Mechanics of generating friction during locomotion on rough and smooth arboreal trackways.

Traveling on arboreal substrates is common among most small mammals living anywhere vegetation grows. Because arboreal supports vary considerably in surface texture, animals must be able to adjust their locomotor biomechanics to remain stable on such supports. I examined how gray short-tailed opossums (Monodelphis domestica), which are generalized marsupials living on or near the ground, adjust to travel on rough and smooth 2 cm-diameter arboreal trackways. Limb contact position was determined via high-speed videography, and substrate reaction force was measured by an instrumented section of each branch trackway. Normal and shear forces were calculated from substrate reaction force and limb contact position around the branch trackways. Normal force is greater in forelimbs, probably because of the forelimb's greater weight support role. Shear force was identical between limb pairs, most likely because of interactions between vertical force, limb placement, mediolateral force, and torque. The opossums adjusted to the smooth trackway mainly by reducing speed, changing footfall patterns and increasing normal force. I predict that arboreal specialists will show less change in performance between rough and smooth arboreal trackways because of their greater ability to grasp or maintain contact with arboreal substrates.