, E. et al. Tail-assisted pitch control in lizards, robots and dinosaurs. Nature 481, 181–184 (2012) doi:10.1038/nature10710
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