IRSCHICK, D. J.*; FULL, R. J.; Tulane University; University of California at Berkeley: Running without a back: Experimental manipulations of axial bending in geckos and effects on kinematics and force dynamics

We investigated the role of axial bending on force dynamics and kinematics during steady-speed horizontal locomotion in the gecko Hemidactylus garnoti. Axial bending has been hypothesized to increase maximum speed in lower tetrapods by increasing stride length, but no study has experimentally manipulated axial bending. We induced geckos to run without bending their back by employing an experimental design in which we placed small plexiglas constraints over the axial portion of geckos. We compared constrained locomotion to that for geckos with a weight equaling the constraint (23% body weight), as well as unconstrained locomotion (4 geckos, 46 runs total). Maximum speeds did not differ significantly between unconstrained (mean=174 cm/s) and weighted (159 cm/s) runs, but declined substantially (~50%) with the presence of the constraint (85 cm/s). The presence of the constraint dramatically diminished the degree of axial bending, but the presence of the weight did not significantly affect axial bending. Indeed, kinematics and force dynamics in weighted and unconstrained lizards were very similar, but constrained lizards exhibited significantly lower whole-body and single-leg lateral forces. For example, the average single-leg lateral forces for constrained lizards were 0.018 N (forelimb) and 0.021 N (hindlimb) compared to 0.025 N (forelimb) and 0.032 N (hindlimb) for weighted lizards. These data show that axial bending provides a performance advantage for lizards, and the lateral forces may play a key role in power production during steady-speed locomotion. Further, they also highlight the remarkable ability of geckos to run with relatively large loads. Supported by NSF grant IBN 9983003 to D. Irschick.