Meeting Abstract

138.6  Monday, Jan. 7  Coordination between inertial and impulsive mechanisms during rapid turns in lizards. LIBBY, T*; GUDENUS, V; HARRINGTON, P; FULL, R.J.; Univ. of California, Berkeley tlibby@berkeley.edu

Animals induce aerial reorientation by swinging appendages or bending torsos. Inertial torques also play a role during terrestrial locomotion, but ground reaction impulses can change angular momentum. To examine the role of back bending and tail swinging during rapid terrestrial turns in lizards (Agama agama), we developed a six-link, planar, rigid-body dynamics model. Informed by the morphometrics of lizards, our model enabled estimation of total angular momentum about the animal’s center of mass (COM) from high-speed video kinematics. We derived the model to represent the expression for angular momentum about the COM of a chain of rigid bodies for an arbitrary number of segments. By writing angular momentum in terms of shape coordinates, we decomposed body velocity into two components revealing the extent to which shape change and impulsive force each contribute towards turning the body. During escape responses, lizards started from a standstill, executed a rapid turn and then ran away from the stimulus. Escape turns typically began with curling and pivoting about the hind legs, followed by an acceleration through the second stride in a maneuver analogous to a C-start in fish. Turns averaged 112°. 86% of the turn was completed within the first stride. Our model predicts that 57% of the rotation during the first stride can be attributed to inertial torques due to curling. Systematically reducing the number of segments in the zero angular momentum model revealed that over 70% of the shape change induced rotation was due to the tail, with the remaining fraction due to back bending. During the second stride, angular velocity from impulsive ground contact countered the tendency towards backwards rotation when the tail uncurled as the animal transitioned to steady running.