Kinetics of Symmetrical Versus Asymmetrical In-Phase Gaits During Arboreal Locomotion

Quadrupedal animals traveling on arboreal supports change aspects of locomotion to avoid slipping and falls. This study compares locomotor biomechanics in two small mammals: first, the gray short-tailed opossum (Monodelphis domestica) predominantly trots, which is a symmetrical gait. The second species, the Siberian chipmunk (Tamias sibiricus), primarily bounds or half-bounds. Trotting and bounding differ fundamentally in three aspects: location and timing of hand and foot placement; in the way that the trunk bends (trotting, mediolateral bending; bounding, flexion, and extension); and in the dynamics of the center of mass. Both species ran on a flat track and a 2 cm diameter cylindrical track, instrumented with a force plate or pole. For bounding chipmunks, the force pole was modified to measure force only on the right side. We measured speed, duty factor, and force, and calculated vertical, braking, propulsive, and net mediolateral impulses. Vertical and fore-aft impulses were different between trotting opossums and bounding chipmunks, but between trackway types, these impulses were similar within each species. The modifications used by each species to travel on arboreal supports were similar, except in one important respect. Net mediolateral impulse in opossums changed from laterally directed on the flat trackway to medial on the arboreal. But in chipmunks, these impulses on the flat track were medially-directed, and on the arboreal track, the amount of variability was substantially greater. We conclude that chipmunks-and perhaps any bounding animal-are less consistent from stride to stride in their locomotion. This inconsistency requires constant medial and lateral impulses to correct their trajectory when traveling on arboreal surfaces.