IRIARTE-DIAZ, J.*; VASQUEZ, R.A.; BOZINOVIC, F.; Brown University; Universidad de Chile; Pontificia Universidad Catolica de Chile: The trot-gallop transition in a small mammal

The transition from trot to gallop in quadruped mammals has been widely hypothesized to be a strategy to minimize the energetic costs of running. Experimental evidence from horses running on treadmills, however, suggests that gait change is not triggered by energetic factors but by mechanical factors, when the musculoskeletal forces reach a critical value. An alternative hypothesis to the previous two to explain gait transition has been proposed based on the relationship among the forces that act on a body while moving. For terrestrial locomotion, gait transition is predicted to occur at speeds when the ratio of inertial to gravitational forces, known as the Froude number, reaches a critical value. All experiments to test these hypotheses have used relatively large species and the results, therefore, may not be applicable to small mammals, as they are probably constrained in their morphology and locomotion in different ways than larger mammals. In this study we evaluate the effects of loads on the locomotor energetics and gait transitions of degus (Octodon degus; Rodentia: Caviomorpha) running on a treadmill. The trot-gallop transition speed did not vary when rodents were experimentally loaded indicating that the forces applied to the musculoskeletal system do not trigger the gait transition. On the other hand, the transition speed was not associated to an energetic minimum indicating that changes of gait are not energetically determined. Experimentally loaded individuals switched to gallop at the same Froude number as controls, suggesting that the relationship between inertial and gravitational forces are important in determining the trot-gallop transition, even when the Froude number alone is not enough to describe the kinetics and kinematics of either walking or running gaits.